OOP Intermediate
Interfaces
Contracts for Behavior
A class can only extend one parent. But a Dog can be both an Animal and a Pet. Interfaces define contracts - a class can implement multiple interfaces, promising to provide certain methods without inheriting implementation.
Basic interface
Define a contract that classes must fulfill.
BasicInterface.java
Replay: real traced execution (multi-file project)
// Basic Interface Declaration and Implementation
public class BasicInterface {
public static void main(String[] args) {
System.out.println("=== Basic Interface ===\n");
// Cannot instantiate interface!
// Drawable d = new Drawable(); // ERROR
// Create implementing classes
double circleRadius = 5;
double rectangleWidth = 4;
Circle circle = new Circle(circleRadius);
Rectangle rectangle = new Rectangle(rectangleWidth, 6);
// Call interface methods
circle.draw();
rectangle.draw();
System.out.println("\n=== Interface as Type ===");
// Use interface type for reference
Drawable shape1 = new Circle(3);
Drawable shape2 = new Rectangle(2, 5);
shape1.draw();
shape2.draw();
System.out.println("\n=== Array of Interface Type ===");
Drawable[] shapes = {
new Circle(1),
new Rectangle(2, 3),
new Circle(4)
};
for (Drawable shape : shapes) {
shape.draw();
}
System.out.println("\n=== Interface Characteristics ===");
System.out.println("""
Interface:
- Cannot be instantiated
- Methods are public abstract by default
- Fields are public static final (constants)
- Classes use 'implements' keyword
- Implementing class MUST provide all methods
""");
}
}
// Interface declaration
interface Drawable {
// Method declaration - implicitly public abstract
void draw();
// Can have multiple methods
void resize(double factor);
// Constants - implicitly public static final
int DEFAULT_SIZE = 10;
}
// Class implements interface
class Circle implements Drawable {
private double radius;
Circle(double radius) {
this.radius = radius;
}
// MUST implement all interface methods
@Override
public void draw() {
System.out.println("Drawing circle with radius " + radius);
}
@Override
public void resize(double factor) {
radius *= factor;
System.out.println("Circle resized to radius " + radius);
}
}
class Rectangle implements Drawable {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public void draw() {
System.out.println("Drawing rectangle " + width + " x " + height);
}
@Override
public void resize(double factor) {
width *= factor;
height *= factor;
System.out.println("Rectangle resized to " + width + " x " + height);
}
}
// Basic Interface Declaration and Implementation
public class BasicInterface {
public static void main(String[] args) {
System.out.println("=== Basic Interface ===\n");
// Cannot instantiate interface!
// Drawable d = new Drawable(); // ERROR
// Create implementing classes
double circleRadius = 2;
double rectangleWidth = 4;
Circle circle = new Circle(circleRadius);
Rectangle rectangle = new Rectangle(rectangleWidth, 6);
// Call interface methods
circle.draw();
rectangle.draw();
System.out.println("\n=== Interface as Type ===");
// Use interface type for reference
Drawable shape1 = new Circle(3);
Drawable shape2 = new Rectangle(2, 5);
shape1.draw();
shape2.draw();
System.out.println("\n=== Array of Interface Type ===");
Drawable[] shapes = {
new Circle(1),
new Rectangle(2, 3),
new Circle(4)
};
for (Drawable shape : shapes) {
shape.draw();
}
System.out.println("\n=== Interface Characteristics ===");
System.out.println("""
Interface:
- Cannot be instantiated
- Methods are public abstract by default
- Fields are public static final (constants)
- Classes use 'implements' keyword
- Implementing class MUST provide all methods
""");
}
}
// Interface declaration
interface Drawable {
// Method declaration - implicitly public abstract
void draw();
// Can have multiple methods
void resize(double factor);
// Constants - implicitly public static final
int DEFAULT_SIZE = 10;
}
// Class implements interface
class Circle implements Drawable {
private double radius;
Circle(double radius) {
this.radius = radius;
}
// MUST implement all interface methods
@Override
public void draw() {
System.out.println("Drawing circle with radius " + radius);
}
@Override
public void resize(double factor) {
radius *= factor;
System.out.println("Circle resized to radius " + radius);
}
}
class Rectangle implements Drawable {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public void draw() {
System.out.println("Drawing rectangle " + width + " x " + height);
}
@Override
public void resize(double factor) {
width *= factor;
height *= factor;
System.out.println("Rectangle resized to " + width + " x " + height);
}
}
// Basic Interface Declaration and Implementation
public class BasicInterface {
public static void main(String[] args) {
System.out.println("=== Basic Interface ===\n");
// Cannot instantiate interface!
// Drawable d = new Drawable(); // ERROR
// Create implementing classes
double circleRadius = 8;
double rectangleWidth = 4;
Circle circle = new Circle(circleRadius);
Rectangle rectangle = new Rectangle(rectangleWidth, 6);
// Call interface methods
circle.draw();
rectangle.draw();
System.out.println("\n=== Interface as Type ===");
// Use interface type for reference
Drawable shape1 = new Circle(3);
Drawable shape2 = new Rectangle(2, 5);
shape1.draw();
shape2.draw();
System.out.println("\n=== Array of Interface Type ===");
Drawable[] shapes = {
new Circle(1),
new Rectangle(2, 3),
new Circle(4)
};
for (Drawable shape : shapes) {
shape.draw();
}
System.out.println("\n=== Interface Characteristics ===");
System.out.println("""
Interface:
- Cannot be instantiated
- Methods are public abstract by default
- Fields are public static final (constants)
- Classes use 'implements' keyword
- Implementing class MUST provide all methods
""");
}
}
// Interface declaration
interface Drawable {
// Method declaration - implicitly public abstract
void draw();
// Can have multiple methods
void resize(double factor);
// Constants - implicitly public static final
int DEFAULT_SIZE = 10;
}
// Class implements interface
class Circle implements Drawable {
private double radius;
Circle(double radius) {
this.radius = radius;
}
// MUST implement all interface methods
@Override
public void draw() {
System.out.println("Drawing circle with radius " + radius);
}
@Override
public void resize(double factor) {
radius *= factor;
System.out.println("Circle resized to radius " + radius);
}
}
class Rectangle implements Drawable {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public void draw() {
System.out.println("Drawing rectangle " + width + " x " + height);
}
@Override
public void resize(double factor) {
width *= factor;
height *= factor;
System.out.println("Rectangle resized to " + width + " x " + height);
}
}
// Basic Interface Declaration and Implementation
public class BasicInterface {
public static void main(String[] args) {
System.out.println("=== Basic Interface ===\n");
// Cannot instantiate interface!
// Drawable d = new Drawable(); // ERROR
// Create implementing classes
double circleRadius = 5;
double rectangleWidth = 3;
Circle circle = new Circle(circleRadius);
Rectangle rectangle = new Rectangle(rectangleWidth, 6);
// Call interface methods
circle.draw();
rectangle.draw();
System.out.println("\n=== Interface as Type ===");
// Use interface type for reference
Drawable shape1 = new Circle(3);
Drawable shape2 = new Rectangle(2, 5);
shape1.draw();
shape2.draw();
System.out.println("\n=== Array of Interface Type ===");
Drawable[] shapes = {
new Circle(1),
new Rectangle(2, 3),
new Circle(4)
};
for (Drawable shape : shapes) {
shape.draw();
}
System.out.println("\n=== Interface Characteristics ===");
System.out.println("""
Interface:
- Cannot be instantiated
- Methods are public abstract by default
- Fields are public static final (constants)
- Classes use 'implements' keyword
- Implementing class MUST provide all methods
""");
}
}
// Interface declaration
interface Drawable {
// Method declaration - implicitly public abstract
void draw();
// Can have multiple methods
void resize(double factor);
// Constants - implicitly public static final
int DEFAULT_SIZE = 10;
}
// Class implements interface
class Circle implements Drawable {
private double radius;
Circle(double radius) {
this.radius = radius;
}
// MUST implement all interface methods
@Override
public void draw() {
System.out.println("Drawing circle with radius " + radius);
}
@Override
public void resize(double factor) {
radius *= factor;
System.out.println("Circle resized to radius " + radius);
}
}
class Rectangle implements Drawable {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public void draw() {
System.out.println("Drawing rectangle " + width + " x " + height);
}
@Override
public void resize(double factor) {
width *= factor;
height *= factor;
System.out.println("Rectangle resized to " + width + " x " + height);
}
}
// Basic Interface Declaration and Implementation
public class BasicInterface {
public static void main(String[] args) {
System.out.println("=== Basic Interface ===\n");
// Cannot instantiate interface!
// Drawable d = new Drawable(); // ERROR
// Create implementing classes
double circleRadius = 5;
double rectangleWidth = 7;
Circle circle = new Circle(circleRadius);
Rectangle rectangle = new Rectangle(rectangleWidth, 6);
// Call interface methods
circle.draw();
rectangle.draw();
System.out.println("\n=== Interface as Type ===");
// Use interface type for reference
Drawable shape1 = new Circle(3);
Drawable shape2 = new Rectangle(2, 5);
shape1.draw();
shape2.draw();
System.out.println("\n=== Array of Interface Type ===");
Drawable[] shapes = {
new Circle(1),
new Rectangle(2, 3),
new Circle(4)
};
for (Drawable shape : shapes) {
shape.draw();
}
System.out.println("\n=== Interface Characteristics ===");
System.out.println("""
Interface:
- Cannot be instantiated
- Methods are public abstract by default
- Fields are public static final (constants)
- Classes use 'implements' keyword
- Implementing class MUST provide all methods
""");
}
}
// Interface declaration
interface Drawable {
// Method declaration - implicitly public abstract
void draw();
// Can have multiple methods
void resize(double factor);
// Constants - implicitly public static final
int DEFAULT_SIZE = 10;
}
// Class implements interface
class Circle implements Drawable {
private double radius;
Circle(double radius) {
this.radius = radius;
}
// MUST implement all interface methods
@Override
public void draw() {
System.out.println("Drawing circle with radius " + radius);
}
@Override
public void resize(double factor) {
radius *= factor;
System.out.println("Circle resized to radius " + radius);
}
}
class Rectangle implements Drawable {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public void draw() {
System.out.println("Drawing rectangle " + width + " x " + height);
}
@Override
public void resize(double factor) {
width *= factor;
height *= factor;
System.out.println("Rectangle resized to " + width + " x " + height);
}
}
circleRadius ← 5.0, rectangleWidth ← 4.0
3public class BasicInterface {4 public static void main(String[] args) {5 System.out.println("=== Basic Interface ===\n");6 7 // Cannot instantiate interface! //?noinstantiate8 // Drawable d = new Drawable(); // ERROR9 10 // Create implementing classes //?createimpl11 double circleRadius→ 5.0 = 5; //@circleRadius=2, 812 double rectangleWidth→ 4.0 = 4; //@rectangleWidth=3, 713 Circle circle = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);output=== Basic Interface ===this.radius ← 5.0, circle ← ⟨Circle A⟩
pass 1 of 412 double rectangleWidth = 4; //@rectangleWidth=3, 713 Circle circle→ ⟨Circle A⟩ = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods //?callmethods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference //?interfacetype23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = { //?interfacearray32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw(); //?polymorphism39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration //?interfacedecl54interface Drawable {55 // Method declaration - implicitly public abstract //?methoddecl56 void draw(); //?implicitpublic57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final //?constant62 int DEFAULT_SIZE = 10; //?implicitfinal63}6465// Class implements interface //?classimpl66class Circle implements Drawable { //?implements67 private double radius;68 69 Circle(double radius5.0) {70 this.radius→ 5.0 = radius5.0;71 }All 4 passes — pass 1 is the card above pass radiusthis.radiuscircleshape11 5.0 5.0 ⟨Circle A⟩ — 2 3.0 3.0 — ⟨Circle B⟩ 3 1.0 1.0 — — 4 4.0 4.0 — — this.width ← 4.0, this.height ← 6.0, rectangle ← ⟨Rectangle C⟩
pass 1 of 313 Circle circle = new Circle(circleRadius);14 Rectangle rectangle→ ⟨Rectangle C⟩ = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods //?callmethods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference //?interfacetype23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = { //?interfacearray32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw(); //?polymorphism39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration //?interfacedecl54interface Drawable {55 // Method declaration - implicitly public abstract //?methoddecl56 void draw(); //?implicitpublic57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final //?constant62 int DEFAULT_SIZE = 10; //?implicitfinal63}6465// Class implements interface //?classimpl66class Circle implements Drawable { //?implements67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods //?mustimplement74 @Override75 public void draw() { //?publicrequired76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width4.0, double height6.0) {90 this.width→ 4.0 = width4.0;91 this.height→ 6.0 = height6.0;92 }All 3 passes — pass 1 is the card above pass widthheightthis.widththis.heightrectangleshape21 4.0 6.0 4.0 6.0 ⟨Rectangle C⟩ — 2 2.0 5.0 2.0 5.0 — ⟨Rectangle D⟩ 3 2.0 3.0 2.0 3.0 — — @Override public void draw()
pass 1 of 416 // Call interface methods //?callmethods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference //?interfacetype23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = { //?interfacearray32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw(); //?polymorphism39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration //?interfacedecl54interface Drawable {55 // Method declaration - implicitly public abstract //?methoddecl56 void draw(); //?implicitpublic57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final //?constant62 int DEFAULT_SIZE = 10; //?implicitfinal63}6465// Class implements interface //?classimpl66class Circle implements Drawable { //?implements67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods //?mustimplement74 @Override75 public void draw() { //?publicrequired76 System.out.println("Drawing circle with radius " + radius5.0);77 }outputDrawing circle with radius 5.0All 4 passes — pass 1 is the card above pass radius1 5.0 2 3.0 3 1.0 4 4.0 @Override public void draw()
pass 1 of 317 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference //?interfacetype23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = { //?interfacearray32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw(); //?polymorphism39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration //?interfacedecl54interface Drawable {55 // Method declaration - implicitly public abstract //?methoddecl56 void draw(); //?implicitpublic57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final //?constant62 int DEFAULT_SIZE = 10; //?implicitfinal63}6465// Class implements interface //?classimpl66class Circle implements Drawable { //?implements67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods //?mustimplement74 @Override75 public void draw() { //?publicrequired76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width, double height) {90 this.width = width;91 this.height = height;92 }93 94 @Override95 public void draw() {96 System.out.println("Drawing rectangle " + width4.0 + " x " + height6.0);97 }outputDrawing rectangle 4.0 x 6.0 === Interface as Type ===All 3 passes — pass 1 is the card above pass widthheight1 4.0 6.0 2 2.0 5.0 3 2.0 3.0 for (Drawable shape : shapes)
pass 1 of 337for (Drawable shape⟨Circle E⟩ : shapes) {38 shape.draw(); //?polymorphism39}All 3 passes — pass 1 is the card above pass shape1 ⟨Circle E⟩ 2 ⟨Rectangle F⟩ 3 ⟨Circle G⟩
circleRadius ← 2.0, rectangleWidth ← 4.0
3public class BasicInterface {4 public static void main(String[] args) {5 System.out.println("=== Basic Interface ===\n");6 7 // Cannot instantiate interface!8 // Drawable d = new Drawable(); // ERROR9 10 // Create implementing classes11 double circleRadius→ 2.0 = 2;12 double rectangleWidth→ 4.0 = 4;13 Circle circle = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);output=== Basic Interface ===this.radius ← 2.0, circle ← ⟨Circle A⟩
pass 1 of 412 double rectangleWidth = 4;13 Circle circle→ ⟨Circle A⟩ = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius2.0) {70 this.radius→ 2.0 = radius2.0;71 }All 4 passes — pass 1 is the card above pass radiusthis.radiuscircleshape11 2.0 2.0 ⟨Circle A⟩ — 2 3.0 3.0 — ⟨Circle B⟩ 3 1.0 1.0 — — 4 4.0 4.0 — — this.width ← 4.0, this.height ← 6.0, rectangle ← ⟨Rectangle C⟩
pass 1 of 313 Circle circle = new Circle(circleRadius);14 Rectangle rectangle→ ⟨Rectangle C⟩ = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width4.0, double height6.0) {90 this.width→ 4.0 = width4.0;91 this.height→ 6.0 = height6.0;92 }All 3 passes — pass 1 is the card above pass widthheightthis.widththis.heightrectangleshape21 4.0 6.0 4.0 6.0 ⟨Rectangle C⟩ — 2 2.0 5.0 2.0 5.0 — ⟨Rectangle D⟩ 3 2.0 3.0 2.0 3.0 — — @Override public void draw()
pass 1 of 416 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius2.0);77 }outputDrawing circle with radius 2.0All 4 passes — pass 1 is the card above pass radius1 2.0 2 3.0 3 1.0 4 4.0 @Override public void draw()
pass 1 of 317 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width, double height) {90 this.width = width;91 this.height = height;92 }93 94 @Override95 public void draw() {96 System.out.println("Drawing rectangle " + width4.0 + " x " + height6.0);97 }outputDrawing rectangle 4.0 x 6.0 === Interface as Type ===All 3 passes — pass 1 is the card above pass widthheight1 4.0 6.0 2 2.0 5.0 3 2.0 3.0 for (Drawable shape : shapes)
pass 1 of 337for (Drawable shape⟨Circle E⟩ : shapes) {38 shape.draw();39}All 3 passes — pass 1 is the card above pass shape1 ⟨Circle E⟩ 2 ⟨Rectangle F⟩ 3 ⟨Circle G⟩
circleRadius ← 8.0, rectangleWidth ← 4.0
3public class BasicInterface {4 public static void main(String[] args) {5 System.out.println("=== Basic Interface ===\n");6 7 // Cannot instantiate interface!8 // Drawable d = new Drawable(); // ERROR9 10 // Create implementing classes11 double circleRadius→ 8.0 = 8;12 double rectangleWidth→ 4.0 = 4;13 Circle circle = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);output=== Basic Interface ===this.radius ← 8.0, circle ← ⟨Circle A⟩
pass 1 of 412 double rectangleWidth = 4;13 Circle circle→ ⟨Circle A⟩ = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius8.0) {70 this.radius→ 8.0 = radius8.0;71 }All 4 passes — pass 1 is the card above pass radiusthis.radiuscircleshape11 8.0 8.0 ⟨Circle A⟩ — 2 3.0 3.0 — ⟨Circle B⟩ 3 1.0 1.0 — — 4 4.0 4.0 — — this.width ← 4.0, this.height ← 6.0, rectangle ← ⟨Rectangle C⟩
pass 1 of 313 Circle circle = new Circle(circleRadius);14 Rectangle rectangle→ ⟨Rectangle C⟩ = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width4.0, double height6.0) {90 this.width→ 4.0 = width4.0;91 this.height→ 6.0 = height6.0;92 }All 3 passes — pass 1 is the card above pass widthheightthis.widththis.heightrectangleshape21 4.0 6.0 4.0 6.0 ⟨Rectangle C⟩ — 2 2.0 5.0 2.0 5.0 — ⟨Rectangle D⟩ 3 2.0 3.0 2.0 3.0 — — @Override public void draw()
pass 1 of 416 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius8.0);77 }outputDrawing circle with radius 8.0All 4 passes — pass 1 is the card above pass radius1 8.0 2 3.0 3 1.0 4 4.0 @Override public void draw()
pass 1 of 317 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width, double height) {90 this.width = width;91 this.height = height;92 }93 94 @Override95 public void draw() {96 System.out.println("Drawing rectangle " + width4.0 + " x " + height6.0);97 }outputDrawing rectangle 4.0 x 6.0 === Interface as Type ===All 3 passes — pass 1 is the card above pass widthheight1 4.0 6.0 2 2.0 5.0 3 2.0 3.0 for (Drawable shape : shapes)
pass 1 of 337for (Drawable shape⟨Circle E⟩ : shapes) {38 shape.draw();39}All 3 passes — pass 1 is the card above pass shape1 ⟨Circle E⟩ 2 ⟨Rectangle F⟩ 3 ⟨Circle G⟩
circleRadius ← 5.0, rectangleWidth ← 3.0
3public class BasicInterface {4 public static void main(String[] args) {5 System.out.println("=== Basic Interface ===\n");6 7 // Cannot instantiate interface!8 // Drawable d = new Drawable(); // ERROR9 10 // Create implementing classes11 double circleRadius→ 5.0 = 5;12 double rectangleWidth→ 3.0 = 3;13 Circle circle = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);output=== Basic Interface ===this.radius ← 5.0, circle ← ⟨Circle A⟩
pass 1 of 412 double rectangleWidth = 3;13 Circle circle→ ⟨Circle A⟩ = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius5.0) {70 this.radius→ 5.0 = radius5.0;71 }All 4 passes — pass 1 is the card above pass radiusthis.radiuscircleshape11 5.0 5.0 ⟨Circle A⟩ — 2 3.0 3.0 — ⟨Circle B⟩ 3 1.0 1.0 — — 4 4.0 4.0 — — this.width ← 3.0, this.height ← 6.0, rectangle ← ⟨Rectangle C⟩
pass 1 of 313 Circle circle = new Circle(circleRadius);14 Rectangle rectangle→ ⟨Rectangle C⟩ = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width3.0, double height6.0) {90 this.width→ 3.0 = width3.0;91 this.height→ 6.0 = height6.0;92 }All 3 passes — pass 1 is the card above pass widthheightthis.widththis.heightrectangleshape21 3.0 6.0 3.0 6.0 ⟨Rectangle C⟩ — 2 2.0 5.0 2.0 5.0 — ⟨Rectangle D⟩ 3 2.0 3.0 2.0 3.0 — — @Override public void draw()
pass 1 of 416 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius5.0);77 }outputDrawing circle with radius 5.0All 4 passes — pass 1 is the card above pass radius1 5.0 2 3.0 3 1.0 4 4.0 @Override public void draw()
pass 1 of 317 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width, double height) {90 this.width = width;91 this.height = height;92 }93 94 @Override95 public void draw() {96 System.out.println("Drawing rectangle " + width3.0 + " x " + height6.0);97 }outputDrawing rectangle 3.0 x 6.0 === Interface as Type ===All 3 passes — pass 1 is the card above pass widthheight1 3.0 6.0 2 2.0 5.0 3 2.0 3.0 for (Drawable shape : shapes)
pass 1 of 337for (Drawable shape⟨Circle E⟩ : shapes) {38 shape.draw();39}All 3 passes — pass 1 is the card above pass shape1 ⟨Circle E⟩ 2 ⟨Rectangle F⟩ 3 ⟨Circle G⟩
circleRadius ← 5.0, rectangleWidth ← 7.0
3public class BasicInterface {4 public static void main(String[] args) {5 System.out.println("=== Basic Interface ===\n");6 7 // Cannot instantiate interface!8 // Drawable d = new Drawable(); // ERROR9 10 // Create implementing classes11 double circleRadius→ 5.0 = 5;12 double rectangleWidth→ 7.0 = 7;13 Circle circle = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);output=== Basic Interface ===this.radius ← 5.0, circle ← ⟨Circle A⟩
pass 1 of 412 double rectangleWidth = 7;13 Circle circle→ ⟨Circle A⟩ = new Circle(circleRadius);14 Rectangle rectangle = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius5.0) {70 this.radius→ 5.0 = radius5.0;71 }All 4 passes — pass 1 is the card above pass radiusthis.radiuscircleshape11 5.0 5.0 ⟨Circle A⟩ — 2 3.0 3.0 — ⟨Circle B⟩ 3 1.0 1.0 — — 4 4.0 4.0 — — this.width ← 7.0, this.height ← 6.0, rectangle ← ⟨Rectangle C⟩
pass 1 of 313 Circle circle = new Circle(circleRadius);14 Rectangle rectangle→ ⟨Rectangle C⟩ = new Rectangle(rectangleWidth, 6);15 16 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width7.0, double height6.0) {90 this.width→ 7.0 = width7.0;91 this.height→ 6.0 = height6.0;92 }All 3 passes — pass 1 is the card above pass widthheightthis.widththis.heightrectangleshape21 7.0 6.0 7.0 6.0 ⟨Rectangle C⟩ — 2 2.0 5.0 2.0 5.0 — ⟨Rectangle D⟩ 3 2.0 3.0 2.0 3.0 — — @Override public void draw()
pass 1 of 416 // Call interface methods17 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius5.0);77 }outputDrawing circle with radius 5.0All 4 passes — pass 1 is the card above pass radius1 5.0 2 3.0 3 1.0 4 4.0 @Override public void draw()
pass 1 of 317 circle.draw();18 rectangle.draw();19 20 System.out.println("\n=== Interface as Type ===");21 22 // Use interface type for reference23 Drawable shape1 = new Circle(3);24 Drawable shape2 = new Rectangle(2, 5);25 26 shape1.draw();27 shape2.draw();28 29 System.out.println("\n=== Array of Interface Type ===");30 31 Drawable[] shapes = {32 new Circle(1),33 new Rectangle(2, 3),34 new Circle(4)35 };36 37 for (Drawable shape : shapes) {38 shape.draw();39 }40 41 System.out.println("\n=== Interface Characteristics ===");42 System.out.println("""43 Interface:44 - Cannot be instantiated45 - Methods are public abstract by default46 - Fields are public static final (constants)47 - Classes use 'implements' keyword48 - Implementing class MUST provide all methods49 """);50 }51}5253// Interface declaration54interface Drawable {55 // Method declaration - implicitly public abstract56 void draw();57 58 // Can have multiple methods59 void resize(double factor);60 61 // Constants - implicitly public static final62 int DEFAULT_SIZE = 10;63}6465// Class implements interface66class Circle implements Drawable {67 private double radius;68 69 Circle(double radius) {70 this.radius = radius;71 }72 73 // MUST implement all interface methods74 @Override75 public void draw() {76 System.out.println("Drawing circle with radius " + radius);77 }78 79 @Override80 public void resize(double factor) {81 radius *= factor;82 System.out.println("Circle resized to radius " + radius);83 }84}8586class Rectangle implements Drawable {87 private double width, height;88 89 Rectangle(double width, double height) {90 this.width = width;91 this.height = height;92 }93 94 @Override95 public void draw() {96 System.out.println("Drawing rectangle " + width7.0 + " x " + height6.0);97 }outputDrawing rectangle 7.0 x 6.0 === Interface as Type ===All 3 passes — pass 1 is the card above pass widthheight1 7.0 6.0 2 2.0 5.0 3 2.0 3.0 for (Drawable shape : shapes)
pass 1 of 337for (Drawable shape⟨Circle E⟩ : shapes) {38 shape.draw();39}All 3 passes — pass 1 is the card above pass shape1 ⟨Circle E⟩ 2 ⟨Rectangle F⟩ 3 ⟨Circle G⟩
interface defines method signatures. No implementation (pre-Java 8).
interface
Contract of methods a class must implement. All methods implicitly public abstract.
Implement multiple interfaces
A class can implement many interfaces.
MultipleInterfaces.java
Replay: real traced execution (multi-file project)
// Implementing Multiple Interfaces
public class MultipleInterfaces {
public static void main(String[] args) {
System.out.println("=== Multiple Interfaces ===\n");
// SmartPhone implements multiple interfaces
SmartPhone phone = new SmartPhone("iPhone 15");
System.out.println("--- Using as Camera ---");
phone.takePhoto();
phone.recordVideo();
System.out.println("\n--- Using as Phone ---");
phone.makeCall("555-1234");
phone.sendText("Hello!");
System.out.println("\n--- Using as MediaPlayer ---");
phone.playMusic("Bohemian Rhapsody");
phone.playVideo("YouTube clip");
System.out.println("\n=== Interface References ===");
// Same object, different interface views
Camera cam = phone;
Callable call = phone;
MediaPlayer player = phone;
cam.takePhoto(); // Only Camera methods visible
call.makeCall("555-5678");
player.playMusic("Song");
System.out.println("\n=== Duck Typing with Interfaces ===");
// Different devices, same interfaces
Camera[] cameras = {
new SmartPhone("Pixel"),
new DigitalCamera("Canon"),
};
for (Camera c : cameras) {
c.takePhoto();
}
}
}
// Interface 1
interface Camera {
void takePhoto();
void recordVideo();
}
// Interface 2
interface Callable {
void makeCall(String number);
void sendText(String message);
}
// Interface 3
interface MediaPlayer {
void playMusic(String song);
void playVideo(String video);
}
// Class implementing MULTIPLE interfaces
class SmartPhone implements Camera, Callable, MediaPlayer {
private String model;
SmartPhone(String model) {
this.model = model;
}
// From Camera
@Override
public void takePhoto() {
System.out.println(model + ": 📷 Taking photo");
}
@Override
public void recordVideo() {
System.out.println(model + ": 🎥 Recording video");
}
// From Callable
@Override
public void makeCall(String number) {
System.out.println(model + ": 📞 Calling " + number);
}
@Override
public void sendText(String message) {
System.out.println(model + ": 💬 Sending: " + message);
}
// From MediaPlayer
@Override
public void playMusic(String song) {
System.out.println(model + ": 🎵 Playing " + song);
}
@Override
public void playVideo(String video) {
System.out.println(model + ": ▶️ Playing " + video);
}
}
// Another class implementing just Camera
class DigitalCamera implements Camera {
private String model;
DigitalCamera(String model) {
this.model = model;
}
@Override
public void takePhoto() {
System.out.println(model + " DSLR: 📷 High-quality photo");
}
@Override
public void recordVideo() {
System.out.println(model + " DSLR: 🎥 4K video recording");
}
// No need to implement Callable or MediaPlayer!
}
public static void main(String[] args)
3public class MultipleInterfaces {4 public static void main(String[] args) {5 System.out.println("=== Multiple Interfaces ===\n");6 7 // SmartPhone implements multiple interfaces //?multiimpl8 SmartPhone phone = new SmartPhone("iPhone 15");output=== Multiple Interfaces ===this.model ← iPhone 15, phone ← ⟨SmartPhone A⟩
pass 1 of 27 // SmartPhone implements multiple interfaces //?multiimpl8 SmartPhone phone→ ⟨SmartPhone A⟩ = new SmartPhone("iPhone 15");9 10 System.out.println("--- Using as Camera ---");11 phone.takePhoto();12 phone.recordVideo();13 14 System.out.println("\n--- Using as Phone ---");15 phone.makeCall("555-1234");16 phone.sendText("Hello!");17 18 System.out.println("\n--- Using as MediaPlayer ---");19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam = phone; //?ascamera26 Callable call = phone; //?ascallable27 MediaPlayer player = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String modeliPhone 15) {70 this.model→ iPhone 15 = modeliPhone 15;71 }output--- Using as Camera ---@Override public void takePhoto()
pass 1 of 310 System.out.println("--- Using as Camera ---");11 phone.takePhoto();12 phone.recordVideo();13 14 System.out.println("\n--- Using as Phone ---");15 phone.makeCall("555-1234");16 phone.sendText("Hello!");17 18 System.out.println("\n--- Using as MediaPlayer ---");19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam = phone; //?ascamera26 Callable call = phone; //?ascallable27 MediaPlayer player = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(modeliPhone 15 + ": 📷 Taking photo");77 }outputiPhone 15: 📷 Taking photoAll 3 passes — pass 1 is the card above pass modelnumbermessagesongvideoccamcallplayerthis.model1 iPhone 15 555-1234 Hello! Bohemian Rhapsody YouTube clip — ⟨SmartPhone A⟩ ⟨SmartPhone A⟩ ⟨SmartPhone A⟩ — 2 iPhone 15 555-5678 — Song — ⟨SmartPhone B⟩ — — — Pixel 3 Pixel — — — — ⟨DigitalCamera C⟩ — — — — @Override public void recordVideo()
11 phone.takePhoto();12 phone.recordVideo();13 14 System.out.println("\n--- Using as Phone ---");15 phone.makeCall("555-1234");16 phone.sendText("Hello!");17 18 System.out.println("\n--- Using as MediaPlayer ---");19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam = phone; //?ascamera26 Callable call = phone; //?ascallable27 MediaPlayer player = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(modeliPhone 15 + ": 🎥 Recording video");82 }outputiPhone 15: 🎥 Recording video --- Using as Phone ---@Override public void makeCall(String number)
pass 1 of 214 System.out.println("\n--- Using as Phone ---");15 phone.makeCall("555-1234");16 phone.sendText("Hello!");17 18 System.out.println("\n--- Using as MediaPlayer ---");19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam = phone; //?ascamera26 Callable call = phone; //?ascallable27 MediaPlayer player = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number555-1234) {87 System.out.println(modeliPhone 15 + ": 📞 Calling " + number555-1234);88 }outputiPhone 15: 📞 Calling 555-1234@Override public void sendText(String message)
15 phone.makeCall("555-1234");16 phone.sendText("Hello!");17 18 System.out.println("\n--- Using as MediaPlayer ---");19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam = phone; //?ascamera26 Callable call = phone; //?ascallable27 MediaPlayer player = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number) {87 System.out.println(model + ": 📞 Calling " + number);88 }89 90 @Override91 public void sendText(String messageHello!) {92 System.out.println(modeliPhone 15 + ": 💬 Sending: " + messageHello!);93 }outputiPhone 15: 💬 Sending: Hello! --- Using as MediaPlayer ---@Override public void playMusic(String song)
pass 1 of 218 System.out.println("\n--- Using as MediaPlayer ---");19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam = phone; //?ascamera26 Callable call = phone; //?ascallable27 MediaPlayer player = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number) {87 System.out.println(model + ": 📞 Calling " + number);88 }89 90 @Override91 public void sendText(String message) {92 System.out.println(model + ": 💬 Sending: " + message);93 }94 95 // From MediaPlayer //?mediaimpls96 @Override97 public void playMusic(String songBohemian Rhapsody) {98 System.out.println(modeliPhone 15 + ": 🎵 Playing " + songBohemian Rhapsody);99 }outputiPhone 15: 🎵 Playing Bohemian Rhapsodycam ← ⟨SmartPhone A⟩, call ← ⟨SmartPhone A⟩, player ← ⟨SmartPhone A⟩
19 phone.playMusic("Bohemian Rhapsody");20 phone.playVideo("YouTube clip");21 22 System.out.println("\n=== Interface References ===");23 24 // Same object, different interface views //?interfaceviews25 Camera cam→ ⟨SmartPhone A⟩ = phone; //?ascamera26 Callable call→ ⟨SmartPhone A⟩ = phone; //?ascallable27 MediaPlayer player→ ⟨SmartPhone A⟩ = phone;28 29 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number) {87 System.out.println(model + ": 📞 Calling " + number);88 }89 90 @Override91 public void sendText(String message) {92 System.out.println(model + ": 💬 Sending: " + message);93 }94 95 // From MediaPlayer //?mediaimpls96 @Override97 public void playMusic(String song) {98 System.out.println(model + ": 🎵 Playing " + song);99 }100 101 @Override102 public void playVideo(String videoYouTube clip) {103 System.out.println(modeliPhone 15 + ": ▶️ Playing " + videoYouTube clip);104 }outputiPhone 15: ▶️ Playing YouTube clip === Interface References ===@Override public void makeCall(String number)
pass 2 of 229 cam.takePhoto(); // Only Camera methods visible30 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number555-5678) {87 System.out.println(modeliPhone 15 + ": 📞 Calling " + number555-5678);88 }outputiPhone 15: 📞 Calling 555-5678@Override public void playMusic(String song)
pass 2 of 230 call.makeCall("555-5678");31 player.playMusic("Song");32 33 System.out.println("\n=== Duck Typing with Interfaces ===");34 35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number) {87 System.out.println(model + ": 📞 Calling " + number);88 }89 90 @Override91 public void sendText(String message) {92 System.out.println(model + ": 💬 Sending: " + message);93 }94 95 // From MediaPlayer //?mediaimpls96 @Override97 public void playMusic(String songSong) {98 System.out.println(modeliPhone 15 + ": 🎵 Playing " + songSong);99 }outputiPhone 15: 🎵 Playing Song === Duck Typing with Interfaces ===this.model ← Pixel
pass 2 of 269SmartPhone(String modelPixel) {70 this.model→ Pixel = modelPixel;71}this.model ← Canon
35 // Different devices, same interfaces //?ducktyping36 Camera[] cameras = {37 new SmartPhone("Pixel"),38 new DigitalCamera("Canon"), //?digitalcam39 };40 41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number) {87 System.out.println(model + ": 📞 Calling " + number);88 }89 90 @Override91 public void sendText(String message) {92 System.out.println(model + ": 💬 Sending: " + message);93 }94 95 // From MediaPlayer //?mediaimpls96 @Override97 public void playMusic(String song) {98 System.out.println(model + ": 🎵 Playing " + song);99 }100 101 @Override102 public void playVideo(String video) {103 System.out.println(model + ": ▶️ Playing " + video);104 }105}106107// Another class implementing just Camera //?partialimpl108class DigitalCamera implements Camera {109 private String model;110 111 DigitalCamera(String modelCanon) {112 this.model→ Canon = modelCanon;113 }for (Camera c : cameras)
pass 1 of 241for (Camera c⟨SmartPhone B⟩ : cameras) {42 c.takePhoto();43}for (Camera c : cameras)
pass 2 of 241for (Camera c⟨DigitalCamera C⟩ : cameras) {42 c.takePhoto();43}@Override public void takePhoto()
41 for (Camera c : cameras) {42 c.takePhoto();43 }44 }45}4647// Interface 1 //?interface148interface Camera {49 void takePhoto();50 void recordVideo();51}5253// Interface 2 //?interface254interface Callable {55 void makeCall(String number);56 void sendText(String message);57}5859// Interface 3 //?interface360interface MediaPlayer {61 void playMusic(String song);62 void playVideo(String video);63}6465// Class implementing MULTIPLE interfaces //?multipleimpl66class SmartPhone implements Camera, Callable, MediaPlayer { //?impllist67 private String model;68 69 SmartPhone(String model) {70 this.model = model;71 }72 73 // From Camera //?cameraimpls74 @Override75 public void takePhoto() {76 System.out.println(model + ": 📷 Taking photo");77 }78 79 @Override80 public void recordVideo() {81 System.out.println(model + ": 🎥 Recording video");82 }83 84 // From Callable //?callableimpls85 @Override86 public void makeCall(String number) {87 System.out.println(model + ": 📞 Calling " + number);88 }89 90 @Override91 public void sendText(String message) {92 System.out.println(model + ": 💬 Sending: " + message);93 }94 95 // From MediaPlayer //?mediaimpls96 @Override97 public void playMusic(String song) {98 System.out.println(model + ": 🎵 Playing " + song);99 }100 101 @Override102 public void playVideo(String video) {103 System.out.println(model + ": ▶️ Playing " + video);104 }105}106107// Another class implementing just Camera //?partialimpl108class DigitalCamera implements Camera {109 private String model;110 111 DigitalCamera(String model) {112 this.model = model;113 }114 115 @Override116 public void takePhoto() {117 System.out.println(modelCanon + " DSLR: 📷 High-quality photo");118 }outputCanon DSLR: 📷 High-quality photo
class X implements A, B, C - must implement all methods from all interfaces.
Interface as type
Use interface type for polymorphism.
InterfaceReference.java
Replay: real traced execution (multi-file project)
// Using Interface as Type
public class InterfaceReference {
public static void main(String[] args) {
System.out.println("=== Interface as Method Parameter ===\n");
Circle circle = new Circle(5);
Rectangle rect = new Rectangle(4, 6);
// Pass different types to same method
printArea(circle);
printArea(rect);
System.out.println("\n=== Interface as Return Type ===");
// Factory method returns interface type
String firstShapeType = "circle";
Shape shape1 = createShape(firstShapeType, 3);
Shape shape2 = createShape("rectangle", 4);
System.out.println("Created: " + shape1.getName());
System.out.println("Created: " + shape2.getName());
System.out.println("\n=== Collection of Interface Type ===");
Shape[] shapes = {
createShape("circle", 5),
createShape("rectangle", 3),
createShape("circle", 2)
};
double totalArea = calculateTotalArea(shapes);
System.out.println("Total area: " + String.format("%.2f", totalArea));
System.out.println("\n=== Interface Extends Interface ===");
// 3DShape extends Shape
Sphere sphere = new Sphere(4);
System.out.println("Sphere:");
System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));
System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));
// Can use as Shape too!
printArea(sphere);
}
// Method with interface parameter
static void printArea(Shape shape) {
System.out.println(shape.getName() + " area: " +
String.format("%.2f", shape.getArea()));
}
// Method with interface return type
static Shape createShape(String type, double size) {
return switch(type.toLowerCase()) {
case "circle" -> new Circle(size);
case "rectangle" -> new Rectangle(size, size);
default -> throw new IllegalArgumentException("Unknown shape: " + type);
};
}
// Method processing array of interfaces
static double calculateTotalArea(Shape[] shapes) {
double total = 0;
for (Shape s : shapes) {
total += s.getArea();
}
return total;
}
}
// Base interface
interface Shape {
double getArea();
String getName();
}
// Interface extending interface
interface Shape3D extends Shape {
double getVolume();
// Also inherits getArea() and getName()!
}
class Circle implements Shape {
private double radius;
Circle(double radius) {
this.radius = radius;
}
@Override
public double getArea() {
return Math.PI * radius * radius;
}
@Override
public String getName() {
return "Circle (r=" + radius + ")";
}
}
class Rectangle implements Shape {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public double getArea() {
return width * height;
}
@Override
public String getName() {
return "Rectangle (" + width + "x" + height + ")";
}
}
// Implements extended interface
class Sphere implements Shape3D {
private double radius;
Sphere(double radius) {
this.radius = radius;
}
// From Shape (inherited by Shape3D)
@Override
public double getArea() {
return 4 * Math.PI * radius * radius; // Surface area
}
@Override
public String getName() {
return "Sphere (r=" + radius + ")";
}
// From Shape3D
@Override
public double getVolume() {
return (4.0 / 3.0) * Math.PI * radius * radius * radius;
}
}
// Using Interface as Type
public class InterfaceReference {
public static void main(String[] args) {
System.out.println("=== Interface as Method Parameter ===\n");
Circle circle = new Circle(5);
Rectangle rect = new Rectangle(4, 6);
// Pass different types to same method
printArea(circle);
printArea(rect);
System.out.println("\n=== Interface as Return Type ===");
// Factory method returns interface type
String firstShapeType = "rectangle";
Shape shape1 = createShape(firstShapeType, 3);
Shape shape2 = createShape("rectangle", 4);
System.out.println("Created: " + shape1.getName());
System.out.println("Created: " + shape2.getName());
System.out.println("\n=== Collection of Interface Type ===");
Shape[] shapes = {
createShape("circle", 5),
createShape("rectangle", 3),
createShape("circle", 2)
};
double totalArea = calculateTotalArea(shapes);
System.out.println("Total area: " + String.format("%.2f", totalArea));
System.out.println("\n=== Interface Extends Interface ===");
// 3DShape extends Shape
Sphere sphere = new Sphere(4);
System.out.println("Sphere:");
System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));
System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));
// Can use as Shape too!
printArea(sphere);
}
// Method with interface parameter
static void printArea(Shape shape) {
System.out.println(shape.getName() + " area: " +
String.format("%.2f", shape.getArea()));
}
// Method with interface return type
static Shape createShape(String type, double size) {
return switch(type.toLowerCase()) {
case "circle" -> new Circle(size);
case "rectangle" -> new Rectangle(size, size);
default -> throw new IllegalArgumentException("Unknown shape: " + type);
};
}
// Method processing array of interfaces
static double calculateTotalArea(Shape[] shapes) {
double total = 0;
for (Shape s : shapes) {
total += s.getArea();
}
return total;
}
}
// Base interface
interface Shape {
double getArea();
String getName();
}
// Interface extending interface
interface Shape3D extends Shape {
double getVolume();
// Also inherits getArea() and getName()!
}
class Circle implements Shape {
private double radius;
Circle(double radius) {
this.radius = radius;
}
@Override
public double getArea() {
return Math.PI * radius * radius;
}
@Override
public String getName() {
return "Circle (r=" + radius + ")";
}
}
class Rectangle implements Shape {
private double width, height;
Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public double getArea() {
return width * height;
}
@Override
public String getName() {
return "Rectangle (" + width + "x" + height + ")";
}
}
// Implements extended interface
class Sphere implements Shape3D {
private double radius;
Sphere(double radius) {
this.radius = radius;
}
// From Shape (inherited by Shape3D)
@Override
public double getArea() {
return 4 * Math.PI * radius * radius; // Surface area
}
@Override
public String getName() {
return "Sphere (r=" + radius + ")";
}
// From Shape3D
@Override
public double getVolume() {
return (4.0 / 3.0) * Math.PI * radius * radius * radius;
}
}
public static void main(String[] args)
3public class InterfaceReference {4 public static void main(String[] args) {5 System.out.println("=== Interface as Method Parameter ===\n");6 7 Circle circle = new Circle(5);8 Rectangle rect = new Rectangle(4, 6);output=== Interface as Method Parameter ===this.radius ← 5.0, circle ← ⟨Circle A⟩
pass 1 of 47 Circle circle→ ⟨Circle A⟩ = new Circle(5);8 Rectangle rect = new Rectangle(4, 6);9 10 // Pass different types to same method //?samemethod11 printArea(circle);12 printArea(rect);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type //?factory17 String firstShapeType = "circle"; //@firstShapeType="circle", "rectangle"18 Shape shape1 = createShape(firstShapeType, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = { //?collection27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes); //?totalarea33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape //?extendsinterface38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too! //?shapereference45 printArea(sphere);46 }47 48 // Method with interface parameter //?interfaceparam49 static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52 }53 54 // Method with interface return type //?interfacereturn55 static Shape createShape(String type, double size) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61 }62 63 // Method processing array of interfaces //?processarray64 static double calculateTotalArea(Shape[] shapes) {65 double total = 0;66 for (Shape s : shapes) {67 total += s.getArea();68 }69 return total;70 }71}7273// Base interface //?shapeinterface74interface Shape {75 double getArea();76 String getName();77}7879// Interface extending interface //?extendedinterface80interface Shape3D extends Shape { //?interfaceextends81 double getVolume();82 // Also inherits getArea() and getName()!83}8485class Circle implements Shape {86 private double radius;87 88 Circle(double radius5.0) {89 this.radius→ 5.0 = radius5.0;90 }All 4 passes — pass 1 is the card above pass radiusfirstShapeTypethis.radiuscircleshape1total1 5.0 — 5.0 ⟨Circle A⟩ — — 2 3.0 circle 3.0 — ⟨Circle B⟩ — 3 5.0 — 5.0 — — — 4 2.0 — 2.0 — — 0.0 this.width ← 4.0, this.height ← 6.0, rect ← ⟨Rectangle C⟩
pass 1 of 37 Circle circle = new Circle(5);8 Rectangle rect→ ⟨Rectangle C⟩ = new Rectangle(4, 6);9 10 // Pass different types to same method //?samemethod11 printArea(circle⟨Circle A⟩);12 printArea(rect);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type //?factory17 String firstShapeType = "circle"; //@firstShapeType="circle", "rectangle"18 Shape shape1 = createShape(firstShapeType, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = { //?collection27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes); //?totalarea33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape //?extendsinterface38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too! //?shapereference45 printArea(sphere);46 }47 48 // Method with interface parameter //?interfaceparam49 static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52 }53 54 // Method with interface return type //?interfacereturn55 static Shape createShape(String type, double size) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61 }62 63 // Method processing array of interfaces //?processarray64 static double calculateTotalArea(Shape[] shapes) {65 double total = 0;66 for (Shape s : shapes) {67 total += s.getArea();68 }69 return total;70 }71}7273// Base interface //?shapeinterface74interface Shape {75 double getArea();76 String getName();77}7879// Interface extending interface //?extendedinterface80interface Shape3D extends Shape { //?interfaceextends81 double getVolume();82 // Also inherits getArea() and getName()!83}8485class Circle implements Shape {86 private double radius;87 88 Circle(double radius) {89 this.radius = radius;90 }91 92 @Override93 public double getArea() {94 return Math.PI * radius * radius;95 }96 97 @Override98 public String getName() {99 return "Circle (r=" + radius + ")";100 }101}102103class Rectangle implements Shape {104 private double width, height;105 106 Rectangle(double width4.0, double height6.0) {107 this.width→ 4.0 = width4.0;108 this.height→ 6.0 = height6.0;109 }All 3 passes — pass 1 is the card above pass widthheightcircleradiusthis.widththis.heightrectshape2total1 4.0 6.0 ⟨Circle A⟩ 5.0 4.0 6.0 ⟨Rectangle C⟩ — — 2 4.0 4.0 — 3.0 4.0 4.0 — ⟨Rectangle D⟩ — 3 3.0 3.0 — — 3.0 3.0 — — 0.0 static void printArea(Shape shape)
pass 1 of 348// Method with interface parameter //?interfaceparam49static void printArea(Shape shape⟨Circle A⟩) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}All 3 passes — pass 1 is the card above pass shaperadiuswidthheight1 ⟨Circle A⟩ 5.0 — — 2 ⟨Rectangle C⟩ — 4.0 6.0 3 ⟨Sphere E⟩ 4.0 — — @Override public String getName()
pass 1 of 297@Override98public String getName() {99 return "Circle (r=" + radius5.0 + ")";100}@Override public double getArea()
pass 1 of 392@Override93public double getArea() {94 return Math.PI * radius5.0 * radius;95}All 3 passes — pass 1 is the card above pass radius1 5.0 2 5.0 3 2.0 printArea(circle);
10 // Pass different types to same method //?samemethod11 printArea(circle⟨Circle A⟩);12 printArea(rect⟨Rectangle C⟩);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type //?factory17 String firstShapeType = "circle"; //@firstShapeType="circle", "rectangle"18 Shape shape1 = createShape(firstShapeType, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = { //?collection27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes); //?totalarea33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape //?extendsinterface38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too! //?shapereference45 printArea(sphere);46}4748// Method with interface parameter //?interfaceparam49static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}outputCircle (r=5.0) area: 78.54@Override public String getName()
pass 1 of 2116@Override117public String getName() {118 return "Rectangle (" + width4.0 + "x" + height6.0 + ")";119}@Override public double getArea()
pass 1 of 2111@Override112public double getArea() {113 return width4.0 * height6.0;114}firstShapeType ← circle
11 printArea(circle);12 printArea(rect⟨Rectangle C⟩);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type //?factory17 String firstShapeType→ circle = "circle"; //@firstShapeType="circle", "rectangle"18 Shape shape1 = createShape(firstShapeTypecircle, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = { //?collection27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes); //?totalarea33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape //?extendsinterface38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too! //?shapereference45 printArea(sphere);46}4748// Method with interface parameter //?interfaceparam49static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}outputRectangle (4.0x6.0) area: 24.00 === Interface as Return Type ===static Shape createShape(String type, double size)
pass 1 of 554// Method with interface return type //?interfacereturn55static Shape createShape(String typecircle, double size3.0) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61}All 5 passes — pass 1 is the card above pass typesizeradiustotal1 circle 3.0 — — 2 rectangle 4.0 3.0 — 3 circle 5.0 — — 4 rectangle 3.0 — — 5 circle 2.0 — 0.0 @Override public String getName()
pass 2 of 297@Override98public String getName() {99 return "Circle (r=" + radius3.0 + ")";100}System.out.println("Created: " + shape1.getName());
21System.out.println("Created: " + shape1.getName());22System.out.println("Created: " + shape2.getName());outputCreated: Circle (r=3.0)@Override public String getName()
pass 2 of 2116@Override117public String getName() {118 return "Rectangle (" + width4.0 + "x" + height4.0 + ")";119}System.out.println("Created: " + shape2.getName());
21System.out.println("Created: " + shape1.getName());22System.out.println("Created: " + shape2.getName());2324System.out.println("\n=== Collection of Interface Type ===");2526Shape[] shapes = { //?collection27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30};outputCreated: Rectangle (4.0x4.0) === Collection of Interface Type ===total ← 0.0
63// Method processing array of interfaces //?processarray64static double calculateTotalArea(Shape[] shapes) {65 double total→ 0.0 = 0;66 for (Shape s : shapes) {for (Shape s : shapes)
pass 1 of 365double total = 0;66for (Shape s⟨Circle F⟩ : shapes) {67 total0.0 += s.getArea();68}All 3 passes — pass 1 is the card above pass stotalwidthheight1 ⟨Circle F⟩ 0.0 — — 2 ⟨Rectangle G⟩ 78.53981633974483 3.0 3.0 3 ⟨Circle H⟩ 87.53981633974483 — — total ← 78.53981633974483
66for (Shape s : shapes) {67 total→ 78.53981633974483 += s.getArea();68}@Override public double getArea()
pass 2 of 2111@Override112public double getArea() {113 return width3.0 * height3.0;114}total ← 87.53981633974483
66for (Shape s : shapes) {67 total→ 87.53981633974483 += s.getArea();68}total ← 100.106186954104
66 for (Shape s : shapes) {67 total→ 100.106186954104 += s.getArea();68 }69 return total100.106186954104;70}totalArea ← 100.106186954104
32double totalArea→ 100.106186954104 = calculateTotalArea(shapes); //?totalarea33System.out.println("Total area: " + String.format("%.2f", totalArea100.106186954104));3435System.out.println("\n=== Interface Extends Interface ===");3637// 3DShape extends Shape //?extendsinterface38Sphere sphere = new Sphere(4);outputTotal area: 100.11 === Interface Extends Interface ===this.radius ← 4.0, sphere ← ⟨Sphere E⟩
37 // 3DShape extends Shape //?extendsinterface38 Sphere sphere→ ⟨Sphere E⟩ = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too! //?shapereference45 printArea(sphere);46 }47 48 // Method with interface parameter //?interfaceparam49 static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52 }53 54 // Method with interface return type //?interfacereturn55 static Shape createShape(String type, double size) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61 }62 63 // Method processing array of interfaces //?processarray64 static double calculateTotalArea(Shape[] shapes) {65 double total = 0;66 for (Shape s : shapes) {67 total += s.getArea();68 }69 return total;70 }71}7273// Base interface //?shapeinterface74interface Shape {75 double getArea();76 String getName();77}7879// Interface extending interface //?extendedinterface80interface Shape3D extends Shape { //?interfaceextends81 double getVolume();82 // Also inherits getArea() and getName()!83}8485class Circle implements Shape {86 private double radius;87 88 Circle(double radius) {89 this.radius = radius;90 }91 92 @Override93 public double getArea() {94 return Math.PI * radius * radius;95 }96 97 @Override98 public String getName() {99 return "Circle (r=" + radius + ")";100 }101}102103class Rectangle implements Shape {104 private double width, height;105 106 Rectangle(double width, double height) {107 this.width = width;108 this.height = height;109 }110 111 @Override112 public double getArea() {113 return width * height;114 }115 116 @Override117 public String getName() {118 return "Rectangle (" + width + "x" + height + ")";119 }120}121122// Implements extended interface //?impl3d123class Sphere implements Shape3D {124 private double radius;125 126 Sphere(double radius4.0) {127 this.radius→ 4.0 = radius4.0;128 }outputSphere:@Override public double getArea()
pass 1 of 2130// From Shape (inherited by Shape3D) //?inheritedfrom131@Override132public double getArea() {133 return 4 * Math.PI * radius4.0 * radius; // Surface area134}System.out.println(" Area: " + String.format("%.2f", sphere.getArea()…
40System.out.println("Sphere:");41System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));output Area: 201.06@Override public double getVolume()
141// From Shape3D //?from3d142@Override143public double getVolume() {144 return (4.0 / 3.0) * Math.PI * radius4.0 * radius * radius;145}printArea(sphere);
41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too! //?shapereference45 printArea(sphere⟨Sphere E⟩);46}output Volume: 268.08@Override public String getName()
136@Override137public String getName() {138 return "Sphere (r=" + radius4.0 + ")";139}@Override public double getArea()
pass 2 of 2130// From Shape (inherited by Shape3D) //?inheritedfrom131@Override132public double getArea() {133 return 4 * Math.PI * radius4.0 * radius; // Surface area134}printArea(sphere);
44 // Can use as Shape too! //?shapereference45 printArea(sphere⟨Sphere E⟩);46}4748// Method with interface parameter //?interfaceparam49static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}outputSphere (r=4.0) area: 201.06
public static void main(String[] args)
3public class InterfaceReference {4 public static void main(String[] args) {5 System.out.println("=== Interface as Method Parameter ===\n");6 7 Circle circle = new Circle(5);8 Rectangle rect = new Rectangle(4, 6);output=== Interface as Method Parameter ===this.radius ← 5.0, circle ← ⟨Circle A⟩
pass 1 of 37 Circle circle→ ⟨Circle A⟩ = new Circle(5);8 Rectangle rect = new Rectangle(4, 6);9 10 // Pass different types to same method11 printArea(circle);12 printArea(rect);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type17 String firstShapeType = "rectangle";18 Shape shape1 = createShape(firstShapeType, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = {27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes);33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too!45 printArea(sphere);46 }47 48 // Method with interface parameter49 static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52 }53 54 // Method with interface return type55 static Shape createShape(String type, double size) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61 }62 63 // Method processing array of interfaces64 static double calculateTotalArea(Shape[] shapes) {65 double total = 0;66 for (Shape s : shapes) {67 total += s.getArea();68 }69 return total;70 }71}7273// Base interface74interface Shape {75 double getArea();76 String getName();77}7879// Interface extending interface80interface Shape3D extends Shape {81 double getVolume();82 // Also inherits getArea() and getName()!83}8485class Circle implements Shape {86 private double radius;87 88 Circle(double radius5.0) {89 this.radius→ 5.0 = radius5.0;90 }All 3 passes — pass 1 is the card above pass radiusthis.radiuscircletotal1 5.0 5.0 ⟨Circle A⟩ — 2 5.0 5.0 — — 3 2.0 2.0 — 0.0 this.width ← 4.0, this.height ← 6.0, rect ← ⟨Rectangle B⟩
pass 1 of 47 Circle circle = new Circle(5);8 Rectangle rect→ ⟨Rectangle B⟩ = new Rectangle(4, 6);9 10 // Pass different types to same method11 printArea(circle⟨Circle A⟩);12 printArea(rect);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type17 String firstShapeType = "rectangle";18 Shape shape1 = createShape(firstShapeType, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = {27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes);33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too!45 printArea(sphere);46 }47 48 // Method with interface parameter49 static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52 }53 54 // Method with interface return type55 static Shape createShape(String type, double size) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61 }62 63 // Method processing array of interfaces64 static double calculateTotalArea(Shape[] shapes) {65 double total = 0;66 for (Shape s : shapes) {67 total += s.getArea();68 }69 return total;70 }71}7273// Base interface74interface Shape {75 double getArea();76 String getName();77}7879// Interface extending interface80interface Shape3D extends Shape {81 double getVolume();82 // Also inherits getArea() and getName()!83}8485class Circle implements Shape {86 private double radius;87 88 Circle(double radius) {89 this.radius = radius;90 }91 92 @Override93 public double getArea() {94 return Math.PI * radius * radius;95 }96 97 @Override98 public String getName() {99 return "Circle (r=" + radius + ")";100 }101}102103class Rectangle implements Shape {104 private double width, height;105 106 Rectangle(double width4.0, double height6.0) {107 this.width→ 4.0 = width4.0;108 this.height→ 6.0 = height6.0;109 }All 4 passes — pass 1 is the card above pass widthheightcircleradiusfirstShapeTypethis.widththis.heightrectshape1shape2total1 4.0 6.0 ⟨Circle A⟩ 5.0 — 4.0 6.0 ⟨Rectangle B⟩ — — — 2 3.0 3.0 — — rectangle 3.0 3.0 — ⟨Rectangle C⟩ — — 3 4.0 4.0 — — — 4.0 4.0 — — ⟨Rectangle D⟩ — 4 3.0 3.0 — — — 3.0 3.0 — — — 0.0 static void printArea(Shape shape)
pass 1 of 348// Method with interface parameter49static void printArea(Shape shape⟨Circle A⟩) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}All 3 passes — pass 1 is the card above pass shaperadiuswidthheight1 ⟨Circle A⟩ 5.0 — — 2 ⟨Rectangle B⟩ — 4.0 6.0 3 ⟨Sphere E⟩ 4.0 — — @Override public String getName()
97@Override98public String getName() {99 return "Circle (r=" + radius5.0 + ")";100}@Override public double getArea()
pass 1 of 392@Override93public double getArea() {94 return Math.PI * radius5.0 * radius;95}All 3 passes — pass 1 is the card above pass radius1 5.0 2 5.0 3 2.0 printArea(circle);
10 // Pass different types to same method11 printArea(circle⟨Circle A⟩);12 printArea(rect⟨Rectangle B⟩);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type17 String firstShapeType = "rectangle";18 Shape shape1 = createShape(firstShapeType, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = {27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes);33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too!45 printArea(sphere);46}4748// Method with interface parameter49static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}outputCircle (r=5.0) area: 78.54@Override public String getName()
pass 1 of 3116@Override117public String getName() {118 return "Rectangle (" + width4.0 + "x" + height6.0 + ")";119}All 3 passes — pass 1 is the card above pass widthheight1 4.0 6.0 2 3.0 3.0 3 4.0 4.0 @Override public double getArea()
pass 1 of 2111@Override112public double getArea() {113 return width4.0 * height6.0;114}firstShapeType ← rectangle
11 printArea(circle);12 printArea(rect⟨Rectangle B⟩);13 14 System.out.println("\n=== Interface as Return Type ===");15 16 // Factory method returns interface type17 String firstShapeType→ rectangle = "rectangle";18 Shape shape1 = createShape(firstShapeTyperectangle, 3);19 Shape shape2 = createShape("rectangle", 4);20 21 System.out.println("Created: " + shape1.getName());22 System.out.println("Created: " + shape2.getName());23 24 System.out.println("\n=== Collection of Interface Type ===");25 26 Shape[] shapes = {27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30 };31 32 double totalArea = calculateTotalArea(shapes);33 System.out.println("Total area: " + String.format("%.2f", totalArea));34 35 System.out.println("\n=== Interface Extends Interface ===");36 37 // 3DShape extends Shape38 Sphere sphere = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too!45 printArea(sphere);46}4748// Method with interface parameter49static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}outputRectangle (4.0x6.0) area: 24.00 === Interface as Return Type ===static Shape createShape(String type, double size)
pass 1 of 554// Method with interface return type55static Shape createShape(String typerectangle, double size3.0) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61}All 5 passes — pass 1 is the card above pass typesizetotal1 rectangle 3.0 — 2 rectangle 4.0 — 3 circle 5.0 — 4 rectangle 3.0 — 5 circle 2.0 0.0 System.out.println("Created: " + shape1.getName());
21System.out.println("Created: " + shape1.getName());22System.out.println("Created: " + shape2.getName());outputCreated: Rectangle (3.0x3.0)System.out.println("Created: " + shape2.getName());
21System.out.println("Created: " + shape1.getName());22System.out.println("Created: " + shape2.getName());2324System.out.println("\n=== Collection of Interface Type ===");2526Shape[] shapes = {27 createShape("circle", 5),28 createShape("rectangle", 3),29 createShape("circle", 2)30};outputCreated: Rectangle (4.0x4.0) === Collection of Interface Type ===total ← 0.0
63// Method processing array of interfaces64static double calculateTotalArea(Shape[] shapes) {65 double total→ 0.0 = 0;66 for (Shape s : shapes) {for (Shape s : shapes)
pass 1 of 365double total = 0;66for (Shape s⟨Circle F⟩ : shapes) {67 total0.0 += s.getArea();68}All 3 passes — pass 1 is the card above pass stotalwidthheight1 ⟨Circle F⟩ 0.0 — — 2 ⟨Rectangle G⟩ 78.53981633974483 3.0 3.0 3 ⟨Circle H⟩ 87.53981633974483 — — total ← 78.53981633974483
66for (Shape s : shapes) {67 total→ 78.53981633974483 += s.getArea();68}@Override public double getArea()
pass 2 of 2111@Override112public double getArea() {113 return width3.0 * height3.0;114}total ← 87.53981633974483
66for (Shape s : shapes) {67 total→ 87.53981633974483 += s.getArea();68}total ← 100.106186954104
66 for (Shape s : shapes) {67 total→ 100.106186954104 += s.getArea();68 }69 return total100.106186954104;70}totalArea ← 100.106186954104
32double totalArea→ 100.106186954104 = calculateTotalArea(shapes);33System.out.println("Total area: " + String.format("%.2f", totalArea100.106186954104));3435System.out.println("\n=== Interface Extends Interface ===");3637// 3DShape extends Shape38Sphere sphere = new Sphere(4);outputTotal area: 100.11 === Interface Extends Interface ===this.radius ← 4.0, sphere ← ⟨Sphere E⟩
37 // 3DShape extends Shape38 Sphere sphere→ ⟨Sphere E⟩ = new Sphere(4);39 40 System.out.println("Sphere:");41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too!45 printArea(sphere);46 }47 48 // Method with interface parameter49 static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52 }53 54 // Method with interface return type55 static Shape createShape(String type, double size) {56 return switch(type.toLowerCase()) {57 case "circle" -> new Circle(size);58 case "rectangle" -> new Rectangle(size, size);59 default -> throw new IllegalArgumentException("Unknown shape: " + type);60 };61 }62 63 // Method processing array of interfaces64 static double calculateTotalArea(Shape[] shapes) {65 double total = 0;66 for (Shape s : shapes) {67 total += s.getArea();68 }69 return total;70 }71}7273// Base interface74interface Shape {75 double getArea();76 String getName();77}7879// Interface extending interface80interface Shape3D extends Shape {81 double getVolume();82 // Also inherits getArea() and getName()!83}8485class Circle implements Shape {86 private double radius;87 88 Circle(double radius) {89 this.radius = radius;90 }91 92 @Override93 public double getArea() {94 return Math.PI * radius * radius;95 }96 97 @Override98 public String getName() {99 return "Circle (r=" + radius + ")";100 }101}102103class Rectangle implements Shape {104 private double width, height;105 106 Rectangle(double width, double height) {107 this.width = width;108 this.height = height;109 }110 111 @Override112 public double getArea() {113 return width * height;114 }115 116 @Override117 public String getName() {118 return "Rectangle (" + width + "x" + height + ")";119 }120}121122// Implements extended interface123class Sphere implements Shape3D {124 private double radius;125 126 Sphere(double radius4.0) {127 this.radius→ 4.0 = radius4.0;128 }outputSphere:@Override public double getArea()
pass 1 of 2130// From Shape (inherited by Shape3D)131@Override132public double getArea() {133 return 4 * Math.PI * radius4.0 * radius; // Surface area134}System.out.println(" Area: " + String.format("%.2f", sphere.getArea()…
40System.out.println("Sphere:");41System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));output Area: 201.06@Override public double getVolume()
141// From Shape3D142@Override143public double getVolume() {144 return (4.0 / 3.0) * Math.PI * radius4.0 * radius * radius;145}printArea(sphere);
41 System.out.println(" Area: " + String.format("%.2f", sphere.getArea()));42 System.out.println(" Volume: " + String.format("%.2f", sphere.getVolume()));43 44 // Can use as Shape too!45 printArea(sphere⟨Sphere E⟩);46}output Volume: 268.08@Override public String getName()
136@Override137public String getName() {138 return "Sphere (r=" + radius4.0 + ")";139}@Override public double getArea()
pass 2 of 2130// From Shape (inherited by Shape3D)131@Override132public double getArea() {133 return 4 * Math.PI * radius4.0 * radius; // Surface area134}printArea(sphere);
44 // Can use as Shape too!45 printArea(sphere⟨Sphere E⟩);46}4748// Method with interface parameter49static void printArea(Shape shape) {50 System.out.println(shape.getName() + " area: " + 51 String.format("%.2f", shape.getArea()));52}outputSphere (r=4.0) area: 201.06
Drawable d = new Circle() - variable type is interface, object is implementing class.
Default methods
Interfaces can provide default implementations (Java 8+).
DefaultMethods.java
Replay: real traced execution (multi-file project)
// Default Methods (Java 8+)
public class DefaultMethods {
public static void main(String[] args) {
System.out.println("=== Default Methods in Interfaces ===\n");
// BasicPrinter uses default log()
BasicPrinter basic = new BasicPrinter();
basic.print("Hello");
basic.log("Starting print job");
System.out.println("\n=== Overriding Default Method ===");
// AdvancedPrinter overrides log()
AdvancedPrinter advanced = new AdvancedPrinter();
advanced.print("Document");
advanced.log("Print initiated");
System.out.println("\n=== Static Methods in Interface ===");
// Call static method on interface
String version = Printable.getVersion();
System.out.println("Printable version: " + version);
System.out.println("\n=== Multiple Interfaces with Same Default ===");
MultiDevice device = new MultiDevice();
device.connect();
System.out.println("\n=== Why Default Methods? ===");
System.out.println("""
Before Java 8:
- Adding method to interface broke all implementations
- Every class needed updating
With default methods:
- Add new methods with default implementation
- Existing classes still work
- Optional override for custom behavior
Real examples:
- List.sort() - added in Java 8
- Collection.stream() - added in Java 8
""");
}
}
// Interface with default method
interface Printable {
// Abstract method - must implement
void print(String content);
// Default method - optional to override
default void log(String message) {
System.out.println("[LOG] " + message);
}
// Another default method
default void preview() {
System.out.println("Showing preview...");
}
// Static method in interface
static String getVersion() {
return "1.0";
}
}
// Uses default implementation
class BasicPrinter implements Printable {
@Override
public void print(String content) {
System.out.println("Printing: " + content);
}
// Does NOT override log() - uses default
// Does NOT override preview() - uses default
}
// Overrides default method
class AdvancedPrinter implements Printable {
@Override
public void print(String content) {
System.out.println("Advanced printing: " + content);
}
@Override
public void log(String message) {
System.out.println("[ADVANCED LOG] " +
java.time.LocalTime.now() + " - " + message);
}
}
// Two interfaces with same default method
interface USB {
default void connect() {
System.out.println("USB connecting...");
}
}
interface Bluetooth {
default void connect() {
System.out.println("Bluetooth connecting...");
}
}
// Must resolve conflict
class MultiDevice implements USB, Bluetooth {
@Override
public void connect() {
System.out.println("Multi-device connecting:");
USB.super.connect();
Bluetooth.super.connect();
}
}
basic ← ⟨BasicPrinter A⟩
3public class DefaultMethods {4 public static void main(String[] args) {5 System.out.println("=== Default Methods in Interfaces ===\n");6 7 // BasicPrinter uses default log() //?usedefault8 BasicPrinter basic→ ⟨BasicPrinter A⟩ = new BasicPrinter();9 basic.print("Hello");10 basic.log("Starting print job"); //?defaultcalloutput=== Default Methods in Interfaces ===@Override public void print(String content)
8 BasicPrinter basic = new BasicPrinter();9 basic.print("Hello");10 basic.log("Starting print job"); //?defaultcall11 12 System.out.println("\n=== Overriding Default Method ===");13 14 // AdvancedPrinter overrides log() //?overridedefault15 AdvancedPrinter advanced = new AdvancedPrinter();16 advanced.print("Document");17 advanced.log("Print initiated"); //?overriddencall18 19 System.out.println("\n=== Static Methods in Interface ===");20 21 // Call static method on interface //?staticmethod22 String version = Printable.getVersion(); //?staticcall23 System.out.println("Printable version: " + version);24 25 System.out.println("\n=== Multiple Interfaces with Same Default ===");26 27 MultiDevice device = new MultiDevice();28 device.connect(); //?conflictresolved29 30 System.out.println("\n=== Why Default Methods? ===");31 System.out.println("""32 Before Java 8:33 - Adding method to interface broke all implementations34 - Every class needed updating35 36 With default methods:37 - Add new methods with default implementation38 - Existing classes still work39 - Optional override for custom behavior40 41 Real examples:42 - List.sort() - added in Java 843 - Collection.stream() - added in Java 844 """);45 }46}4748// Interface with default method //?interfacedefault49interface Printable {50 // Abstract method - must implement //?abstract51 void print(String content);52 53 // Default method - optional to override //?defaultmethod54 default void log(String message) { //?defaultkeyword55 System.out.println("[LOG] " + message);56 }57 58 // Another default method59 default void preview() {60 System.out.println("Showing preview...");61 }62 63 // Static method in interface //?staticinterface64 static String getVersion() {65 return "1.0";66 }67}6869// Uses default implementation //?usesdefault70class BasicPrinter implements Printable {71 72 @Override73 public void print(String contentHello) {74 System.out.println("Printing: " + contentHello);75 }outputPrinting: Helloadvanced ← ⟨AdvancedPrinter B⟩
9 basic.print("Hello");10 basic.log("Starting print job"); //?defaultcall11 12 System.out.println("\n=== Overriding Default Method ===");13 14 // AdvancedPrinter overrides log() //?overridedefault15 AdvancedPrinter advanced→ ⟨AdvancedPrinter B⟩ = new AdvancedPrinter();16 advanced.print("Document");17 advanced.log("Print initiated"); //?overriddencall18 19 System.out.println("\n=== Static Methods in Interface ===");20 21 // Call static method on interface //?staticmethod22 String version = Printable.getVersion(); //?staticcall23 System.out.println("Printable version: " + version);24 25 System.out.println("\n=== Multiple Interfaces with Same Default ===");26 27 MultiDevice device = new MultiDevice();28 device.connect(); //?conflictresolved29 30 System.out.println("\n=== Why Default Methods? ===");31 System.out.println("""32 Before Java 8:33 - Adding method to interface broke all implementations34 - Every class needed updating35 36 With default methods:37 - Add new methods with default implementation38 - Existing classes still work39 - Optional override for custom behavior40 41 Real examples:42 - List.sort() - added in Java 843 - Collection.stream() - added in Java 844 """);45 }46}4748// Interface with default method //?interfacedefault49interface Printable {50 // Abstract method - must implement //?abstract51 void print(String content);52 53 // Default method - optional to override //?defaultmethod54 default void log(String messageStarting print job) { //?defaultkeyword55 System.out.println("[LOG] " + messageStarting print job);56 }output[LOG] Starting print job === Overriding Default Method ===@Override public void print(String content)
15 AdvancedPrinter advanced = new AdvancedPrinter();16 advanced.print("Document");17 advanced.log("Print initiated"); //?overriddencall18 19 System.out.println("\n=== Static Methods in Interface ===");20 21 // Call static method on interface //?staticmethod22 String version = Printable.getVersion(); //?staticcall23 System.out.println("Printable version: " + version);24 25 System.out.println("\n=== Multiple Interfaces with Same Default ===");26 27 MultiDevice device = new MultiDevice();28 device.connect(); //?conflictresolved29 30 System.out.println("\n=== Why Default Methods? ===");31 System.out.println("""32 Before Java 8:33 - Adding method to interface broke all implementations34 - Every class needed updating35 36 With default methods:37 - Add new methods with default implementation38 - Existing classes still work39 - Optional override for custom behavior40 41 Real examples:42 - List.sort() - added in Java 843 - Collection.stream() - added in Java 844 """);45 }46}4748// Interface with default method //?interfacedefault49interface Printable {50 // Abstract method - must implement //?abstract51 void print(String content);52 53 // Default method - optional to override //?defaultmethod54 default void log(String message) { //?defaultkeyword55 System.out.println("[LOG] " + message);56 }57 58 // Another default method59 default void preview() {60 System.out.println("Showing preview...");61 }62 63 // Static method in interface //?staticinterface64 static String getVersion() {65 return "1.0";66 }67}6869// Uses default implementation //?usesdefault70class BasicPrinter implements Printable {71 72 @Override73 public void print(String content) {74 System.out.println("Printing: " + content);75 }76 77 // Does NOT override log() - uses default78 // Does NOT override preview() - uses default79}8081// Overrides default method //?overridesdefault82class AdvancedPrinter implements Printable {83 84 @Override85 public void print(String contentDocument) {86 System.out.println("Advanced printing: " + contentDocument);87 }outputAdvanced printing: Document@Override public void log(String message)
16 advanced.print("Document");17 advanced.log("Print initiated"); //?overriddencall18 19 System.out.println("\n=== Static Methods in Interface ===");20 21 // Call static method on interface //?staticmethod22 String version = Printable.getVersion(); //?staticcall23 System.out.println("Printable version: " + version);24 25 System.out.println("\n=== Multiple Interfaces with Same Default ===");26 27 MultiDevice device = new MultiDevice();28 device.connect(); //?conflictresolved29 30 System.out.println("\n=== Why Default Methods? ===");31 System.out.println("""32 Before Java 8:33 - Adding method to interface broke all implementations34 - Every class needed updating35 36 With default methods:37 - Add new methods with default implementation38 - Existing classes still work39 - Optional override for custom behavior40 41 Real examples:42 - List.sort() - added in Java 843 - Collection.stream() - added in Java 844 """);45 }46}4748// Interface with default method //?interfacedefault49interface Printable {50 // Abstract method - must implement //?abstract51 void print(String content);52 53 // Default method - optional to override //?defaultmethod54 default void log(String message) { //?defaultkeyword55 System.out.println("[LOG] " + message);56 }57 58 // Another default method59 default void preview() {60 System.out.println("Showing preview...");61 }62 63 // Static method in interface //?staticinterface64 static String getVersion() {65 return "1.0";66 }67}6869// Uses default implementation //?usesdefault70class BasicPrinter implements Printable {71 72 @Override73 public void print(String content) {74 System.out.println("Printing: " + content);75 }76 77 // Does NOT override log() - uses default78 // Does NOT override preview() - uses default79}8081// Overrides default method //?overridesdefault82class AdvancedPrinter implements Printable {83 84 @Override85 public void print(String content) {86 System.out.println("Advanced printing: " + content);87 }88 89 @Override90 public void log(String messagePrint initiated) { //?overridelog91 System.out.println("[ADVANCED LOG] " + 92 java.time.LocalTime.now() + " - " + messagePrint initiated);93 }output[ADVANCED LOG] 23:01:20.249419147 - Print initiated === Static Methods in Interface ===version ← 1.0, device ← ⟨MultiDevice C⟩
21// Call static method on interface //?staticmethod22String version→ 1.0 = Printable.getVersion(); //?staticcall23System.out.println("Printable version: " + version1.0);2425System.out.println("\n=== Multiple Interfaces with Same Default ===");2627MultiDevice device→ ⟨MultiDevice C⟩ = new MultiDevice();28device.connect(); //?conflictresolvedoutputPrintable version: 1.0 === Multiple Interfaces with Same Default ===@Override public void connect()
112@Override113public void connect() { //?mustoverride114 System.out.println("Multi-device connecting:");115 USB.super.connect(); //?superusb116 Bluetooth.super.connect(); //?superbluetoothoutputMulti-device connecting:default void connect()
97interface USB {98 default void connect() {99 System.out.println("USB connecting...");100 }outputUSB connecting...USB.super.connect(); //?superusb
114 System.out.println("Multi-device connecting:");115 USB.super.connect(); //?superusb116 Bluetooth.super.connect(); //?superbluetooth117}default void connect()
103interface Bluetooth {104 default void connect() {105 System.out.println("Bluetooth connecting...");106 }outputBluetooth connecting...Bluetooth.super.connect(); //?superbluetooth
27 MultiDevice device = new MultiDevice();28 device.connect(); //?conflictresolved29 30 System.out.println("\n=== Why Default Methods? ===");31 System.out.println("""32 Before Java 8:33 - Adding method to interface broke all implementations34 - Every class needed updating35 36 With default methods:37 - Add new methods with default implementation38 - Existing classes still work39 - Optional override for custom behavior40 41 Real examples:42 - List.sort() - added in Java 843 - Collection.stream() - added in Java 844 """);45 }46}4748// Interface with default method //?interfacedefault49interface Printable {50 // Abstract method - must implement //?abstract51 void print(String content);52 53 // Default method - optional to override //?defaultmethod54 default void log(String message) { //?defaultkeyword55 System.out.println("[LOG] " + message);56 }57 58 // Another default method59 default void preview() {60 System.out.println("Showing preview...");61 }62 63 // Static method in interface //?staticinterface64 static String getVersion() {65 return "1.0";66 }67}6869// Uses default implementation //?usesdefault70class BasicPrinter implements Printable {71 72 @Override73 public void print(String content) {74 System.out.println("Printing: " + content);75 }76 77 // Does NOT override log() - uses default78 // Does NOT override preview() - uses default79}8081// Overrides default method //?overridesdefault82class AdvancedPrinter implements Printable {83 84 @Override85 public void print(String content) {86 System.out.println("Advanced printing: " + content);87 }88 89 @Override90 public void log(String message) { //?overridelog91 System.out.println("[ADVANCED LOG] " + 92 java.time.LocalTime.now() + " - " + message);93 }94}9596// Two interfaces with same default method //?conflict97interface USB {98 default void connect() {99 System.out.println("USB connecting...");100 }101}102103interface Bluetooth {104 default void connect() {105 System.out.println("Bluetooth connecting...");106 }107}108109// Must resolve conflict //?resolveconflict110class MultiDevice implements USB, Bluetooth {111 112 @Override113 public void connect() { //?mustoverride114 System.out.println("Multi-device connecting:");115 USB.super.connect(); //?superusb116 Bluetooth.super.connect(); //?superbluetooth117 }output === Why Default Methods? === Before Java 8: - Adding method to interface broke all implementations - Every class needed updating With default methods: - Add new methods with default implementation - Existing classes still work - Optional override for custom behavior Real examples: - List.sort() - added in Java 8 - Collection.stream() - added in Java 8
default void method() { } - provides implementation that classes can inherit or override.
default method
Interface method with implementation. Classes inherit it unless they override.
Functional interfaces
Interfaces with exactly one abstract method work with lambdas.
FunctionalInterfaceDemo.java
Replay: real traced execution (multi-file project)
// Functional Interfaces and Lambdas
public class FunctionalInterfaceDemo {
public static void main(String[] args) {
System.out.println("=== Functional Interface ===\n");
// Traditional: Anonymous inner class
Calculator add1 = new Calculator() {
@Override
public int calculate(int a, int b) {
return a + b;
}
};
System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));
// Modern: Lambda expression
Calculator add2 = (a, b) -> a + b;
System.out.println("Lambda: 5 + 3 = " + add2.calculate(5, 3));
System.out.println("\n=== Multiple Operations ===");
// Different operations, same interface
Calculator subtract = (a, b) -> a - b;
Calculator multiply = (a, b) -> a * b;
Calculator divide = (a, b) -> a / b;
Calculator max = (a, b) -> a > b ? a : b;
int x = 10;
int y = 4;
System.out.println(x + " + " + y + " = " + add2.calculate(x, y));
System.out.println(x + " - " + y + " = " + subtract.calculate(x, y));
System.out.println(x + " * " + y + " = " + multiply.calculate(x, y));
System.out.println(x + " / " + y + " = " + divide.calculate(x, y));
System.out.println("max(" + x + ", " + y + ") = " + max.calculate(x, y));
System.out.println("\n=== Passing Lambda to Method ===");
// Pass behavior as parameter
applyOperation("Addition", 7, 3, (a, b) -> a + b);
applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));
System.out.println("\n=== Lambda Variations ===");
// Single parameter
Greeter hello = name -> "Hello, " + name + "!";
System.out.println(hello.greet("World"));
// Multi-line body
Greeter formal = name -> {
String title = "Dear " + name;
return title + ", welcome!";
};
System.out.println(formal.greet("Guest"));
// No parameters
MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;
System.out.println(supplier.get());
System.out.println("\n=== @FunctionalInterface ===");
System.out.println("""
@FunctionalInterface annotation:
- Documents intent
- Compiler enforces single abstract method
- Enables lambda usage
Common built-in functional interfaces:
- Runnable: () -> void
- Comparator<T>: (T, T) -> int
- Consumer<T>: (T) -> void
- Supplier<T>: () -> T
- Function<T, R>: (T) -> R
- Predicate<T>: (T) -> boolean
""");
}
// Method accepting functional interface
static void applyOperation(String name, int a, int b, Calculator calc) {
int result = calc.calculate(a, b);
System.out.println(name + ": " + a + " op " + b + " = " + result);
}
}
// Functional interface - exactly ONE abstract method
@FunctionalInterface
interface Calculator {
int calculate(int a, int b);
// Default methods allowed!
default void printResult(int a, int b) {
System.out.println("Result: " + calculate(a, b));
}
}
// Another functional interface
@FunctionalInterface
interface Greeter {
String greet(String name);
}
@FunctionalInterface
interface MessageSupplier {
String get();
}
// Functional Interfaces and Lambdas
public class FunctionalInterfaceDemo {
public static void main(String[] args) {
System.out.println("=== Functional Interface ===\n");
// Traditional: Anonymous inner class
Calculator add1 = new Calculator() {
@Override
public int calculate(int a, int b) {
return a + b;
}
};
System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));
// Modern: Lambda expression
Calculator add2 = (a, b) -> a + b;
System.out.println("Lambda: 5 + 3 = " + add2.calculate(5, 3));
System.out.println("\n=== Multiple Operations ===");
// Different operations, same interface
Calculator subtract = (a, b) -> a - b;
Calculator multiply = (a, b) -> a * b;
Calculator divide = (a, b) -> a / b;
Calculator max = (a, b) -> a > b ? a : b;
int x = 6;
int y = 4;
System.out.println(x + " + " + y + " = " + add2.calculate(x, y));
System.out.println(x + " - " + y + " = " + subtract.calculate(x, y));
System.out.println(x + " * " + y + " = " + multiply.calculate(x, y));
System.out.println(x + " / " + y + " = " + divide.calculate(x, y));
System.out.println("max(" + x + ", " + y + ") = " + max.calculate(x, y));
System.out.println("\n=== Passing Lambda to Method ===");
// Pass behavior as parameter
applyOperation("Addition", 7, 3, (a, b) -> a + b);
applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));
System.out.println("\n=== Lambda Variations ===");
// Single parameter
Greeter hello = name -> "Hello, " + name + "!";
System.out.println(hello.greet("World"));
// Multi-line body
Greeter formal = name -> {
String title = "Dear " + name;
return title + ", welcome!";
};
System.out.println(formal.greet("Guest"));
// No parameters
MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;
System.out.println(supplier.get());
System.out.println("\n=== @FunctionalInterface ===");
System.out.println("""
@FunctionalInterface annotation:
- Documents intent
- Compiler enforces single abstract method
- Enables lambda usage
Common built-in functional interfaces:
- Runnable: () -> void
- Comparator<T>: (T, T) -> int
- Consumer<T>: (T) -> void
- Supplier<T>: () -> T
- Function<T, R>: (T) -> R
- Predicate<T>: (T) -> boolean
""");
}
// Method accepting functional interface
static void applyOperation(String name, int a, int b, Calculator calc) {
int result = calc.calculate(a, b);
System.out.println(name + ": " + a + " op " + b + " = " + result);
}
}
// Functional interface - exactly ONE abstract method
@FunctionalInterface
interface Calculator {
int calculate(int a, int b);
// Default methods allowed!
default void printResult(int a, int b) {
System.out.println("Result: " + calculate(a, b));
}
}
// Another functional interface
@FunctionalInterface
interface Greeter {
String greet(String name);
}
@FunctionalInterface
interface MessageSupplier {
String get();
}
// Functional Interfaces and Lambdas
public class FunctionalInterfaceDemo {
public static void main(String[] args) {
System.out.println("=== Functional Interface ===\n");
// Traditional: Anonymous inner class
Calculator add1 = new Calculator() {
@Override
public int calculate(int a, int b) {
return a + b;
}
};
System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));
// Modern: Lambda expression
Calculator add2 = (a, b) -> a + b;
System.out.println("Lambda: 5 + 3 = " + add2.calculate(5, 3));
System.out.println("\n=== Multiple Operations ===");
// Different operations, same interface
Calculator subtract = (a, b) -> a - b;
Calculator multiply = (a, b) -> a * b;
Calculator divide = (a, b) -> a / b;
Calculator max = (a, b) -> a > b ? a : b;
int x = 10;
int y = 2;
System.out.println(x + " + " + y + " = " + add2.calculate(x, y));
System.out.println(x + " - " + y + " = " + subtract.calculate(x, y));
System.out.println(x + " * " + y + " = " + multiply.calculate(x, y));
System.out.println(x + " / " + y + " = " + divide.calculate(x, y));
System.out.println("max(" + x + ", " + y + ") = " + max.calculate(x, y));
System.out.println("\n=== Passing Lambda to Method ===");
// Pass behavior as parameter
applyOperation("Addition", 7, 3, (a, b) -> a + b);
applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));
System.out.println("\n=== Lambda Variations ===");
// Single parameter
Greeter hello = name -> "Hello, " + name + "!";
System.out.println(hello.greet("World"));
// Multi-line body
Greeter formal = name -> {
String title = "Dear " + name;
return title + ", welcome!";
};
System.out.println(formal.greet("Guest"));
// No parameters
MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;
System.out.println(supplier.get());
System.out.println("\n=== @FunctionalInterface ===");
System.out.println("""
@FunctionalInterface annotation:
- Documents intent
- Compiler enforces single abstract method
- Enables lambda usage
Common built-in functional interfaces:
- Runnable: () -> void
- Comparator<T>: (T, T) -> int
- Consumer<T>: (T) -> void
- Supplier<T>: () -> T
- Function<T, R>: (T) -> R
- Predicate<T>: (T) -> boolean
""");
}
// Method accepting functional interface
static void applyOperation(String name, int a, int b, Calculator calc) {
int result = calc.calculate(a, b);
System.out.println(name + ": " + a + " op " + b + " = " + result);
}
}
// Functional interface - exactly ONE abstract method
@FunctionalInterface
interface Calculator {
int calculate(int a, int b);
// Default methods allowed!
default void printResult(int a, int b) {
System.out.println("Result: " + calculate(a, b));
}
}
// Another functional interface
@FunctionalInterface
interface Greeter {
String greet(String name);
}
@FunctionalInterface
interface MessageSupplier {
String get();
}
add1 ← ⟨FunctionalInterfaceDemo$1 A⟩
3public class FunctionalInterfaceDemo {4 public static void main(String[] args) {5 System.out.println("=== Functional Interface ===\n");6 7 // Traditional: Anonymous inner class //?anonymous8 Calculator add1→ ⟨FunctionalInterfaceDemo$1 A⟩ = new Calculator() {9 @Override10 public int calculate(int a, int b) {11 return a + b;12 }13 };14 15 System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));output=== Functional Interface ===@Override public int calculate(int a, int b)
8Calculator add1 = new Calculator() {9 @Override10 public int calculate(int a5, int b3) {11 return a5 + b3;12 }add2 ← ⟨FunctionalInterfaceDemo lambda B⟩, subtract ← ⟨FunctionalInterfaceDemo lambda C⟩
15System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));1617// Modern: Lambda expression //?lambda18Calculator add2→ ⟨FunctionalInterfaceDemo lambda B⟩ = (a, b) -> a + b; //?lambdasyntax1920System.out.println("Lambda: 5 + 3 = " + add2.calculate(5, 3));2122System.out.println("\n=== Multiple Operations ===");2324// Different operations, same interface //?operations25Calculator subtract→ ⟨FunctionalInterfaceDemo lambda C⟩ = (a, b) -> a - b;26Calculator multiply→ ⟨FunctionalInterfaceDemo lambda D⟩ = (a, b) -> a * b;27Calculator divide→ ⟨FunctionalInterfaceDemo lambda E⟩ = (a, b) -> a / b;28Calculator max→ ⟨FunctionalInterfaceDemo lambda F⟩ = (a, b) -> a > b ? a : b; //?ternary2930int x→ 10 = 10; //@x=631int y→ 4 = 4; //@y=232System.out.println(x10 + " + " + y4 + " = " + add2.calculate(x, y));33System.out.println(x10 + " - " + y4 + " = " + subtract.calculate(x, y));34System.out.println(x10 + " * " + y4 + " = " + multiply.calculate(x, y));35System.out.println(x10 + " / " + y4 + " = " + divide.calculate(x, y));36System.out.println("max(" + x10 + ", " + y4 + ") = " + max.calculate(x, y));3738System.out.println("\n=== Passing Lambda to Method ===");3940// Pass behavior as parameter //?passlambda41applyOperation("Addition", 7, 3, (a, b) -> a + b);42applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));outputAnonymous class: 5 + 3 = 8 Lambda: 5 + 3 = 8 === Multiple Operations === 10 + 4 = 14 10 - 4 = 6 10 * 4 = 40 10 / 4 = 2 max(10, 4) = 10 === Passing Lambda to Method ===result ← 10
pass 1 of 240 // Pass behavior as parameter //?passlambda41 applyOperation("Addition", 7, 3, (a, b) -> a + b);42 applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));43 44 System.out.println("\n=== Lambda Variations ===");45 46 // Single parameter //?singleparam47 Greeter hello = name -> "Hello, " + name + "!"; //?noparens48 System.out.println(hello.greet("World"));49 50 // Multi-line body //?multiline51 Greeter formal = name -> {52 String title = "Dear " + name;53 return title + ", welcome!";54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters //?noparam58 MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}7778// Method accepting functional interface //?methodaccept79static void applyOperation(String nameAddition, int a7, int b3, Calculator calc⟨FunctionalInterfaceDemo lambda G⟩) {80 int result→ 10 = calc.calculate(a7, b3);81 System.out.println(nameAddition + ": " + a7 + " op " + b3 + " = " + result10);82}outputAddition: 7 op 3 = 10result ← 256, hello ← ⟨FunctionalInterfaceDemo lambda H⟩, formal ← ⟨FunctionalInterfaceDemo lambda I⟩
pass 2 of 241 applyOperation("Addition", 7, 3, (a, b) -> a + b);42 applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));43 44 System.out.println("\n=== Lambda Variations ===");45 46 // Single parameter //?singleparam47 Greeter hello→ ⟨FunctionalInterfaceDemo lambda H⟩ = name -> "Hello, " + name + "!"; //?noparens48 System.out.println(hello.greet("World"));49 50 // Multi-line body //?multiline51 Greeter formal→ ⟨FunctionalInterfaceDemo lambda I⟩ = name -> {52 String title = "Dear " + name;53 return title + ", welcome!";54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters //?noparam58 MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}7778// Method accepting functional interface //?methodaccept79static void applyOperation(String namePower, int a2, int b8, Calculator calc⟨FunctionalInterfaceDemo lambda J⟩) {80 int result→ 256 = calc.calculate(a2, b8);81 System.out.println(namePower + ": " + a2 + " op " + b8 + " = " + result256);82}outputPower: 2 op 8 = 256 === Lambda Variations === Hello, World!supplier ← ⟨FunctionalInterfaceDemo lambda K⟩
54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters //?noparam58 MessageSupplier supplier→ ⟨FunctionalInterfaceDemo lambda K⟩ = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}outputDear Guest, welcome! Sample message at 1736937000000 === @FunctionalInterface === @FunctionalInterface annotation: - Documents intent - Compiler enforces single abstract method - Enables lambda usage Common built-in functional interfaces: - Runnable: () -> void - Comparator<T>: (T, T) -> int - Consumer<T>: (T) -> void - Supplier<T>: () -> T - Function<T, R>: (T) -> R - Predicate<T>: (T) -> boolean
add1 ← ⟨FunctionalInterfaceDemo$1 A⟩
3public class FunctionalInterfaceDemo {4 public static void main(String[] args) {5 System.out.println("=== Functional Interface ===\n");6 7 // Traditional: Anonymous inner class8 Calculator add1→ ⟨FunctionalInterfaceDemo$1 A⟩ = new Calculator() {9 @Override10 public int calculate(int a, int b) {11 return a + b;12 }13 };14 15 System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));output=== Functional Interface ===@Override public int calculate(int a, int b)
8Calculator add1 = new Calculator() {9 @Override10 public int calculate(int a5, int b3) {11 return a5 + b3;12 }add2 ← ⟨FunctionalInterfaceDemo lambda B⟩, subtract ← ⟨FunctionalInterfaceDemo lambda C⟩
15System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));1617// Modern: Lambda expression18Calculator add2→ ⟨FunctionalInterfaceDemo lambda B⟩ = (a, b) -> a + b;1920System.out.println("Lambda: 5 + 3 = " + add2.calculate(5, 3));2122System.out.println("\n=== Multiple Operations ===");2324// Different operations, same interface25Calculator subtract→ ⟨FunctionalInterfaceDemo lambda C⟩ = (a, b) -> a - b;26Calculator multiply→ ⟨FunctionalInterfaceDemo lambda D⟩ = (a, b) -> a * b;27Calculator divide→ ⟨FunctionalInterfaceDemo lambda E⟩ = (a, b) -> a / b;28Calculator max→ ⟨FunctionalInterfaceDemo lambda F⟩ = (a, b) -> a > b ? a : b;2930int x→ 6 = 6;31int y→ 4 = 4;32System.out.println(x6 + " + " + y4 + " = " + add2.calculate(x, y));33System.out.println(x6 + " - " + y4 + " = " + subtract.calculate(x, y));34System.out.println(x6 + " * " + y4 + " = " + multiply.calculate(x, y));35System.out.println(x6 + " / " + y4 + " = " + divide.calculate(x, y));36System.out.println("max(" + x6 + ", " + y4 + ") = " + max.calculate(x, y));3738System.out.println("\n=== Passing Lambda to Method ===");3940// Pass behavior as parameter41applyOperation("Addition", 7, 3, (a, b) -> a + b);42applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));outputAnonymous class: 5 + 3 = 8 Lambda: 5 + 3 = 8 === Multiple Operations === 6 + 4 = 10 6 - 4 = 2 6 * 4 = 24 6 / 4 = 1 max(6, 4) = 6 === Passing Lambda to Method ===result ← 10
pass 1 of 240 // Pass behavior as parameter41 applyOperation("Addition", 7, 3, (a, b) -> a + b);42 applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));43 44 System.out.println("\n=== Lambda Variations ===");45 46 // Single parameter47 Greeter hello = name -> "Hello, " + name + "!";48 System.out.println(hello.greet("World"));49 50 // Multi-line body51 Greeter formal = name -> {52 String title = "Dear " + name;53 return title + ", welcome!";54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters58 MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}7778// Method accepting functional interface79static void applyOperation(String nameAddition, int a7, int b3, Calculator calc⟨FunctionalInterfaceDemo lambda G⟩) {80 int result→ 10 = calc.calculate(a7, b3);81 System.out.println(nameAddition + ": " + a7 + " op " + b3 + " = " + result10);82}outputAddition: 7 op 3 = 10result ← 256, hello ← ⟨FunctionalInterfaceDemo lambda H⟩, formal ← ⟨FunctionalInterfaceDemo lambda I⟩
pass 2 of 241 applyOperation("Addition", 7, 3, (a, b) -> a + b);42 applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));43 44 System.out.println("\n=== Lambda Variations ===");45 46 // Single parameter47 Greeter hello→ ⟨FunctionalInterfaceDemo lambda H⟩ = name -> "Hello, " + name + "!";48 System.out.println(hello.greet("World"));49 50 // Multi-line body51 Greeter formal→ ⟨FunctionalInterfaceDemo lambda I⟩ = name -> {52 String title = "Dear " + name;53 return title + ", welcome!";54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters58 MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}7778// Method accepting functional interface79static void applyOperation(String namePower, int a2, int b8, Calculator calc⟨FunctionalInterfaceDemo lambda J⟩) {80 int result→ 256 = calc.calculate(a2, b8);81 System.out.println(namePower + ": " + a2 + " op " + b8 + " = " + result256);82}outputPower: 2 op 8 = 256 === Lambda Variations === Hello, World!supplier ← ⟨FunctionalInterfaceDemo lambda K⟩
54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters58 MessageSupplier supplier→ ⟨FunctionalInterfaceDemo lambda K⟩ = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}outputDear Guest, welcome! Sample message at 1736937000000 === @FunctionalInterface === @FunctionalInterface annotation: - Documents intent - Compiler enforces single abstract method - Enables lambda usage Common built-in functional interfaces: - Runnable: () -> void - Comparator<T>: (T, T) -> int - Consumer<T>: (T) -> void - Supplier<T>: () -> T - Function<T, R>: (T) -> R - Predicate<T>: (T) -> boolean
add1 ← ⟨FunctionalInterfaceDemo$1 A⟩
3public class FunctionalInterfaceDemo {4 public static void main(String[] args) {5 System.out.println("=== Functional Interface ===\n");6 7 // Traditional: Anonymous inner class8 Calculator add1→ ⟨FunctionalInterfaceDemo$1 A⟩ = new Calculator() {9 @Override10 public int calculate(int a, int b) {11 return a + b;12 }13 };14 15 System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));output=== Functional Interface ===@Override public int calculate(int a, int b)
8Calculator add1 = new Calculator() {9 @Override10 public int calculate(int a5, int b3) {11 return a5 + b3;12 }add2 ← ⟨FunctionalInterfaceDemo lambda B⟩, subtract ← ⟨FunctionalInterfaceDemo lambda C⟩
15System.out.println("Anonymous class: 5 + 3 = " + add1.calculate(5, 3));1617// Modern: Lambda expression18Calculator add2→ ⟨FunctionalInterfaceDemo lambda B⟩ = (a, b) -> a + b;1920System.out.println("Lambda: 5 + 3 = " + add2.calculate(5, 3));2122System.out.println("\n=== Multiple Operations ===");2324// Different operations, same interface25Calculator subtract→ ⟨FunctionalInterfaceDemo lambda C⟩ = (a, b) -> a - b;26Calculator multiply→ ⟨FunctionalInterfaceDemo lambda D⟩ = (a, b) -> a * b;27Calculator divide→ ⟨FunctionalInterfaceDemo lambda E⟩ = (a, b) -> a / b;28Calculator max→ ⟨FunctionalInterfaceDemo lambda F⟩ = (a, b) -> a > b ? a : b;2930int x→ 10 = 10;31int y→ 2 = 2;32System.out.println(x10 + " + " + y2 + " = " + add2.calculate(x, y));33System.out.println(x10 + " - " + y2 + " = " + subtract.calculate(x, y));34System.out.println(x10 + " * " + y2 + " = " + multiply.calculate(x, y));35System.out.println(x10 + " / " + y2 + " = " + divide.calculate(x, y));36System.out.println("max(" + x10 + ", " + y2 + ") = " + max.calculate(x, y));3738System.out.println("\n=== Passing Lambda to Method ===");3940// Pass behavior as parameter41applyOperation("Addition", 7, 3, (a, b) -> a + b);42applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));outputAnonymous class: 5 + 3 = 8 Lambda: 5 + 3 = 8 === Multiple Operations === 10 + 2 = 12 10 - 2 = 8 10 * 2 = 20 10 / 2 = 5 max(10, 2) = 10 === Passing Lambda to Method ===result ← 10
pass 1 of 240 // Pass behavior as parameter41 applyOperation("Addition", 7, 3, (a, b) -> a + b);42 applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));43 44 System.out.println("\n=== Lambda Variations ===");45 46 // Single parameter47 Greeter hello = name -> "Hello, " + name + "!";48 System.out.println(hello.greet("World"));49 50 // Multi-line body51 Greeter formal = name -> {52 String title = "Dear " + name;53 return title + ", welcome!";54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters58 MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}7778// Method accepting functional interface79static void applyOperation(String nameAddition, int a7, int b3, Calculator calc⟨FunctionalInterfaceDemo lambda G⟩) {80 int result→ 10 = calc.calculate(a7, b3);81 System.out.println(nameAddition + ": " + a7 + " op " + b3 + " = " + result10);82}outputAddition: 7 op 3 = 10result ← 256, hello ← ⟨FunctionalInterfaceDemo lambda H⟩, formal ← ⟨FunctionalInterfaceDemo lambda I⟩
pass 2 of 241 applyOperation("Addition", 7, 3, (a, b) -> a + b);42 applyOperation("Power", 2, 8, (a, b) -> (int) Math.pow(a, b));43 44 System.out.println("\n=== Lambda Variations ===");45 46 // Single parameter47 Greeter hello→ ⟨FunctionalInterfaceDemo lambda H⟩ = name -> "Hello, " + name + "!";48 System.out.println(hello.greet("World"));49 50 // Multi-line body51 Greeter formal→ ⟨FunctionalInterfaceDemo lambda I⟩ = name -> {52 String title = "Dear " + name;53 return title + ", welcome!";54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters58 MessageSupplier supplier = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}7778// Method accepting functional interface79static void applyOperation(String namePower, int a2, int b8, Calculator calc⟨FunctionalInterfaceDemo lambda J⟩) {80 int result→ 256 = calc.calculate(a2, b8);81 System.out.println(namePower + ": " + a2 + " op " + b8 + " = " + result256);82}outputPower: 2 op 8 = 256 === Lambda Variations === Hello, World!supplier ← ⟨FunctionalInterfaceDemo lambda K⟩
54 };55 System.out.println(formal.greet("Guest"));56 57 // No parameters58 MessageSupplier supplier→ ⟨FunctionalInterfaceDemo lambda K⟩ = () -> "Sample message at " + 1736937000000L;59 System.out.println(supplier.get());60 61 System.out.println("\n=== @FunctionalInterface ===");62 System.out.println("""63 @FunctionalInterface annotation:64 - Documents intent65 - Compiler enforces single abstract method66 - Enables lambda usage67 68 Common built-in functional interfaces:69 - Runnable: () -> void70 - Comparator<T>: (T, T) -> int71 - Consumer<T>: (T) -> void72 - Supplier<T>: () -> T73 - Function<T, R>: (T) -> R74 - Predicate<T>: (T) -> boolean75 """);76}outputDear Guest, welcome! Sample message at 1736937000000 === @FunctionalInterface === @FunctionalInterface annotation: - Documents intent - Compiler enforces single abstract method - Enables lambda usage Common built-in functional interfaces: - Runnable: () -> void - Comparator<T>: (T, T) -> int - Consumer<T>: (T) -> void - Supplier<T>: () -> T - Function<T, R>: (T) -> R - Predicate<T>: (T) -> boolean
@FunctionalInterface ensures single abstract method. Use with lambdas.
functional interface
Interface with one abstract method. Can be implemented with lambda expression.
Exercise: Practical.java
Build a plugin system using interfaces