Static Interface Methods - Java Programming

You want List.of(1, 2, 3) to create a list. Static interface methods let interfaces have utility functions without needing a separate utility class. The method belongs to the interface itself, not to implementing classes.

Basic static method

Define a utility method on an interface.

                            StaticBasics.java
                        
Replay: real traced execution (multi-file project)

                            // Basic Static Method Syntax

interface MathUtils {
    // Static method in interface
    static int square(int n) {
        return n * n;
    }

    static int cube(int n) {
        return n * n * n;
    }

    static boolean isEven(int n) {
        return n % 2 == 0;
    }

    static boolean isPositive(int n) {
        return n > 0;
    }

    // Static methods can call other static methods
    static boolean isPositiveEven(int n) {
        return isPositive(n) && isEven(n);
    }
}

// Class implementing interface
class Calculator implements MathUtils {
    // Does NOT inherit static methods!

    public int add(int a, int b) {
        return a + b;
    }
}

public class StaticBasics {
    public static void main(String[] args) {
        System.out.println("=== Static Interface Methods ===\n");

        // Call via interface name
        System.out.println("MathUtils.square(5) = " + MathUtils.square(5));
        System.out.println("MathUtils.cube(3) = " + MathUtils.cube(3));
        System.out.println("MathUtils.isEven(4) = " + MathUtils.isEven(4));
        System.out.println("MathUtils.isEven(7) = " + MathUtils.isEven(7));
        System.out.println("MathUtils.isPositiveEven(6) = " + MathUtils.isPositiveEven(6));

        System.out.println("\n=== Using with Calculator ===");
        Calculator calc = new Calculator();
        System.out.println("calc.add(2, 3) = " + calc.add(2, 3));

        // Cannot call static via instance!
        // calc.square(5);  // COMPILE ERROR!
        // Must use interface name
        System.out.println("MathUtils.square(5) = " + MathUtils.square(5));

        System.out.println("\n=== Static vs Instance ===");
        System.out.println("""
            Static methods in interfaces:
            - Belong to the interface itself
            - Called via InterfaceName.method()
            - NOT inherited by implementing classes
            - Cannot be overridden
            - Good for utility functions

            Default methods (for comparison):
            - Belong to instances
            - Called via object.method()
            - ARE inherited by implementing classes
            - CAN be overridden
            """);
    }
}

                        
1public static void main(String[] args)⋯36public class StaticBasics {37    public static void main(String[] args) {38        System.out.println("=== Static Interface Methods ===\n");39        40        // Call via interface name //?callviainterface41        System.out.println("MathUtils.square(5) = " + MathUtils.square(5));42        System.out.println("MathUtils.cube(3) = " + MathUtils.cube(3));⋯output=== Static Interface Methods ===
2static int square(int n)pass 1 of 2⋯4// Static method in interface //?staticmethod5static int square(int n5) { //?statickeyword6    return n5 * n;7}⋯
3System.out.println("MathUtils.square(5) = " + MathUtils.square(5));⋯40// Call via interface name //?callviainterface41System.out.println("MathUtils.square(5) = " + MathUtils.square(5));42System.out.println("MathUtils.cube(3) = " + MathUtils.cube(3));43System.out.println("MathUtils.isEven(4) = " + MathUtils.isEven(4));⋯outputMathUtils.square(5) = 25
4static int cube(int n)⋯9static int cube(int n3) {10    return n3 * n * n;11}⋯
5System.out.println("MathUtils.cube(3) = " + MathUtils.cube(3));⋯41System.out.println("MathUtils.square(5) = " + MathUtils.square(5));42System.out.println("MathUtils.cube(3) = " + MathUtils.cube(3));43System.out.println("MathUtils.isEven(4) = " + MathUtils.isEven(4));44System.out.println("MathUtils.isEven(7) = " + MathUtils.isEven(7));⋯outputMathUtils.cube(3) = 27
6static boolean isEven(int n)pass 1 of 3⋯13static boolean isEven(int n4) { //?iseven14    return n4 % 2 == 0;15}⋯All 3 passes — pass 1 is the card abovepassn
14
27
36
7System.out.println("MathUtils.isEven(4) = " + MathUtils.isEven(4));⋯42System.out.println("MathUtils.cube(3) = " + MathUtils.cube(3));43System.out.println("MathUtils.isEven(4) = " + MathUtils.isEven(4));44System.out.println("MathUtils.isEven(7) = " + MathUtils.isEven(7));45System.out.println("MathUtils.isPositiveEven(6) = " + MathUtils.isPositiveEven(6));⋯outputMathUtils.isEven(4) = true
8System.out.println("MathUtils.isEven(7) = " + MathUtils.isEven(7));⋯43System.out.println("MathUtils.isEven(4) = " + MathUtils.isEven(4));44System.out.println("MathUtils.isEven(7) = " + MathUtils.isEven(7));45System.out.println("MathUtils.isPositiveEven(6) = " + MathUtils.isPositiveEven(6));⋯outputMathUtils.isEven(7) = false
9static boolean isPositiveEven(int n)⋯21// Static methods can call other static methods //?callstatic22static boolean isPositiveEven(int n6) {23    return isPositive(n6) && isEven(n);24}⋯
10static boolean isPositive(int n)⋯17static boolean isPositive(int n6) {18    return n6 > 0;19}⋯
11calc ← ⟨Calculator A⟩⋯44System.out.println("MathUtils.isEven(7) = " + MathUtils.isEven(7));45System.out.println("MathUtils.isPositiveEven(6) = " + MathUtils.isPositiveEven(6));4647System.out.println("\n=== Using with Calculator ===");48Calculator calc→ ⟨Calculator A⟩ = new Calculator();49System.out.println("calc.add(2, 3) = " + calc.add(2, 3));⋯outputMathUtils.isPositiveEven(6) = true

=== Using with Calculator ===
12public int add(int a, int b)⋯31public int add(int a2, int b3) {32    return a2 + b3;33}⋯
13System.out.println("calc.add(2, 3) = " + calc.add(2, 3));⋯48Calculator calc = new Calculator();49System.out.println("calc.add(2, 3) = " + calc.add(2, 3));5051// Cannot call static via instance! //?cannotinstance52// calc.square(5);  // COMPILE ERROR!53// Must use interface name54System.out.println("MathUtils.square(5) = " + MathUtils.square(5));⋯outputcalc.add(2, 3) = 5
14static int square(int n)pass 2 of 2⋯4// Static method in interface //?staticmethod5static int square(int n5) { //?statickeyword6    return n5 * n;7}⋯
15System.out.println("MathUtils.square(5) = " + MathUtils.square(5));⋯53    // Must use interface name54    System.out.println("MathUtils.square(5) = " + MathUtils.square(5));55    56    System.out.println("\n=== Static vs Instance ===");57    System.out.println("""58        Static methods in interfaces:59        - Belong to the interface itself60        - Called via InterfaceName.method()61        - NOT inherited by implementing classes62        - Cannot be overridden63        - Good for utility functions64        65        Default methods (for comparison):66        - Belong to instances67        - Called via object.method()68        - ARE inherited by implementing classes69        - CAN be overridden70        """);71}⋯outputMathUtils.square(5) = 25

=== Static vs Instance ===
Static methods in interfaces:
- Belong to the interface itself
- Called via InterfaceName.method()
- NOT inherited by implementing classes
- Cannot be overridden
- Good for utility functions

Default methods (for comparison):
- Belong to instances
- Called via object.method()
- ARE inherited by implementing classes
- CAN be overridden

All 3 passes — pass 1 is the card above
pass	`n`
1	4
2	7
3	6

static methods belong to interface. Call via InterfaceName.method().

static interface method Method that belongs to interface, not implementations. Not inherited.

Factory methods

Static methods that create instances.

                            circleRadius
                            circleRadius = 5.0 (default)circleRadius = 2.5circleRadius = 8.0
                        

                            FactoryMethods.java
                        
Replay: real traced execution (multi-file project)

                            // Factory Methods in Interfaces

interface Shape {
    String getName();
    double getArea();

    // Static factory methods
    static Shape circle(double radius) {
        return new Circle(radius);
    }

    static Shape rectangle(double width, double height) {
        return new Rectangle(width, height);
    }

    static Shape square(double side) {
        return new Rectangle(side, side);  // Square is special rectangle
    }
}

// Implementation classes can be package-private
class Circle implements Shape {
    private double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public String getName() {
        return "Circle(r=" + radius + ")";
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

class Rectangle implements Shape {
    private double width, height;

    Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public String getName() {
        return "Rectangle(" + width + "x" + height + ")";
    }

    @Override
    public double getArea() {
        return width * height;
    }
}

// Another example: immutable collections pattern
interface ImmutableList<T> {
    int size();
    T get(int index);

    // Factory methods like List.of()
    static <T> ImmutableList<T> of() {
        return new EmptyList<>();
    }

    static <T> ImmutableList<T> of(T item) {
        return new SingleList<>(item);
    }

    @SafeVarargs
    static <T> ImmutableList<T> of(T... items) {
        return new ArrayBackedList<>(items.clone());
    }
}

class EmptyList<T> implements ImmutableList<T> {
    @Override public int size() { return 0; }
    @Override public T get(int index) { throw new IndexOutOfBoundsException(); }
}

class SingleList<T> implements ImmutableList<T> {
    private T item;
    SingleList(T item) { this.item = item; }
    @Override public int size() { return 1; }
    @Override public T get(int index) {
        if (index != 0) throw new IndexOutOfBoundsException();
        return item;
    }
}

class ArrayBackedList<T> implements ImmutableList<T> {
    private Object[] items;
    ArrayBackedList(Object[] items) { this.items = items; }
    @Override public int size() { return items.length; }
    @Override @SuppressWarnings("unchecked")
    public T get(int index) { return (T) items[index]; }
}

public class FactoryMethods {
    public static void main(String[] args) {
        System.out.println("=== Shape Factory Methods ===\n");

        // Create shapes via interface
        double circleRadius = 5.0;
        Shape circle = Shape.circle(circleRadius);
        Shape rect = Shape.rectangle(4, 6);
        Shape square = Shape.square(3);

        System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));
        System.out.println(rect.getName() + " area: " + rect.getArea());
        System.out.println(square.getName() + " area: " + square.getArea());

        System.out.println("\n=== Benefits of Factory Methods ===");
        System.out.println("""
            1. Hide implementations - users don't know about Circle/Rectangle
            2. Return appropriate subtype - square() returns Rectangle
            3. Caching possible - can reuse instances
            4. Validation - can check parameters
            """);

        System.out.println("=== Immutable List Factory ===\n");

        // Like Java's List.of()
        ImmutableList<String> empty = ImmutableList.of();
        ImmutableList<String> single = ImmutableList.of("hello");
        ImmutableList<String> multi = ImmutableList.of("a", "b", "c");

        System.out.println("empty.size() = " + empty.size());
        System.out.println("single.get(0) = " + single.get(0));
        System.out.println("multi.size() = " + multi.size());

        for (int i = 0; i < multi.size(); i++) {
            System.out.println("multi.get(" + i + ") = " + multi.get(i));
        }

        System.out.println("\n=== Real Java Examples ===");
        System.out.println("""
            Java's static factory methods:
            - List.of("a", "b", "c")
            - Set.of(1, 2, 3)
            - Map.of("key", "value")
            - Optional.of(value)
            - Optional.empty()
            - Stream.of(items)
            - Comparator.comparing(keyExtractor)
            """);
    }
}
// Factory Methods in Interfaces

interface Shape {
    String getName();
    double getArea();

    // Static factory methods
    static Shape circle(double radius) {
        return new Circle(radius);
    }

    static Shape rectangle(double width, double height) {
        return new Rectangle(width, height);
    }

    static Shape square(double side) {
        return new Rectangle(side, side);  // Square is special rectangle
    }
}

// Implementation classes can be package-private
class Circle implements Shape {
    private double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public String getName() {
        return "Circle(r=" + radius + ")";
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

class Rectangle implements Shape {
    private double width, height;

    Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public String getName() {
        return "Rectangle(" + width + "x" + height + ")";
    }

    @Override
    public double getArea() {
        return width * height;
    }
}

// Another example: immutable collections pattern
interface ImmutableList<T> {
    int size();
    T get(int index);

    // Factory methods like List.of()
    static <T> ImmutableList<T> of() {
        return new EmptyList<>();
    }

    static <T> ImmutableList<T> of(T item) {
        return new SingleList<>(item);
    }

    @SafeVarargs
    static <T> ImmutableList<T> of(T... items) {
        return new ArrayBackedList<>(items.clone());
    }
}

class EmptyList<T> implements ImmutableList<T> {
    @Override public int size() { return 0; }
    @Override public T get(int index) { throw new IndexOutOfBoundsException(); }
}

class SingleList<T> implements ImmutableList<T> {
    private T item;
    SingleList(T item) { this.item = item; }
    @Override public int size() { return 1; }
    @Override public T get(int index) {
        if (index != 0) throw new IndexOutOfBoundsException();
        return item;
    }
}

class ArrayBackedList<T> implements ImmutableList<T> {
    private Object[] items;
    ArrayBackedList(Object[] items) { this.items = items; }
    @Override public int size() { return items.length; }
    @Override @SuppressWarnings("unchecked")
    public T get(int index) { return (T) items[index]; }
}

public class FactoryMethods {
    public static void main(String[] args) {
        System.out.println("=== Shape Factory Methods ===\n");

        // Create shapes via interface
        double circleRadius = 2.5;
        Shape circle = Shape.circle(circleRadius);
        Shape rect = Shape.rectangle(4, 6);
        Shape square = Shape.square(3);

        System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));
        System.out.println(rect.getName() + " area: " + rect.getArea());
        System.out.println(square.getName() + " area: " + square.getArea());

        System.out.println("\n=== Benefits of Factory Methods ===");
        System.out.println("""
            1. Hide implementations - users don't know about Circle/Rectangle
            2. Return appropriate subtype - square() returns Rectangle
            3. Caching possible - can reuse instances
            4. Validation - can check parameters
            """);

        System.out.println("=== Immutable List Factory ===\n");

        // Like Java's List.of()
        ImmutableList<String> empty = ImmutableList.of();
        ImmutableList<String> single = ImmutableList.of("hello");
        ImmutableList<String> multi = ImmutableList.of("a", "b", "c");

        System.out.println("empty.size() = " + empty.size());
        System.out.println("single.get(0) = " + single.get(0));
        System.out.println("multi.size() = " + multi.size());

        for (int i = 0; i < multi.size(); i++) {
            System.out.println("multi.get(" + i + ") = " + multi.get(i));
        }

        System.out.println("\n=== Real Java Examples ===");
        System.out.println("""
            Java's static factory methods:
            - List.of("a", "b", "c")
            - Set.of(1, 2, 3)
            - Map.of("key", "value")
            - Optional.of(value)
            - Optional.empty()
            - Stream.of(items)
            - Comparator.comparing(keyExtractor)
            """);
    }
}
// Factory Methods in Interfaces

interface Shape {
    String getName();
    double getArea();

    // Static factory methods
    static Shape circle(double radius) {
        return new Circle(radius);
    }

    static Shape rectangle(double width, double height) {
        return new Rectangle(width, height);
    }

    static Shape square(double side) {
        return new Rectangle(side, side);  // Square is special rectangle
    }
}

// Implementation classes can be package-private
class Circle implements Shape {
    private double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public String getName() {
        return "Circle(r=" + radius + ")";
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

class Rectangle implements Shape {
    private double width, height;

    Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public String getName() {
        return "Rectangle(" + width + "x" + height + ")";
    }

    @Override
    public double getArea() {
        return width * height;
    }
}

// Another example: immutable collections pattern
interface ImmutableList<T> {
    int size();
    T get(int index);

    // Factory methods like List.of()
    static <T> ImmutableList<T> of() {
        return new EmptyList<>();
    }

    static <T> ImmutableList<T> of(T item) {
        return new SingleList<>(item);
    }

    @SafeVarargs
    static <T> ImmutableList<T> of(T... items) {
        return new ArrayBackedList<>(items.clone());
    }
}

class EmptyList<T> implements ImmutableList<T> {
    @Override public int size() { return 0; }
    @Override public T get(int index) { throw new IndexOutOfBoundsException(); }
}

class SingleList<T> implements ImmutableList<T> {
    private T item;
    SingleList(T item) { this.item = item; }
    @Override public int size() { return 1; }
    @Override public T get(int index) {
        if (index != 0) throw new IndexOutOfBoundsException();
        return item;
    }
}

class ArrayBackedList<T> implements ImmutableList<T> {
    private Object[] items;
    ArrayBackedList(Object[] items) { this.items = items; }
    @Override public int size() { return items.length; }
    @Override @SuppressWarnings("unchecked")
    public T get(int index) { return (T) items[index]; }
}

public class FactoryMethods {
    public static void main(String[] args) {
        System.out.println("=== Shape Factory Methods ===\n");

        // Create shapes via interface
        double circleRadius = 8.0;
        Shape circle = Shape.circle(circleRadius);
        Shape rect = Shape.rectangle(4, 6);
        Shape square = Shape.square(3);

        System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));
        System.out.println(rect.getName() + " area: " + rect.getArea());
        System.out.println(square.getName() + " area: " + square.getArea());

        System.out.println("\n=== Benefits of Factory Methods ===");
        System.out.println("""
            1. Hide implementations - users don't know about Circle/Rectangle
            2. Return appropriate subtype - square() returns Rectangle
            3. Caching possible - can reuse instances
            4. Validation - can check parameters
            """);

        System.out.println("=== Immutable List Factory ===\n");

        // Like Java's List.of()
        ImmutableList<String> empty = ImmutableList.of();
        ImmutableList<String> single = ImmutableList.of("hello");
        ImmutableList<String> multi = ImmutableList.of("a", "b", "c");

        System.out.println("empty.size() = " + empty.size());
        System.out.println("single.get(0) = " + single.get(0));
        System.out.println("multi.size() = " + multi.size());

        for (int i = 0; i < multi.size(); i++) {
            System.out.println("multi.get(" + i + ") = " + multi.get(i));
        }

        System.out.println("\n=== Real Java Examples ===");
        System.out.println("""
            Java's static factory methods:
            - List.of("a", "b", "c")
            - Set.of(1, 2, 3)
            - Map.of("key", "value")
            - Optional.of(value)
            - Optional.empty()
            - Stream.of(items)
            - Comparator.comparing(keyExtractor)
            """);
    }
}

                        
1circleRadius ← 5.0⋯102public class FactoryMethods {103    public static void main(String[] args) {104        System.out.println("=== Shape Factory Methods ===\n");105        106        // Create shapes via interface //?usefactory107        double circleRadius→ 5.0 = 5.0;  //@circleRadius=5.0, 2.5, 8.0108        Shape circle = Shape.circle(circleRadius5.0);109        Shape rect = Shape.rectangle(4, 6);⋯output=== Shape Factory Methods ===
2static Shape circle(double radius)⋯7// Static factory methods //?factory8static Shape circle(double radius5.0) { //?circlefactory9    return new Circle(radius);10}⋯
3this.radius ← 5.0⋯25Circle(double radius5.0) { //?packageconstructor26    this.radius→ 5.0 = radius5.0;27}⋯
4circle ← ⟨Circle A⟩⋯107double circleRadius = 5.0;  //@circleRadius=5.0, 2.5, 8.0108Shape circle→ ⟨Circle A⟩ = Shape.circle(circleRadius5.0);109Shape rect = Shape.rectangle(4, 6);110Shape square = Shape.square(3);⋯
5static Shape rectangle(double width, double height)⋯12static Shape rectangle(double width4.0, double height6.0) { //?rectanglefactory13    return new Rectangle(width, height);14}⋯
6this.width ← 4.0, this.height ← 6.0pass 1 of 2⋯43Rectangle(double width4.0, double height6.0) {44    this.width→ 4.0 = width4.0;45    this.height→ 6.0 = height6.0;46}⋯
7rect ← ⟨Rectangle B⟩⋯108Shape circle = Shape.circle(circleRadius);109Shape rect→ ⟨Rectangle B⟩ = Shape.rectangle(4, 6);110Shape square = Shape.square(3);⋯
8static Shape square(double side)⋯16static Shape square(double side3.0) { //?squarefactory17    return new Rectangle(side, side);  // Square is special rectangle18}⋯
9this.width ← 3.0, this.height ← 3.0pass 2 of 2⋯43Rectangle(double width3.0, double height3.0) {44    this.width→ 3.0 = width3.0;45    this.height→ 3.0 = height3.0;46}⋯
10square ← ⟨Rectangle C⟩⋯109Shape rect = Shape.rectangle(4, 6);110Shape square→ ⟨Rectangle C⟩ = Shape.square(3);111112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());⋯
11@Override
    public String getName()⋯29@Override30public String getName() {31    return "Circle(r=" + radius5.0 + ")";32}⋯
12@Override
    public double getArea()⋯34@Override35public double getArea() {36    return Math.PI * radius5.0 * radius;37}⋯
13System.out.println(circle.getName() + " area: " + String.format("%.2f"…⋯112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());⋯outputCircle(r=5.0) area: 78.54
14@Override
    public String getName()pass 1 of 2⋯48@Override49public String getName() {50    return "Rectangle(" + width4.0 + "x" + height6.0 + ")";51}⋯
15@Override
    public double getArea()pass 1 of 2⋯53@Override54public double getArea() {55    return width4.0 * height6.0;56}⋯
16System.out.println(rect.getName() + " area: " + rect.getArea());⋯112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());⋯outputRectangle(4.0x6.0) area: 24.0
17@Override
    public String getName()pass 2 of 2⋯48@Override49public String getName() {50    return "Rectangle(" + width3.0 + "x" + height3.0 + ")";51}⋯
18@Override
    public double getArea()pass 2 of 2⋯53@Override54public double getArea() {55    return width3.0 * height3.0;56}⋯
19System.out.println(square.getName() + " area: " + square.getArea());⋯113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());115116System.out.println("\n=== Benefits of Factory Methods ===");117System.out.println("""118    1. Hide implementations - users don't know about Circle/Rectangle119    2. Return appropriate subtype - square() returns Rectangle120    3. Caching possible - can reuse instances121    4. Validation - can check parameters122    """);123124System.out.println("=== Immutable List Factory ===\n");125126// Like Java's List.of() //?likelistof127ImmutableList<String> empty = ImmutableList.of();128ImmutableList<String> single = ImmutableList.of("hello");⋯outputRectangle(3.0x3.0) area: 9.0

=== Benefits of Factory Methods ===
1. Hide implementations - users don't know about Circle/Rectangle
2. Return appropriate subtype - square() returns Rectangle
3. Caching possible - can reuse instances
4. Validation - can check parameters
=== Immutable List Factory ===
20empty ← ⟨EmptyList D⟩⋯126// Like Java's List.of() //?likelistof127ImmutableList<String> empty→ ⟨EmptyList D⟩ = ImmutableList.of();128ImmutableList<String> single = ImmutableList.of("hello");129ImmutableList<String> multi = ImmutableList.of("a", "b", "c");⋯
21static <T> ImmutableList<T> of(T item)⋯69static <T> ImmutableList<T> of(T itemhello) { //?singlelist70    return new SingleList<>(item);71}⋯
22this.item ← hello⋯85private T item;86SingleList(T itemhello) { this.item→ hello = item; }87@Override public int size() { return 1; }⋯
23single ← ⟨SingleList E⟩⋯127ImmutableList<String> empty = ImmutableList.of();128ImmutableList<String> single→ ⟨SingleList E⟩ = ImmutableList.of("hello");129ImmutableList<String> multi = ImmutableList.of("a", "b", "c");⋯
24@SafeVarargs
    static <T> ImmutableList<T> of(T... items)⋯73@SafeVarargs74static <T> ImmutableList<T> of(T... items) { //?vararglist75    return new ArrayBackedList<>(items.clone());76}⋯
25ArrayBackedList(Object[] items)⋯95private Object[] items;96ArrayBackedList(Object[] items) { this.items = items; }97@Override public int size() { return items.length; }⋯
26multi ← ⟨ArrayBackedList F⟩⋯128ImmutableList<String> single = ImmutableList.of("hello");129ImmutableList<String> multi→ ⟨ArrayBackedList F⟩ = ImmutableList.of("a", "b", "c");130131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));⋯
27System.out.println("empty.size() = " + empty.size());⋯131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputempty.size() = 0
28@Override public T get(int index)⋯87@Override public int size() { return 1; }88@Override public T get(int index0) {89    if (index != 0) throw new IndexOutOfBoundsException();90    return itemhello;91}⋯
29System.out.println("single.get(0) = " + single.get(0));⋯131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputsingle.get(0) = hello
30@Override public int size()pass 1 of 5⋯96ArrayBackedList(Object[] items) { this.items = items; }97@Override public int size() { return items.length3; }98@Override @SuppressWarnings("unchecked")⋯
31System.out.println("multi.size() = " + multi.size());⋯132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputmulti.size() = 3
32for (int i = 0; i < multi.size(); i++)pass 1 of 3⋯135for (int i0 = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i0 + ") = " + multi.get(i));137}⋯All 3 passes — pass 1 is the card abovepassi
10
21
32
33@Override @SuppressWarnings("unchecked")
    public T get(int index)pass 1 of 3⋯97    @Override public int size() { return items.length; }98    @Override @SuppressWarnings("unchecked")99    public T get(int index0) { return (T) items[index]a; }100}⋯All 3 passes — pass 1 is the card abovepassindexitems[index]
10a
21b
32c
34System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i0 + ") = " + multi.get(i));137}⋯outputmulti.get(0) = a
35System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i1 + ") = " + multi.get(i));137}⋯outputmulti.get(1) = b
36System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i2 + ") = " + multi.get(i));137}⋯outputmulti.get(2) = c
37System.out.println(" === Real Java Examples ===");⋯139    System.out.println("\n=== Real Java Examples ===");140    System.out.println("""141        Java's static factory methods:142        - List.of("a", "b", "c")143        - Set.of(1, 2, 3)144        - Map.of("key", "value")145        - Optional.of(value)146        - Optional.empty()147        - Stream.of(items)148        - Comparator.comparing(keyExtractor)149        """);150}⋯output
=== Real Java Examples ===
Java's static factory methods:
- List.of("a", "b", "c")
- Set.of(1, 2, 3)
- Map.of("key", "value")
- Optional.of(value)
- Optional.empty()
- Stream.of(items)
- Comparator.comparing(keyExtractor)
1circleRadius ← 2.5⋯102public class FactoryMethods {103    public static void main(String[] args) {104        System.out.println("=== Shape Factory Methods ===\n");105        106        // Create shapes via interface107        double circleRadius→ 2.5 = 2.5;108        Shape circle = Shape.circle(circleRadius2.5);109        Shape rect = Shape.rectangle(4, 6);⋯output=== Shape Factory Methods ===
2static Shape circle(double radius)⋯7// Static factory methods8static Shape circle(double radius2.5) {9    return new Circle(radius);10}⋯
3this.radius ← 2.5⋯25Circle(double radius2.5) {26    this.radius→ 2.5 = radius2.5;27}⋯
4circle ← ⟨Circle A⟩⋯107double circleRadius = 2.5;108Shape circle→ ⟨Circle A⟩ = Shape.circle(circleRadius2.5);109Shape rect = Shape.rectangle(4, 6);110Shape square = Shape.square(3);⋯
5static Shape rectangle(double width, double height)⋯12static Shape rectangle(double width4.0, double height6.0) {13    return new Rectangle(width, height);14}⋯
6this.width ← 4.0, this.height ← 6.0pass 1 of 2⋯43Rectangle(double width4.0, double height6.0) {44    this.width→ 4.0 = width4.0;45    this.height→ 6.0 = height6.0;46}⋯
7rect ← ⟨Rectangle B⟩⋯108Shape circle = Shape.circle(circleRadius);109Shape rect→ ⟨Rectangle B⟩ = Shape.rectangle(4, 6);110Shape square = Shape.square(3);⋯
8static Shape square(double side)⋯16static Shape square(double side3.0) {17    return new Rectangle(side, side);  // Square is special rectangle18}⋯
9this.width ← 3.0, this.height ← 3.0pass 2 of 2⋯43Rectangle(double width3.0, double height3.0) {44    this.width→ 3.0 = width3.0;45    this.height→ 3.0 = height3.0;46}⋯
10square ← ⟨Rectangle C⟩⋯109Shape rect = Shape.rectangle(4, 6);110Shape square→ ⟨Rectangle C⟩ = Shape.square(3);111112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());⋯
11@Override
    public String getName()⋯29@Override30public String getName() {31    return "Circle(r=" + radius2.5 + ")";32}⋯
12@Override
    public double getArea()⋯34@Override35public double getArea() {36    return Math.PI * radius2.5 * radius;37}⋯
13System.out.println(circle.getName() + " area: " + String.format("%.2f"…⋯112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());⋯outputCircle(r=2.5) area: 19.63
14@Override
    public String getName()pass 1 of 2⋯48@Override49public String getName() {50    return "Rectangle(" + width4.0 + "x" + height6.0 + ")";51}⋯
15@Override
    public double getArea()pass 1 of 2⋯53@Override54public double getArea() {55    return width4.0 * height6.0;56}⋯
16System.out.println(rect.getName() + " area: " + rect.getArea());⋯112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());⋯outputRectangle(4.0x6.0) area: 24.0
17@Override
    public String getName()pass 2 of 2⋯48@Override49public String getName() {50    return "Rectangle(" + width3.0 + "x" + height3.0 + ")";51}⋯
18@Override
    public double getArea()pass 2 of 2⋯53@Override54public double getArea() {55    return width3.0 * height3.0;56}⋯
19System.out.println(square.getName() + " area: " + square.getArea());⋯113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());115116System.out.println("\n=== Benefits of Factory Methods ===");117System.out.println("""118    1. Hide implementations - users don't know about Circle/Rectangle119    2. Return appropriate subtype - square() returns Rectangle120    3. Caching possible - can reuse instances121    4. Validation - can check parameters122    """);123124System.out.println("=== Immutable List Factory ===\n");125126// Like Java's List.of()127ImmutableList<String> empty = ImmutableList.of();128ImmutableList<String> single = ImmutableList.of("hello");⋯outputRectangle(3.0x3.0) area: 9.0

=== Benefits of Factory Methods ===
1. Hide implementations - users don't know about Circle/Rectangle
2. Return appropriate subtype - square() returns Rectangle
3. Caching possible - can reuse instances
4. Validation - can check parameters
=== Immutable List Factory ===
20empty ← ⟨EmptyList D⟩⋯126// Like Java's List.of()127ImmutableList<String> empty→ ⟨EmptyList D⟩ = ImmutableList.of();128ImmutableList<String> single = ImmutableList.of("hello");129ImmutableList<String> multi = ImmutableList.of("a", "b", "c");⋯
21static <T> ImmutableList<T> of(T item)⋯69static <T> ImmutableList<T> of(T itemhello) {70    return new SingleList<>(item);71}⋯
22this.item ← hello⋯85private T item;86SingleList(T itemhello) { this.item→ hello = item; }87@Override public int size() { return 1; }⋯
23single ← ⟨SingleList E⟩⋯127ImmutableList<String> empty = ImmutableList.of();128ImmutableList<String> single→ ⟨SingleList E⟩ = ImmutableList.of("hello");129ImmutableList<String> multi = ImmutableList.of("a", "b", "c");⋯
24@SafeVarargs
    static <T> ImmutableList<T> of(T... items)⋯73@SafeVarargs74static <T> ImmutableList<T> of(T... items) {75    return new ArrayBackedList<>(items.clone());76}⋯
25ArrayBackedList(Object[] items)⋯95private Object[] items;96ArrayBackedList(Object[] items) { this.items = items; }97@Override public int size() { return items.length; }⋯
26multi ← ⟨ArrayBackedList F⟩⋯128ImmutableList<String> single = ImmutableList.of("hello");129ImmutableList<String> multi→ ⟨ArrayBackedList F⟩ = ImmutableList.of("a", "b", "c");130131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));⋯
27System.out.println("empty.size() = " + empty.size());⋯131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputempty.size() = 0
28@Override public T get(int index)⋯87@Override public int size() { return 1; }88@Override public T get(int index0) {89    if (index != 0) throw new IndexOutOfBoundsException();90    return itemhello;91}⋯
29System.out.println("single.get(0) = " + single.get(0));⋯131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputsingle.get(0) = hello
30@Override public int size()pass 1 of 5⋯96ArrayBackedList(Object[] items) { this.items = items; }97@Override public int size() { return items.length3; }98@Override @SuppressWarnings("unchecked")⋯
31System.out.println("multi.size() = " + multi.size());⋯132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputmulti.size() = 3
32for (int i = 0; i < multi.size(); i++)pass 1 of 3⋯135for (int i0 = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i0 + ") = " + multi.get(i));137}⋯All 3 passes — pass 1 is the card abovepassi
10
21
32
33@Override @SuppressWarnings("unchecked")
    public T get(int index)pass 1 of 3⋯97    @Override public int size() { return items.length; }98    @Override @SuppressWarnings("unchecked")99    public T get(int index0) { return (T) items[index]a; }100}⋯All 3 passes — pass 1 is the card abovepassindexitems[index]
10a
21b
32c
34System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i0 + ") = " + multi.get(i));137}⋯outputmulti.get(0) = a
35System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i1 + ") = " + multi.get(i));137}⋯outputmulti.get(1) = b
36System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i2 + ") = " + multi.get(i));137}⋯outputmulti.get(2) = c
37System.out.println(" === Real Java Examples ===");⋯139    System.out.println("\n=== Real Java Examples ===");140    System.out.println("""141        Java's static factory methods:142        - List.of("a", "b", "c")143        - Set.of(1, 2, 3)144        - Map.of("key", "value")145        - Optional.of(value)146        - Optional.empty()147        - Stream.of(items)148        - Comparator.comparing(keyExtractor)149        """);150}⋯output
=== Real Java Examples ===
Java's static factory methods:
- List.of("a", "b", "c")
- Set.of(1, 2, 3)
- Map.of("key", "value")
- Optional.of(value)
- Optional.empty()
- Stream.of(items)
- Comparator.comparing(keyExtractor)
1circleRadius ← 8.0⋯102public class FactoryMethods {103    public static void main(String[] args) {104        System.out.println("=== Shape Factory Methods ===\n");105        106        // Create shapes via interface107        double circleRadius→ 8.0 = 8.0;108        Shape circle = Shape.circle(circleRadius8.0);109        Shape rect = Shape.rectangle(4, 6);⋯output=== Shape Factory Methods ===
2static Shape circle(double radius)⋯7// Static factory methods8static Shape circle(double radius8.0) {9    return new Circle(radius);10}⋯
3this.radius ← 8.0⋯25Circle(double radius8.0) {26    this.radius→ 8.0 = radius8.0;27}⋯
4circle ← ⟨Circle A⟩⋯107double circleRadius = 8.0;108Shape circle→ ⟨Circle A⟩ = Shape.circle(circleRadius8.0);109Shape rect = Shape.rectangle(4, 6);110Shape square = Shape.square(3);⋯
5static Shape rectangle(double width, double height)⋯12static Shape rectangle(double width4.0, double height6.0) {13    return new Rectangle(width, height);14}⋯
6this.width ← 4.0, this.height ← 6.0pass 1 of 2⋯43Rectangle(double width4.0, double height6.0) {44    this.width→ 4.0 = width4.0;45    this.height→ 6.0 = height6.0;46}⋯
7rect ← ⟨Rectangle B⟩⋯108Shape circle = Shape.circle(circleRadius);109Shape rect→ ⟨Rectangle B⟩ = Shape.rectangle(4, 6);110Shape square = Shape.square(3);⋯
8static Shape square(double side)⋯16static Shape square(double side3.0) {17    return new Rectangle(side, side);  // Square is special rectangle18}⋯
9this.width ← 3.0, this.height ← 3.0pass 2 of 2⋯43Rectangle(double width3.0, double height3.0) {44    this.width→ 3.0 = width3.0;45    this.height→ 3.0 = height3.0;46}⋯
10square ← ⟨Rectangle C⟩⋯109Shape rect = Shape.rectangle(4, 6);110Shape square→ ⟨Rectangle C⟩ = Shape.square(3);111112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());⋯
11@Override
    public String getName()⋯29@Override30public String getName() {31    return "Circle(r=" + radius8.0 + ")";32}⋯
12@Override
    public double getArea()⋯34@Override35public double getArea() {36    return Math.PI * radius8.0 * radius;37}⋯
13System.out.println(circle.getName() + " area: " + String.format("%.2f"…⋯112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());⋯outputCircle(r=8.0) area: 201.06
14@Override
    public String getName()pass 1 of 2⋯48@Override49public String getName() {50    return "Rectangle(" + width4.0 + "x" + height6.0 + ")";51}⋯
15@Override
    public double getArea()pass 1 of 2⋯53@Override54public double getArea() {55    return width4.0 * height6.0;56}⋯
16System.out.println(rect.getName() + " area: " + rect.getArea());⋯112System.out.println(circle.getName() + " area: " + String.format("%.2f", circle.getArea()));113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());⋯outputRectangle(4.0x6.0) area: 24.0
17@Override
    public String getName()pass 2 of 2⋯48@Override49public String getName() {50    return "Rectangle(" + width3.0 + "x" + height3.0 + ")";51}⋯
18@Override
    public double getArea()pass 2 of 2⋯53@Override54public double getArea() {55    return width3.0 * height3.0;56}⋯
19System.out.println(square.getName() + " area: " + square.getArea());⋯113System.out.println(rect.getName() + " area: " + rect.getArea());114System.out.println(square.getName() + " area: " + square.getArea());115116System.out.println("\n=== Benefits of Factory Methods ===");117System.out.println("""118    1. Hide implementations - users don't know about Circle/Rectangle119    2. Return appropriate subtype - square() returns Rectangle120    3. Caching possible - can reuse instances121    4. Validation - can check parameters122    """);123124System.out.println("=== Immutable List Factory ===\n");125126// Like Java's List.of()127ImmutableList<String> empty = ImmutableList.of();128ImmutableList<String> single = ImmutableList.of("hello");⋯outputRectangle(3.0x3.0) area: 9.0

=== Benefits of Factory Methods ===
1. Hide implementations - users don't know about Circle/Rectangle
2. Return appropriate subtype - square() returns Rectangle
3. Caching possible - can reuse instances
4. Validation - can check parameters
=== Immutable List Factory ===
20empty ← ⟨EmptyList D⟩⋯126// Like Java's List.of()127ImmutableList<String> empty→ ⟨EmptyList D⟩ = ImmutableList.of();128ImmutableList<String> single = ImmutableList.of("hello");129ImmutableList<String> multi = ImmutableList.of("a", "b", "c");⋯
21static <T> ImmutableList<T> of(T item)⋯69static <T> ImmutableList<T> of(T itemhello) {70    return new SingleList<>(item);71}⋯
22this.item ← hello⋯85private T item;86SingleList(T itemhello) { this.item→ hello = item; }87@Override public int size() { return 1; }⋯
23single ← ⟨SingleList E⟩⋯127ImmutableList<String> empty = ImmutableList.of();128ImmutableList<String> single→ ⟨SingleList E⟩ = ImmutableList.of("hello");129ImmutableList<String> multi = ImmutableList.of("a", "b", "c");⋯
24@SafeVarargs
    static <T> ImmutableList<T> of(T... items)⋯73@SafeVarargs74static <T> ImmutableList<T> of(T... items) {75    return new ArrayBackedList<>(items.clone());76}⋯
25ArrayBackedList(Object[] items)⋯95private Object[] items;96ArrayBackedList(Object[] items) { this.items = items; }97@Override public int size() { return items.length; }⋯
26multi ← ⟨ArrayBackedList F⟩⋯128ImmutableList<String> single = ImmutableList.of("hello");129ImmutableList<String> multi→ ⟨ArrayBackedList F⟩ = ImmutableList.of("a", "b", "c");130131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));⋯
27System.out.println("empty.size() = " + empty.size());⋯131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputempty.size() = 0
28@Override public T get(int index)⋯87@Override public int size() { return 1; }88@Override public T get(int index0) {89    if (index != 0) throw new IndexOutOfBoundsException();90    return itemhello;91}⋯
29System.out.println("single.get(0) = " + single.get(0));⋯131System.out.println("empty.size() = " + empty.size());132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputsingle.get(0) = hello
30@Override public int size()pass 1 of 5⋯96ArrayBackedList(Object[] items) { this.items = items; }97@Override public int size() { return items.length3; }98@Override @SuppressWarnings("unchecked")⋯
31System.out.println("multi.size() = " + multi.size());⋯132System.out.println("single.get(0) = " + single.get(0));133System.out.println("multi.size() = " + multi.size());⋯outputmulti.size() = 3
32for (int i = 0; i < multi.size(); i++)pass 1 of 3⋯135for (int i0 = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i0 + ") = " + multi.get(i));137}⋯All 3 passes — pass 1 is the card abovepassi
10
21
32
33@Override @SuppressWarnings("unchecked")
    public T get(int index)pass 1 of 3⋯97    @Override public int size() { return items.length; }98    @Override @SuppressWarnings("unchecked")99    public T get(int index0) { return (T) items[index]a; }100}⋯All 3 passes — pass 1 is the card abovepassindexitems[index]
10a
21b
32c
34System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i0 + ") = " + multi.get(i));137}⋯outputmulti.get(0) = a
35System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i1 + ") = " + multi.get(i));137}⋯outputmulti.get(1) = b
36System.out.println("multi.get(" + i + ") = " + multi.get(i));⋯135for (int i = 0; i < multi.size(); i++) {136    System.out.println("multi.get(" + i2 + ") = " + multi.get(i));137}⋯outputmulti.get(2) = c
37System.out.println(" === Real Java Examples ===");⋯139    System.out.println("\n=== Real Java Examples ===");140    System.out.println("""141        Java's static factory methods:142        - List.of("a", "b", "c")143        - Set.of(1, 2, 3)144        - Map.of("key", "value")145        - Optional.of(value)146        - Optional.empty()147        - Stream.of(items)148        - Comparator.comparing(keyExtractor)149        """);150}⋯output
=== Real Java Examples ===
Java's static factory methods:
- List.of("a", "b", "c")
- Set.of(1, 2, 3)
- Map.of("key", "value")
- Optional.of(value)
- Optional.empty()
- Stream.of(items)
- Comparator.comparing(keyExtractor)

All 3 passes — pass 1 is the card above
pass	`i`
1	0
2	1
3	2

All 3 passes — pass 1 is the card above
pass	`index`	`items[index]`
1	0	a
2	1	b
3	2	c

All 3 passes — pass 1 is the card above
pass	`i`
1	0
2	1
3	2

All 3 passes — pass 1 is the card above
pass	`index`	`items[index]`
1	0	a
2	1	b
3	2	c

All 3 passes — pass 1 is the card above
pass	`i`
1	0
2	1
3	2

All 3 passes — pass 1 is the card above
pass	`index`	`items[index]`
1	0	a
2	1	b
3	2	c

List.of(), Map.of() are static factory methods on interfaces.

Utility methods

Helper functions related to the interface's purpose.

                            UtilityMethods.java
                        
Replay: real traced execution (multi-file project)

                            // Utility Methods in Interfaces

interface StringUtils {
    // Null-safe operations
    static boolean isEmpty(String s) {
        return s == null || s.isEmpty();
    }

    static boolean isBlank(String s) {
        return s == null || s.isBlank();
    }

    static String nullToEmpty(String s) {
        return s == null ? "" : s;
    }

    // String transformations
    static String reverse(String s) {
        if (isEmpty(s)) return s;
        return new StringBuilder(s).reverse().toString();
    }

    static String capitalize(String s) {
        if (isEmpty(s)) return s;
        return Character.toUpperCase(s.charAt(0)) + s.substring(1).toLowerCase();
    }

    static String repeat(String s, int times) {
        if (isEmpty(s) || times <= 0) return "";
        return s.repeat(times);
    }

    // Validation
    static boolean isAlpha(String s) {
        if (isEmpty(s)) return false;
        return s.chars().allMatch(Character::isLetter);
    }

    static boolean isNumeric(String s) {
        if (isEmpty(s)) return false;
        return s.chars().allMatch(Character::isDigit);
    }
}

interface ArrayUtils {
    // Array checks
    static boolean isEmpty(int[] arr) {
        return arr == null || arr.length == 0;
    }

    static boolean contains(int[] arr, int value) {
        if (isEmpty(arr)) return false;
        for (int item : arr) {
            if (item == value) return true;
        }
        return false;
    }

    // Statistics
    static int sum(int[] arr) {
        if (isEmpty(arr)) return 0;
        int total = 0;
        for (int item : arr) total += item;
        return total;
    }

    static double average(int[] arr) {
        if (isEmpty(arr)) return 0.0;
        return (double) sum(arr) / arr.length;
    }

    static int max(int[] arr) {
        if (isEmpty(arr)) throw new IllegalArgumentException("Array is empty");
        int result = arr[0];
        for (int item : arr) {
            if (item > result) result = item;
        }
        return result;
    }

    static int min(int[] arr) {
        if (isEmpty(arr)) throw new IllegalArgumentException("Array is empty");
        int result = arr[0];
        for (int item : arr) {
            if (item < result) result = item;
        }
        return result;
    }

    // Transformations
    static int[] reverse(int[] arr) {
        if (isEmpty(arr)) return arr;
        int[] result = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            result[i] = arr[arr.length - 1 - i];
        }
        return result;
    }
}

public class UtilityMethods {
    public static void main(String[] args) {
        System.out.println("=== StringUtils ===\n");

        // Null safety
        System.out.println("isEmpty(null): " + StringUtils.isEmpty(null));
        System.out.println("isEmpty(\"\"): " + StringUtils.isEmpty(""));
        System.out.println("isEmpty(\"hello\"): " + StringUtils.isEmpty("hello"));
        System.out.println("isBlank(\"   \"): " + StringUtils.isBlank("   "));

        // Transformations
        System.out.println("\nreverse(\"hello\"): " + StringUtils.reverse("hello"));
        System.out.println("capitalize(\"jOHN\"): " + StringUtils.capitalize("jOHN"));
        System.out.println("repeat(\"ab\", 3): " + StringUtils.repeat("ab", 3));

        // Validation
        System.out.println("\nisAlpha(\"Hello\"): " + StringUtils.isAlpha("Hello"));
        System.out.println("isAlpha(\"Hello123\"): " + StringUtils.isAlpha("Hello123"));
        System.out.println("isNumeric(\"12345\"): " + StringUtils.isNumeric("12345"));

        System.out.println("\n=== ArrayUtils ===\n");

        int[] numbers = {5, 2, 8, 1, 9, 3};
        int[] empty = {};

        // Checks
        System.out.println("isEmpty(numbers): " + ArrayUtils.isEmpty(numbers));
        System.out.println("isEmpty(empty): " + ArrayUtils.isEmpty(empty));
        System.out.println("contains(numbers, 8): " + ArrayUtils.contains(numbers, 8));
        System.out.println("contains(numbers, 7): " + ArrayUtils.contains(numbers, 7));

        // Statistics
        System.out.println("\nsum(numbers): " + ArrayUtils.sum(numbers));
        System.out.println("average(numbers): " + ArrayUtils.average(numbers));
        System.out.println("max(numbers): " + ArrayUtils.max(numbers));
        System.out.println("min(numbers): " + ArrayUtils.min(numbers));

        // Transformations
        System.out.print("\nreverse(numbers): ");
        for (int n : ArrayUtils.reverse(numbers)) {
            System.out.print(n + " ");
        }
        System.out.println();

        System.out.println("\n=== Why Interfaces for Utilities? ===");
        System.out.println("""
            Before Java 8:
            - Utility classes with private constructor
            - All static methods
            - Couldn't use interface

            With Java 8:
            - Interfaces can have static methods
            - Cleaner organization
            - Can combine with default/abstract methods

            Benefits:
            - No need for private constructor hack
            - Cleaner than abstract class
            - Groups related utilities
            """);
    }
}

                        
1public static void main(String[] args)⋯101public class UtilityMethods {102    public static void main(String[] args) {103        System.out.println("=== StringUtils ===\n");104        105        // Null safety //?testnullsafe106        System.out.println("isEmpty(null): " + StringUtils.isEmpty(null));107        System.out.println("isEmpty(\"\"): " + StringUtils.isEmpty(""));⋯output=== StringUtils ===
2static boolean isEmpty(String s)pass 1 of 9⋯4// Null-safe operations //?nullsafe5static boolean isEmpty(String snull) {6    return snull == null || s.isEmpty();7}⋯All 9 passes — pass 1 is the card abovepasss
1null
2(empty)
3hello
4hello
5jOHN
6ab
7Hello
8Hello123
912345
3System.out.println("isEmpty(null): " + StringUtils.isEmpty(null));⋯105// Null safety //?testnullsafe106System.out.println("isEmpty(null): " + StringUtils.isEmpty(null));107System.out.println("isEmpty(\"\"): " + StringUtils.isEmpty(""));108System.out.println("isEmpty(\"hello\"): " + StringUtils.isEmpty("hello"));⋯outputisEmpty(null): true
4System.out.println("isEmpty(\"\"): " + StringUtils.isEmpty(""));⋯106System.out.println("isEmpty(null): " + StringUtils.isEmpty(null));107System.out.println("isEmpty(\"\"): " + StringUtils.isEmpty(""));108System.out.println("isEmpty(\"hello\"): " + StringUtils.isEmpty("hello"));109System.out.println("isBlank(\"   \"): " + StringUtils.isBlank("   "));⋯outputisEmpty(""): true
5System.out.println("isEmpty(\"hello\"): " + StringUtils.isEmpty("hello…⋯107System.out.println("isEmpty(\"\"): " + StringUtils.isEmpty(""));108System.out.println("isEmpty(\"hello\"): " + StringUtils.isEmpty("hello"));109System.out.println("isBlank(\"   \"): " + StringUtils.isBlank("   "));⋯outputisEmpty("hello"): false
6static boolean isBlank(String s)⋯9static boolean isBlank(String s   ) {10    return s    == null || s.isBlank();11}⋯
7System.out.println("isBlank(\"   \"): " + StringUtils.isBlank("   "));⋯108System.out.println("isEmpty(\"hello\"): " + StringUtils.isEmpty("hello"));109System.out.println("isBlank(\"   \"): " + StringUtils.isBlank("   "));110111// Transformations //?testtransform112System.out.println("\nreverse(\"hello\"): " + StringUtils.reverse("hello"));113System.out.println("capitalize(\"jOHN\"): " + StringUtils.capitalize("jOHN"));⋯outputisBlank("   "): true
8static String reverse(String s)⋯17// String transformations //?transformations18static String reverse(String shello) {19    if (isEmpty(s)) return s;⋯
9return new StringBuilder(s).reverse().toString();⋯19    if (isEmpty(s)) return s;20    return new StringBuilder(s).reverse().toString();21}⋯
10System.out.println(" reverse(\"hello\"): " + StringUtils.reverse("hell…⋯111// Transformations //?testtransform112System.out.println("\nreverse(\"hello\"): " + StringUtils.reverse("hello"));113System.out.println("capitalize(\"jOHN\"): " + StringUtils.capitalize("jOHN"));114System.out.println("repeat(\"ab\", 3): " + StringUtils.repeat("ab", 3));⋯output
reverse("hello"): olleh
11static String capitalize(String s)⋯23static String capitalize(String sjOHN) {24    if (isEmpty(s)) return s;⋯
12return Character.toUpperCase(s.charAt(0)) + s.substring(1).toLowerCase…⋯24    if (isEmpty(s)) return s;25    return Character.toUpperCase(s.charAt(0)) + s.substring(1).toLowerCase();26}⋯
13System.out.println("capitalize(\"jOHN\"): " + StringUtils.capitalize("…⋯112System.out.println("\nreverse(\"hello\"): " + StringUtils.reverse("hello"));113System.out.println("capitalize(\"jOHN\"): " + StringUtils.capitalize("jOHN"));114System.out.println("repeat(\"ab\", 3): " + StringUtils.repeat("ab", 3));⋯outputcapitalize("jOHN"): John
14static String repeat(String s, int times)⋯28static String repeat(String sab, int times3) { //?repeat29    if (isEmpty(s) || times <= 0) return "";⋯
15return s.repeat(times);⋯29    if (isEmpty(s) || times <= 0) return "";30    return s.repeat(times3);31}⋯
16System.out.println("repeat(\"ab\", 3): " + StringUtils.repeat("ab", 3)…⋯113System.out.println("capitalize(\"jOHN\"): " + StringUtils.capitalize("jOHN"));114System.out.println("repeat(\"ab\", 3): " + StringUtils.repeat("ab", 3));115116// Validation //?testvalidation117System.out.println("\nisAlpha(\"Hello\"): " + StringUtils.isAlpha("Hello"));118System.out.println("isAlpha(\"Hello123\"): " + StringUtils.isAlpha("Hello123"));⋯outputrepeat("ab", 3): ababab
17static boolean isAlpha(String s)pass 1 of 2⋯33// Validation //?validation34static boolean isAlpha(String sHello) {35    if (isEmpty(s)) return false;⋯
18return s.chars().allMatch(Character::isLetter);⋯35    if (isEmpty(s)) return false;36    return s.chars().allMatch(Character::isLetter);37}⋯
19System.out.println(" isAlpha(\"Hello\"): " + StringUtils.isAlpha("Hell…⋯116// Validation //?testvalidation117System.out.println("\nisAlpha(\"Hello\"): " + StringUtils.isAlpha("Hello"));118System.out.println("isAlpha(\"Hello123\"): " + StringUtils.isAlpha("Hello123"));119System.out.println("isNumeric(\"12345\"): " + StringUtils.isNumeric("12345"));⋯output
isAlpha("Hello"): true
20static boolean isAlpha(String s)pass 2 of 2⋯33// Validation //?validation34static boolean isAlpha(String sHello123) {35    if (isEmpty(s)) return false;⋯
21return s.chars().allMatch(Character::isLetter);⋯35    if (isEmpty(s)) return false;36    return s.chars().allMatch(Character::isLetter);37}⋯
22System.out.println("isAlpha(\"Hello123\"): " + StringUtils.isAlpha("He…⋯117System.out.println("\nisAlpha(\"Hello\"): " + StringUtils.isAlpha("Hello"));118System.out.println("isAlpha(\"Hello123\"): " + StringUtils.isAlpha("Hello123"));119System.out.println("isNumeric(\"12345\"): " + StringUtils.isNumeric("12345"));⋯outputisAlpha("Hello123"): false
23static boolean isNumeric(String s)⋯39static boolean isNumeric(String s12345) {40    if (isEmpty(s)) return false;⋯
24return s.chars().allMatch(Character::isDigit);⋯40    if (isEmpty(s)) return false;41    return s.chars().allMatch(Character::isDigit);42}⋯
25int[] numbers = {5, 2, 8, 1, 9, 3};⋯118System.out.println("isAlpha(\"Hello123\"): " + StringUtils.isAlpha("Hello123"));119System.out.println("isNumeric(\"12345\"): " + StringUtils.isNumeric("12345"));120121System.out.println("\n=== ArrayUtils ===\n");122123int[] numbers = {5, 2, 8, 1, 9, 3};124int[] empty = {};125126// Checks //?testchecks127System.out.println("isEmpty(numbers): " + ArrayUtils.isEmpty(numbers));128System.out.println("isEmpty(empty): " + ArrayUtils.isEmpty(empty));⋯outputisNumeric("12345"): true

=== ArrayUtils ===
26static boolean isEmpty(int[] arr)pass 1 of 10⋯46// Array checks //?arraychecks47static boolean isEmpty(int[] arr) {48    return arr == null || arr.length6 == 0;49}⋯All 10 passes — pass 1 is the card abovepassarr.lengthitemvalue
16——
20——
3688
46——
56——
66——
76——
86——
96——
106——
27System.out.println("isEmpty(numbers): " + ArrayUtils.isEmpty(numbers))…⋯126// Checks //?testchecks127System.out.println("isEmpty(numbers): " + ArrayUtils.isEmpty(numbers));128System.out.println("isEmpty(empty): " + ArrayUtils.isEmpty(empty));129System.out.println("contains(numbers, 8): " + ArrayUtils.contains(numbers, 8));⋯outputisEmpty(numbers): false
28System.out.println("isEmpty(empty): " + ArrayUtils.isEmpty(empty));⋯127System.out.println("isEmpty(numbers): " + ArrayUtils.isEmpty(numbers));128System.out.println("isEmpty(empty): " + ArrayUtils.isEmpty(empty));129System.out.println("contains(numbers, 8): " + ArrayUtils.contains(numbers, 8));130System.out.println("contains(numbers, 7): " + ArrayUtils.contains(numbers, 7));⋯outputisEmpty(empty): true
29static boolean contains(int[] arr, int value)pass 1 of 2⋯51static boolean contains(int[] arr, int value8) { //?contains52    if (isEmpty(arr)) return false;⋯
30for (int item : arr)pass 1 of 9⋯52if (isEmpty(arr)) return false;53for (int item5 : arr) {54    if (item == value) return true;⋯All 9 passes — pass 1 is the card abovepassitemvalue
15—
22—
388
45—
52—
68—
71—
89—
93—
31if (item == value)⋯53for (int item : arr) {54    if (item8 == value8) return true;55}⋯
32System.out.println("contains(numbers, 8): " + ArrayUtils.contains(numb…⋯128System.out.println("isEmpty(empty): " + ArrayUtils.isEmpty(empty));129System.out.println("contains(numbers, 8): " + ArrayUtils.contains(numbers, 8));130System.out.println("contains(numbers, 7): " + ArrayUtils.contains(numbers, 7));⋯outputcontains(numbers, 8): true
33static boolean contains(int[] arr, int value)pass 2 of 2⋯51static boolean contains(int[] arr, int value7) { //?contains52    if (isEmpty(arr)) return false;⋯
34return false;⋯55    }56    return false;57}⋯
35System.out.println("contains(numbers, 7): " + ArrayUtils.contains(numb…⋯129System.out.println("contains(numbers, 8): " + ArrayUtils.contains(numbers, 8));130System.out.println("contains(numbers, 7): " + ArrayUtils.contains(numbers, 7));131132// Statistics //?teststatistics133System.out.println("\nsum(numbers): " + ArrayUtils.sum(numbers));134System.out.println("average(numbers): " + ArrayUtils.average(numbers));⋯outputcontains(numbers, 7): false
36static int sum(int[] arr)pass 1 of 2⋯59// Statistics //?statistics60static int sum(int[] arr) {61    if (isEmpty(arr)) return 0;⋯
37total ← 0⋯61if (isEmpty(arr)) return 0;62int total→ 0 = 0;63for (int item : arr) total += item;⋯
38total ← 5pass 1 of 12⋯62int total = 0;63for (int item5 : arr) total→ 5 += item;64return total;⋯All 12 passes — pass 1 is the card abovepassitemtotal
150 → 5
225 → 7
387 → 15
4115 → 16
5916 → 25
6325 → 28
750 → 5
825 → 7
987 → 15
10115 → 16
11916 → 25
12325 → 28
39return total;⋯63    for (int item : arr) total += item;64    return total28;65}⋯
40System.out.println(" sum(numbers): " + ArrayUtils.sum(numbers));⋯132// Statistics //?teststatistics133System.out.println("\nsum(numbers): " + ArrayUtils.sum(numbers));134System.out.println("average(numbers): " + ArrayUtils.average(numbers));135System.out.println("max(numbers): " + ArrayUtils.max(numbers));⋯output
sum(numbers): 28
41static double average(int[] arr)⋯67static double average(int[] arr) { //?average68    if (isEmpty(arr)) return 0.0;⋯
42return (double) sum(arr) / arr.length;⋯68    if (isEmpty(arr)) return 0.0;69    return (double) sum(arr) / arr.length6;70}⋯
43static int sum(int[] arr)pass 2 of 2⋯59// Statistics //?statistics60static int sum(int[] arr) {61    if (isEmpty(arr)) return 0;⋯
44total ← 0⋯61if (isEmpty(arr)) return 0;62int total→ 0 = 0;63for (int item : arr) total += item;⋯
45return total;⋯63    for (int item : arr) total += item;64    return total28;65}⋯
46System.out.println("average(numbers): " + ArrayUtils.average(numbers))…⋯133System.out.println("\nsum(numbers): " + ArrayUtils.sum(numbers));134System.out.println("average(numbers): " + ArrayUtils.average(numbers));135System.out.println("max(numbers): " + ArrayUtils.max(numbers));136System.out.println("min(numbers): " + ArrayUtils.min(numbers));⋯outputaverage(numbers): 4.666666666666667
47static int max(int[] arr)⋯72static int max(int[] arr) { //?max73    if (isEmpty(arr)) throw new IllegalArgumentException("Array is empty");⋯
48result ← 5⋯73if (isEmpty(arr)) throw new IllegalArgumentException("Array is empty");74int result→ 5 = arr[0]5;75for (int item : arr) {⋯
49for (int item : arr)pass 1 of 6⋯74int result = arr[0];75for (int item5 : arr) {76    if (item > result) result = item;⋯All 6 passes — pass 1 is the card abovepassitemresult
15—
22—
385 → 8
41—
598 → 9
63—
50result ← 8pass 1 of 2⋯75for (int item : arr) {76    if (item8 > result5) result = item;77}⋯
51result ← 9pass 2 of 2⋯75for (int item : arr) {76    if (item9 > result8) result = item;77}⋯
52return result;⋯77    }78    return result9;79}⋯
53System.out.println("max(numbers): " + ArrayUtils.max(numbers));⋯134System.out.println("average(numbers): " + ArrayUtils.average(numbers));135System.out.println("max(numbers): " + ArrayUtils.max(numbers));136System.out.println("min(numbers): " + ArrayUtils.min(numbers));⋯outputmax(numbers): 9
54static int min(int[] arr)⋯81static int min(int[] arr) {82    if (isEmpty(arr)) throw new IllegalArgumentException("Array is empty");⋯
55result ← 5⋯82if (isEmpty(arr)) throw new IllegalArgumentException("Array is empty");83int result→ 5 = arr[0]5;84for (int item : arr) {⋯
56for (int item : arr)pass 1 of 6⋯83int result = arr[0];84for (int item5 : arr) {85    if (item < result) result = item;⋯All 6 passes — pass 1 is the card abovepassitemresult
15—
225 → 2
38—
412 → 1
59—
63—
57result ← 2pass 1 of 2⋯84for (int item : arr) {85    if (item2 < result5) result = item;86}⋯
58result ← 1pass 2 of 2⋯84for (int item : arr) {85    if (item1 < result2) result = item;86}⋯
59return result;⋯86    }87    return result1;88}⋯
60System.out.println("min(numbers): " + ArrayUtils.min(numbers));⋯135System.out.println("max(numbers): " + ArrayUtils.max(numbers));136System.out.println("min(numbers): " + ArrayUtils.min(numbers));137138// Transformations //?testreverse139System.out.print("\nreverse(numbers): ");140for (int n : ArrayUtils.reverse(numbers)) {⋯outputmin(numbers): 1

reverse(numbers): 
61static int[] reverse(int[] arr)⋯90// Transformations //?arraytransform91static int[] reverse(int[] arr) {92    if (isEmpty(arr)) return arr;⋯
62int[] result = new int[arr.length];⋯92if (isEmpty(arr)) return arr;93int[] result = new int[arr.length6];94for (int i = 0; i < arr.length; i++) {⋯
63result[i] ← 3pass 1 of 6⋯93int[] result = new int[arr.length];94for (int i0 = 0; i < arr.length6; i++) {95    result[i]→ 3 = arr[arr.length - 1 - i]3;96}⋯All 6 passes — pass 1 is the card abovepassiarr[arr.length - 1 - i]result[i]
1030 → 3
2190 → 9
3210 → 1
4380 → 8
5420 → 2
6550 → 5
64return result;⋯96    }97    return result;98}⋯
65for (int n : ArrayUtils.reverse(numbers))pass 1 of 6⋯139System.out.print("\nreverse(numbers): ");140for (int n3 : ArrayUtils.reverse(numbers)) {141    System.out.print(n3 + " ");142}⋯output3 All 6 passes — pass 1 is the card abovepassn
13
29
31
48
52
65
66System.out.println();⋯142    }143    System.out.println();144    145    System.out.println("\n=== Why Interfaces for Utilities? ===");146    System.out.println("""147        Before Java 8:148        - Utility classes with private constructor149        - All static methods150        - Couldn't use interface151        152        With Java 8:153        - Interfaces can have static methods154        - Cleaner organization155        - Can combine with default/abstract methods156        157        Benefits:158        - No need for private constructor hack159        - Cleaner than abstract class160        - Groups related utilities161        """);162}⋯output
=== Why Interfaces for Utilities? ===
Before Java 8:
- Utility classes with private constructor
- All static methods
- Couldn't use interface

With Java 8:
- Interfaces can have static methods
- Cleaner organization
- Can combine with default/abstract methods

Benefits:
- No need for private constructor hack
- Cleaner than abstract class
- Groups related utilities

All 9 passes — pass 1 is the card above
pass	`s`
1	null
2	(empty)
3	hello
4	hello
5	jOHN
6	ab
7	Hello
8	Hello123
9	12345

All 10 passes — pass 1 is the card above
pass	`arr.length`	`item`	`value`
1	6	—	—
2	0	—	—
3	6	8	8
4	6	—	—
5	6	—	—
6	6	—	—
7	6	—	—
8	6	—	—
9	6	—	—
10	6	—	—

All 9 passes — pass 1 is the card above
pass	`item`	`value`
1	5	—
2	2	—
3	8	8
4	5	—
5	2	—
6	8	—
7	1	—
8	9	—
9	3	—

All 12 passes — pass 1 is the card above
pass	`item`	`total`
1	5	0 → 5
2	2	5 → 7
3	8	7 → 15
4	1	15 → 16
5	9	16 → 25
6	3	25 → 28
7	5	0 → 5
8	2	5 → 7
9	8	7 → 15
10	1	15 → 16
11	9	16 → 25
12	3	25 → 28

All 6 passes — pass 1 is the card above
pass	`item`	`result`
1	5	—
2	2	—
3	8	5 → 8
4	1	—
5	9	8 → 9
6	3	—

All 6 passes — pass 1 is the card above
pass	`item`	`result`
1	5	—
2	2	5 → 2
3	8	—
4	1	2 → 1
5	9	—
6	3	—

All 6 passes — pass 1 is the card above
pass	`i`	`arr[arr.length - 1 - i]`	`result[i]`
1	0	3	0 → 3
2	1	9	0 → 9
3	2	1	0 → 1
4	3	8	0 → 8
5	4	2	0 → 2
6	5	5	0 → 5

All 6 passes — pass 1 is the card above
pass	`n`
1	3
2	9
3	1
4	8
5	2
6	5

Group related utilities on the interface instead of separate helper class.

Static with default

Combine static and default methods in one interface.

                            minChars
                            minChars = 5 (default)minChars = 3minChars = 8
                        

                            StaticWithDefault.java
                        
Replay: real traced execution (multi-file project)

                            // Combining Static and Default Methods

interface Validator<T> {
    // Abstract - each validator implements differently
    boolean isValid(T value);
    String getErrorMessage();

    // Default - common behavior
    default ValidationResult validate(T value) {
        if (isValid(value)) {
            return ValidationResult.success();
        }
        return ValidationResult.failure(getErrorMessage());
    }

    // Static factory methods for common validators
    static Validator<String> notEmpty() {
        return new Validator<>() {
            @Override
            public boolean isValid(String value) {
                return value != null && !value.isEmpty();
            }

            @Override
            public String getErrorMessage() {
                return "Value cannot be empty";
            }
        };
    }

    static Validator<String> minLength(int min) {
        return new Validator<>() {
            @Override
            public boolean isValid(String value) {
                return value != null && value.length() >= min;
            }

            @Override
            public String getErrorMessage() {
                return "Value must be at least " + min + " characters";
            }
        };
    }

    static Validator<Integer> range(int min, int max) {
        return new Validator<>() {
            @Override
            public boolean isValid(Integer value) {
                return value != null && value >= min && value <= max;
            }

            @Override
            public String getErrorMessage() {
                return "Value must be between " + min + " and " + max;
            }
        };
    }

    // Static combinator methods
    static <T> Validator<T> and(Validator<T> v1, Validator<T> v2) {
        return new Validator<>() {
            @Override
            public boolean isValid(T value) {
                return v1.isValid(value) && v2.isValid(value);
            }

            @Override
            public String getErrorMessage() {
                return v1.getErrorMessage() + " AND " + v2.getErrorMessage();
            }
        };
    }
}

// Simple result class
class ValidationResult {
    private final boolean valid;
    private final String message;

    private ValidationResult(boolean valid, String message) {
        this.valid = valid;
        this.message = message;
    }

    static ValidationResult success() {
        return new ValidationResult(true, "OK");
    }

    static ValidationResult failure(String message) {
        return new ValidationResult(false, message);
    }

    @Override
    public String toString() {
        return valid ? "✓ Valid" : "✗ Invalid: " + message;
    }
}

// Custom validator
class EmailValidator implements Validator<String> {
    @Override
    public boolean isValid(String value) {
        return value != null && value.contains("@") && value.contains(".");
    }

    @Override
    public String getErrorMessage() {
        return "Invalid email format";
    }
}

public class StaticWithDefault {
    public static void main(String[] args) {
        System.out.println("=== Validator Interface ===\n");

        // Using static factory methods
        Validator<String> notEmpty = Validator.notEmpty();
        int minChars = 5;
        Validator<String> minLengthRule = Validator.minLength(minChars);
        Validator<Integer> ageRange = Validator.range(18, 65);

        System.out.println("--- notEmpty validator ---");
        System.out.println("\"hello\": " + notEmpty.validate("hello"));
        System.out.println("\"\": " + notEmpty.validate(""));
        System.out.println("null: " + notEmpty.validate(null));

        System.out.println("\n--- minLength(" + minChars + ") validator ---");
        System.out.println("\"hello\": " + minLengthRule.validate("hello"));
        System.out.println("\"hi\": " + minLengthRule.validate("hi"));

        System.out.println("\n--- range(18, 65) validator ---");
        System.out.println("25: " + ageRange.validate(25));
        System.out.println("15: " + ageRange.validate(15));
        System.out.println("70: " + ageRange.validate(70));

        // Using combinator
        System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");
        Validator<String> combined = Validator.and(notEmpty, minLengthRule);
        System.out.println("\"hello world\": " + combined.validate("hello world"));
        System.out.println("\"hi\": " + combined.validate("hi"));
        System.out.println("\"\": " + combined.validate(""));

        // Custom validator uses default method
        System.out.println("\n--- Custom EmailValidator ---");
        Validator<String> email = new EmailValidator();
        System.out.println("\"test@example.com\": " + email.validate("test@example.com"));
        System.out.println("\"invalid-email\": " + email.validate("invalid-email"));

        System.out.println("\n=== Design Summary ===");
        System.out.println("""
            Static methods:
            - Validator.notEmpty() - factory for common validator
            - Validator.minLength(n) - configurable factory
            - Validator.range(min, max) - another factory
            - Validator.and(v1, v2) - combinator

            Default method:
            - validate(value) - uses isValid() and getErrorMessage()

            Abstract methods:
            - isValid() - must implement
            - getErrorMessage() - must implement

            This pattern:
            - Factories create pre-built validators
            - Custom validators implement interface
            - All get validate() for free
            """);
    }
}
// Combining Static and Default Methods

interface Validator<T> {
    // Abstract - each validator implements differently
    boolean isValid(T value);
    String getErrorMessage();

    // Default - common behavior
    default ValidationResult validate(T value) {
        if (isValid(value)) {
            return ValidationResult.success();
        }
        return ValidationResult.failure(getErrorMessage());
    }

    // Static factory methods for common validators
    static Validator<String> notEmpty() {
        return new Validator<>() {
            @Override
            public boolean isValid(String value) {
                return value != null && !value.isEmpty();
            }

            @Override
            public String getErrorMessage() {
                return "Value cannot be empty";
            }
        };
    }

    static Validator<String> minLength(int min) {
        return new Validator<>() {
            @Override
            public boolean isValid(String value) {
                return value != null && value.length() >= min;
            }

            @Override
            public String getErrorMessage() {
                return "Value must be at least " + min + " characters";
            }
        };
    }

    static Validator<Integer> range(int min, int max) {
        return new Validator<>() {
            @Override
            public boolean isValid(Integer value) {
                return value != null && value >= min && value <= max;
            }

            @Override
            public String getErrorMessage() {
                return "Value must be between " + min + " and " + max;
            }
        };
    }

    // Static combinator methods
    static <T> Validator<T> and(Validator<T> v1, Validator<T> v2) {
        return new Validator<>() {
            @Override
            public boolean isValid(T value) {
                return v1.isValid(value) && v2.isValid(value);
            }

            @Override
            public String getErrorMessage() {
                return v1.getErrorMessage() + " AND " + v2.getErrorMessage();
            }
        };
    }
}

// Simple result class
class ValidationResult {
    private final boolean valid;
    private final String message;

    private ValidationResult(boolean valid, String message) {
        this.valid = valid;
        this.message = message;
    }

    static ValidationResult success() {
        return new ValidationResult(true, "OK");
    }

    static ValidationResult failure(String message) {
        return new ValidationResult(false, message);
    }

    @Override
    public String toString() {
        return valid ? "✓ Valid" : "✗ Invalid: " + message;
    }
}

// Custom validator
class EmailValidator implements Validator<String> {
    @Override
    public boolean isValid(String value) {
        return value != null && value.contains("@") && value.contains(".");
    }

    @Override
    public String getErrorMessage() {
        return "Invalid email format";
    }
}

public class StaticWithDefault {
    public static void main(String[] args) {
        System.out.println("=== Validator Interface ===\n");

        // Using static factory methods
        Validator<String> notEmpty = Validator.notEmpty();
        int minChars = 3;
        Validator<String> minLengthRule = Validator.minLength(minChars);
        Validator<Integer> ageRange = Validator.range(18, 65);

        System.out.println("--- notEmpty validator ---");
        System.out.println("\"hello\": " + notEmpty.validate("hello"));
        System.out.println("\"\": " + notEmpty.validate(""));
        System.out.println("null: " + notEmpty.validate(null));

        System.out.println("\n--- minLength(" + minChars + ") validator ---");
        System.out.println("\"hello\": " + minLengthRule.validate("hello"));
        System.out.println("\"hi\": " + minLengthRule.validate("hi"));

        System.out.println("\n--- range(18, 65) validator ---");
        System.out.println("25: " + ageRange.validate(25));
        System.out.println("15: " + ageRange.validate(15));
        System.out.println("70: " + ageRange.validate(70));

        // Using combinator
        System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");
        Validator<String> combined = Validator.and(notEmpty, minLengthRule);
        System.out.println("\"hello world\": " + combined.validate("hello world"));
        System.out.println("\"hi\": " + combined.validate("hi"));
        System.out.println("\"\": " + combined.validate(""));

        // Custom validator uses default method
        System.out.println("\n--- Custom EmailValidator ---");
        Validator<String> email = new EmailValidator();
        System.out.println("\"test@example.com\": " + email.validate("test@example.com"));
        System.out.println("\"invalid-email\": " + email.validate("invalid-email"));

        System.out.println("\n=== Design Summary ===");
        System.out.println("""
            Static methods:
            - Validator.notEmpty() - factory for common validator
            - Validator.minLength(n) - configurable factory
            - Validator.range(min, max) - another factory
            - Validator.and(v1, v2) - combinator

            Default method:
            - validate(value) - uses isValid() and getErrorMessage()

            Abstract methods:
            - isValid() - must implement
            - getErrorMessage() - must implement

            This pattern:
            - Factories create pre-built validators
            - Custom validators implement interface
            - All get validate() for free
            """);
    }
}
// Combining Static and Default Methods

interface Validator<T> {
    // Abstract - each validator implements differently
    boolean isValid(T value);
    String getErrorMessage();

    // Default - common behavior
    default ValidationResult validate(T value) {
        if (isValid(value)) {
            return ValidationResult.success();
        }
        return ValidationResult.failure(getErrorMessage());
    }

    // Static factory methods for common validators
    static Validator<String> notEmpty() {
        return new Validator<>() {
            @Override
            public boolean isValid(String value) {
                return value != null && !value.isEmpty();
            }

            @Override
            public String getErrorMessage() {
                return "Value cannot be empty";
            }
        };
    }

    static Validator<String> minLength(int min) {
        return new Validator<>() {
            @Override
            public boolean isValid(String value) {
                return value != null && value.length() >= min;
            }

            @Override
            public String getErrorMessage() {
                return "Value must be at least " + min + " characters";
            }
        };
    }

    static Validator<Integer> range(int min, int max) {
        return new Validator<>() {
            @Override
            public boolean isValid(Integer value) {
                return value != null && value >= min && value <= max;
            }

            @Override
            public String getErrorMessage() {
                return "Value must be between " + min + " and " + max;
            }
        };
    }

    // Static combinator methods
    static <T> Validator<T> and(Validator<T> v1, Validator<T> v2) {
        return new Validator<>() {
            @Override
            public boolean isValid(T value) {
                return v1.isValid(value) && v2.isValid(value);
            }

            @Override
            public String getErrorMessage() {
                return v1.getErrorMessage() + " AND " + v2.getErrorMessage();
            }
        };
    }
}

// Simple result class
class ValidationResult {
    private final boolean valid;
    private final String message;

    private ValidationResult(boolean valid, String message) {
        this.valid = valid;
        this.message = message;
    }

    static ValidationResult success() {
        return new ValidationResult(true, "OK");
    }

    static ValidationResult failure(String message) {
        return new ValidationResult(false, message);
    }

    @Override
    public String toString() {
        return valid ? "✓ Valid" : "✗ Invalid: " + message;
    }
}

// Custom validator
class EmailValidator implements Validator<String> {
    @Override
    public boolean isValid(String value) {
        return value != null && value.contains("@") && value.contains(".");
    }

    @Override
    public String getErrorMessage() {
        return "Invalid email format";
    }
}

public class StaticWithDefault {
    public static void main(String[] args) {
        System.out.println("=== Validator Interface ===\n");

        // Using static factory methods
        Validator<String> notEmpty = Validator.notEmpty();
        int minChars = 8;
        Validator<String> minLengthRule = Validator.minLength(minChars);
        Validator<Integer> ageRange = Validator.range(18, 65);

        System.out.println("--- notEmpty validator ---");
        System.out.println("\"hello\": " + notEmpty.validate("hello"));
        System.out.println("\"\": " + notEmpty.validate(""));
        System.out.println("null: " + notEmpty.validate(null));

        System.out.println("\n--- minLength(" + minChars + ") validator ---");
        System.out.println("\"hello\": " + minLengthRule.validate("hello"));
        System.out.println("\"hi\": " + minLengthRule.validate("hi"));

        System.out.println("\n--- range(18, 65) validator ---");
        System.out.println("25: " + ageRange.validate(25));
        System.out.println("15: " + ageRange.validate(15));
        System.out.println("70: " + ageRange.validate(70));

        // Using combinator
        System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");
        Validator<String> combined = Validator.and(notEmpty, minLengthRule);
        System.out.println("\"hello world\": " + combined.validate("hello world"));
        System.out.println("\"hi\": " + combined.validate("hi"));
        System.out.println("\"\": " + combined.validate(""));

        // Custom validator uses default method
        System.out.println("\n--- Custom EmailValidator ---");
        Validator<String> email = new EmailValidator();
        System.out.println("\"test@example.com\": " + email.validate("test@example.com"));
        System.out.println("\"invalid-email\": " + email.validate("invalid-email"));

        System.out.println("\n=== Design Summary ===");
        System.out.println("""
            Static methods:
            - Validator.notEmpty() - factory for common validator
            - Validator.minLength(n) - configurable factory
            - Validator.range(min, max) - another factory
            - Validator.and(v1, v2) - combinator

            Default method:
            - validate(value) - uses isValid() and getErrorMessage()

            Abstract methods:
            - isValid() - must implement
            - getErrorMessage() - must implement

            This pattern:
            - Factories create pre-built validators
            - Custom validators implement interface
            - All get validate() for free
            """);
    }
}

                        
1public static void main(String[] args)⋯112public class StaticWithDefault {113    public static void main(String[] args) {114        System.out.println("=== Validator Interface ===\n");115        116        // Using static factory methods //?usestatic117        Validator<String> notEmpty = Validator.notEmpty();118        int minChars = 5;  //@minChars=5, 3, 8⋯output=== Validator Interface ===
2notEmpty ← ⟨Validator$1 A⟩, minChars ← 5⋯116// Using static factory methods //?usestatic117Validator<String> notEmpty→ ⟨Validator$1 A⟩ = Validator.notEmpty();118int minChars→ 5 = 5;  //@minChars=5, 3, 8119Validator<String> minLengthRule = Validator.minLength(minChars5);120Validator<Integer> ageRange = Validator.range(18, 65);⋯
3static Validator<String> minLength(int min)⋯31static Validator<String> minLength(int min5) { //?minlength32    return new Validator<>() {33        @Override34        public boolean isValid(String value) {35            return value != null && value.length() >= min;36        }37        38        @Override39        public String getErrorMessage() {40            return "Value must be at least " + min + " characters";41        }42    };43}⋯
4minLengthRule ← ⟨Validator$2 B⟩⋯118int minChars = 5;  //@minChars=5, 3, 8119Validator<String> minLengthRule→ ⟨Validator$2 B⟩ = Validator.minLength(minChars5);120Validator<Integer> ageRange = Validator.range(18, 65);⋯
5static Validator<Integer> range(int min, int max)⋯45static Validator<Integer> range(int min18, int max65) { //?range46    return new Validator<>() {47        @Override48        public boolean isValid(Integer value) {49            return value != null && value >= min && value <= max;50        }51        52        @Override53        public String getErrorMessage() {54            return "Value must be between " + min + " and " + max;55        }56    };57}⋯
6ageRange ← ⟨Validator$3 C⟩⋯119Validator<String> minLengthRule = Validator.minLength(minChars);120Validator<Integer> ageRange→ ⟨Validator$3 C⟩ = Validator.range(18, 65);121122System.out.println("--- notEmpty validator ---");123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));⋯output--- notEmpty validator ---
7default ValidationResult validate(T value)pass 1 of 13⋯8// Default - common behavior //?defaultvalidate9default ValidationResult validate(T valuehello) {10    if (isValid(value)) {⋯13 passes — pass 1 is the card abovepassvalue
1hello
2(empty)
3null
4hello
5hi
625
715
870
9hello world
⋯ 2 more passes ⋯
12test@example.com
13invalid-email
8@Override
            public boolean isValid(String value)pass 1 of 6⋯18return new Validator<>() {19    @Override20    public boolean isValid(String valuehello) {21        return valuehello != null && !value.isEmpty();22    }⋯All 6 passes — pass 1 is the card abovepassvalue
1hello
2(empty)
3null
4hello world
5hi
6(empty)
9if (isValid(value))pass 1 of 5⋯9default ValidationResult validate(T value) {10    if (isValid(valuehello)) {11        return ValidationResult.success();12    }⋯All 5 passes — pass 1 is the card abovepassvalue
1hello
2hello
325
4hello world
5test@example.com
10this.valid ← true, this.message ← OKpass 1 of 13⋯80private ValidationResult(boolean validtrue, String messageOK) {81    this.valid→ true = validtrue;82    this.message→ OK = messageOK;83}⋯13 passes — pass 1 is the card abovepassvalidmessagethis.validthis.message
1trueOKtrueOK
2falseValue cannot be emptyfalseValue cannot be empty
3falseValue cannot be emptyfalseValue cannot be empty
4trueOKtrueOK
5falseValue must be at least 5 charactersfalseValue must be at least 5 characters
6trueOKtrueOK
7falseValue must be between 18 and 65falseValue must be between 18 and 65
8falseValue must be between 18 and 65falseValue must be between 18 and 65
9trueOKtrueOK
⋯ 2 more passes ⋯
12trueOKtrueOK
13falseInvalid email formatfalseInvalid email format
11System.out.println("\"hello\": " + notEmpty.validate("hello"));⋯122System.out.println("--- notEmpty validator ---");123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));⋯output"hello": ✓ Valid
12return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
13static ValidationResult failure(String message)pass 1 of 8⋯89static ValidationResult failure(String messageValue cannot be empty) {90    return new ValidationResult(false, message);91}⋯All 8 passes — pass 1 is the card abovepassmessage
1Value cannot be empty
2Value cannot be empty
3Value must be at least 5 characters
4Value must be between 18 and 65
5Value must be between 18 and 65
6Value cannot be empty AND Value must be at least 5 characters
7Value cannot be empty AND Value must be at least 5 characters
8Invalid email format
14System.out.println("\"\": " + notEmpty.validate(""));⋯123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));⋯output"": ✗ Invalid: Value cannot be empty
15return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
16System.out.println(" --- minLength(" + minChars + ") validator ---");⋯124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));126127System.out.println("\n--- minLength(" + minChars5 + ") validator ---");128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯outputnull: ✗ Invalid: Value cannot be empty

--- minLength(5) validator ---
17@Override
            public boolean isValid(String value)pass 1 of 4⋯32return new Validator<>() {33    @Override34    public boolean isValid(String valuehello) {35        return valuehello != null && value.length() >= min5;36    }⋯All 4 passes — pass 1 is the card abovepassvalue
1hello
2hi
3hello world
4hi
18System.out.println("\"hello\": " + minLengthRule.validate("hello"));⋯127System.out.println("\n--- minLength(" + minChars + ") validator ---");128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯output"hello": ✓ Valid
19return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
20@Override
            public String getErrorMessage()pass 1 of 3⋯38@Override39public String getErrorMessage() {40    return "Value must be at least " + min5 + " characters";41}⋯
21System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));130131System.out.println("\n--- range(18, 65) validator ---");132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));⋯output"hi": ✗ Invalid: Value must be at least 5 characters

--- range(18, 65) validator ---
22@Override
            public boolean isValid(Integer value)pass 1 of 3⋯46return new Validator<>() {47    @Override48    public boolean isValid(Integer value25) {49        return value25 != null && value >= min18 && value <= max65;50    }⋯All 3 passes — pass 1 is the card abovepassvalue
125
215
370
23System.out.println("25: " + ageRange.validate(25));⋯131System.out.println("\n--- range(18, 65) validator ---");132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));⋯output25: ✓ Valid
24return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
25@Override
            public String getErrorMessage()pass 1 of 2⋯52@Override53public String getErrorMessage() {54    return "Value must be between " + min18 + " and " + max65;55}⋯
26System.out.println("15: " + ageRange.validate(15));⋯132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));⋯output15: ✗ Invalid: Value must be between 18 and 65
27return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
28@Override
            public String getErrorMessage()pass 2 of 2⋯52@Override53public String getErrorMessage() {54    return "Value must be between " + min18 + " and " + max65;55}⋯
29Validator<String> combined = Validator.and(notEmpty, minLengthRule);⋯133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));135136// Using combinator //?usecombinator137System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");138Validator<String> combined = Validator.and(notEmpty⟨Validator$1 A⟩, minLengthRule⟨Validator$2 B⟩);139System.out.println("\"hello world\": " + combined.validate("hello world"));⋯output70: ✗ Invalid: Value must be between 18 and 65

--- Combined validator (notEmpty AND minLength) ---
30static <T> Validator<T> and(Validator<T> v1, Validator<T> v2)⋯59// Static combinator methods //?combinators60static <T> Validator<T> and(Validator<T> v1⟨Validator$1 A⟩, Validator<T> v2⟨Validator$2 B⟩) { //?andcombinator61    return new Validator<>() {62        @Override63        public boolean isValid(T value) {64            return v1.isValid(value) && v2.isValid(value);65        }66        67        @Override68        public String getErrorMessage() {69            return v1.getErrorMessage() + " AND " + v2.getErrorMessage();70        }71    };72}⋯
31combined ← ⟨Validator$4 D⟩⋯137System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");138Validator<String> combined→ ⟨Validator$4 D⟩ = Validator.and(notEmpty⟨Validator$1 A⟩, minLengthRule⟨Validator$2 B⟩);139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));⋯
32@Override
            public boolean isValid(T value)pass 1 of 3⋯61return new Validator<>() {62    @Override63    public boolean isValid(T valuehello world) {64        return v1.isValid(valuehello world) && v2.isValid(value);65    }⋯All 3 passes — pass 1 is the card abovepassvalue
1hello world
2hi
3(empty)
33System.out.println("\"hello world\": " + combined.validate("hello worl…⋯138Validator<String> combined = Validator.and(notEmpty, minLengthRule);139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));⋯output"hello world": ✓ Valid
34return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
35System.out.println("\"hi\": " + combined.validate("hi"));⋯139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));⋯output"hi": ✗ Invalid: Value cannot be empty AND Value must be at least 5 characters
36return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
37email ← ⟨EmailValidator E⟩⋯140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));142143// Custom validator uses default method //?usecustom144System.out.println("\n--- Custom EmailValidator ---");145Validator<String> email→ ⟨EmailValidator E⟩ = new EmailValidator();146System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147System.out.println("\"invalid-email\": " + email.validate("invalid-email"));⋯output"": ✗ Invalid: Value cannot be empty AND Value must be at least 5 characters

--- Custom EmailValidator ---
38@Override
    public boolean isValid(String value)pass 1 of 2⋯100class EmailValidator implements Validator<String> {101    @Override102    public boolean isValid(String valuetest@example.com) {103        return valuetest@example.com != null && value.contains("@") && value.contains(".");104    }⋯
39System.out.println("\"test@example.com\": " + email.validate("test@exa…⋯145Validator<String> email = new EmailValidator();146System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147System.out.println("\"invalid-email\": " + email.validate("invalid-email"));⋯output"test@example.com": ✓ Valid
40@Override
    public boolean isValid(String value)pass 2 of 2⋯100class EmailValidator implements Validator<String> {101    @Override102    public boolean isValid(String valueinvalid-email) {103        return valueinvalid-email != null && value.contains("@") && value.contains(".");104    }⋯
41return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
42System.out.println("\"invalid-email\": " + email.validate("invalid-ema…⋯146    System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147    System.out.println("\"invalid-email\": " + email.validate("invalid-email"));148    149    System.out.println("\n=== Design Summary ===");150    System.out.println("""151        Static methods:152        - Validator.notEmpty() - factory for common validator153        - Validator.minLength(n) - configurable factory154        - Validator.range(min, max) - another factory155        - Validator.and(v1, v2) - combinator156        157        Default method:158        - validate(value) - uses isValid() and getErrorMessage()159        160        Abstract methods:161        - isValid() - must implement162        - getErrorMessage() - must implement163        164        This pattern:165        - Factories create pre-built validators166        - Custom validators implement interface167        - All get validate() for free168        """);169}⋯output"invalid-email": ✗ Invalid: Invalid email format

=== Design Summary ===
Static methods:
- Validator.notEmpty() - factory for common validator
- Validator.minLength(n) - configurable factory
- Validator.range(min, max) - another factory
- Validator.and(v1, v2) - combinator

Default method:
- validate(value) - uses isValid() and getErrorMessage()

Abstract methods:
- isValid() - must implement
- getErrorMessage() - must implement

This pattern:
- Factories create pre-built validators
- Custom validators implement interface
- All get validate() for free
1public static void main(String[] args)⋯112public class StaticWithDefault {113    public static void main(String[] args) {114        System.out.println("=== Validator Interface ===\n");115        116        // Using static factory methods117        Validator<String> notEmpty = Validator.notEmpty();118        int minChars = 3;⋯output=== Validator Interface ===
2notEmpty ← ⟨Validator$1 A⟩, minChars ← 3⋯116// Using static factory methods117Validator<String> notEmpty→ ⟨Validator$1 A⟩ = Validator.notEmpty();118int minChars→ 3 = 3;119Validator<String> minLengthRule = Validator.minLength(minChars3);120Validator<Integer> ageRange = Validator.range(18, 65);⋯
3static Validator<String> minLength(int min)⋯31static Validator<String> minLength(int min3) {32    return new Validator<>() {33        @Override34        public boolean isValid(String value) {35            return value != null && value.length() >= min;36        }37        38        @Override39        public String getErrorMessage() {40            return "Value must be at least " + min + " characters";41        }42    };43}⋯
4minLengthRule ← ⟨Validator$2 B⟩⋯118int minChars = 3;119Validator<String> minLengthRule→ ⟨Validator$2 B⟩ = Validator.minLength(minChars3);120Validator<Integer> ageRange = Validator.range(18, 65);⋯
5static Validator<Integer> range(int min, int max)⋯45static Validator<Integer> range(int min18, int max65) {46    return new Validator<>() {47        @Override48        public boolean isValid(Integer value) {49            return value != null && value >= min && value <= max;50        }51        52        @Override53        public String getErrorMessage() {54            return "Value must be between " + min + " and " + max;55        }56    };57}⋯
6ageRange ← ⟨Validator$3 C⟩⋯119Validator<String> minLengthRule = Validator.minLength(minChars);120Validator<Integer> ageRange→ ⟨Validator$3 C⟩ = Validator.range(18, 65);121122System.out.println("--- notEmpty validator ---");123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));⋯output--- notEmpty validator ---
7default ValidationResult validate(T value)pass 1 of 13⋯8// Default - common behavior9default ValidationResult validate(T valuehello) {10    if (isValid(value)) {⋯13 passes — pass 1 is the card abovepassvalue
1hello
2(empty)
3null
4hello
5hi
625
715
870
9hello world
⋯ 2 more passes ⋯
12test@example.com
13invalid-email
8@Override
            public boolean isValid(String value)pass 1 of 6⋯18return new Validator<>() {19    @Override20    public boolean isValid(String valuehello) {21        return valuehello != null && !value.isEmpty();22    }⋯All 6 passes — pass 1 is the card abovepassvalue
1hello
2(empty)
3null
4hello world
5hi
6(empty)
9if (isValid(value))pass 1 of 5⋯9default ValidationResult validate(T value) {10    if (isValid(valuehello)) {11        return ValidationResult.success();12    }⋯All 5 passes — pass 1 is the card abovepassvalue
1hello
2hello
325
4hello world
5test@example.com
10this.valid ← true, this.message ← OKpass 1 of 13⋯80private ValidationResult(boolean validtrue, String messageOK) {81    this.valid→ true = validtrue;82    this.message→ OK = messageOK;83}⋯13 passes — pass 1 is the card abovepassvalidmessagethis.validthis.message
1trueOKtrueOK
2falseValue cannot be emptyfalseValue cannot be empty
3falseValue cannot be emptyfalseValue cannot be empty
4trueOKtrueOK
5falseValue must be at least 3 charactersfalseValue must be at least 3 characters
6trueOKtrueOK
7falseValue must be between 18 and 65falseValue must be between 18 and 65
8falseValue must be between 18 and 65falseValue must be between 18 and 65
9trueOKtrueOK
⋯ 2 more passes ⋯
12trueOKtrueOK
13falseInvalid email formatfalseInvalid email format
11System.out.println("\"hello\": " + notEmpty.validate("hello"));⋯122System.out.println("--- notEmpty validator ---");123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));⋯output"hello": ✓ Valid
12return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
13static ValidationResult failure(String message)pass 1 of 8⋯89static ValidationResult failure(String messageValue cannot be empty) {90    return new ValidationResult(false, message);91}⋯All 8 passes — pass 1 is the card abovepassmessage
1Value cannot be empty
2Value cannot be empty
3Value must be at least 3 characters
4Value must be between 18 and 65
5Value must be between 18 and 65
6Value cannot be empty AND Value must be at least 3 characters
7Value cannot be empty AND Value must be at least 3 characters
8Invalid email format
14System.out.println("\"\": " + notEmpty.validate(""));⋯123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));⋯output"": ✗ Invalid: Value cannot be empty
15return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
16System.out.println(" --- minLength(" + minChars + ") validator ---");⋯124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));126127System.out.println("\n--- minLength(" + minChars3 + ") validator ---");128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯outputnull: ✗ Invalid: Value cannot be empty

--- minLength(3) validator ---
17@Override
            public boolean isValid(String value)pass 1 of 4⋯32return new Validator<>() {33    @Override34    public boolean isValid(String valuehello) {35        return valuehello != null && value.length() >= min3;36    }⋯All 4 passes — pass 1 is the card abovepassvalue
1hello
2hi
3hello world
4hi
18System.out.println("\"hello\": " + minLengthRule.validate("hello"));⋯127System.out.println("\n--- minLength(" + minChars + ") validator ---");128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯output"hello": ✓ Valid
19return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
20@Override
            public String getErrorMessage()pass 1 of 3⋯38@Override39public String getErrorMessage() {40    return "Value must be at least " + min3 + " characters";41}⋯
21System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));130131System.out.println("\n--- range(18, 65) validator ---");132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));⋯output"hi": ✗ Invalid: Value must be at least 3 characters

--- range(18, 65) validator ---
22@Override
            public boolean isValid(Integer value)pass 1 of 3⋯46return new Validator<>() {47    @Override48    public boolean isValid(Integer value25) {49        return value25 != null && value >= min18 && value <= max65;50    }⋯All 3 passes — pass 1 is the card abovepassvalue
125
215
370
23System.out.println("25: " + ageRange.validate(25));⋯131System.out.println("\n--- range(18, 65) validator ---");132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));⋯output25: ✓ Valid
24return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
25@Override
            public String getErrorMessage()pass 1 of 2⋯52@Override53public String getErrorMessage() {54    return "Value must be between " + min18 + " and " + max65;55}⋯
26System.out.println("15: " + ageRange.validate(15));⋯132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));⋯output15: ✗ Invalid: Value must be between 18 and 65
27return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
28@Override
            public String getErrorMessage()pass 2 of 2⋯52@Override53public String getErrorMessage() {54    return "Value must be between " + min18 + " and " + max65;55}⋯
29Validator<String> combined = Validator.and(notEmpty, minLengthRule);⋯133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));135136// Using combinator137System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");138Validator<String> combined = Validator.and(notEmpty⟨Validator$1 A⟩, minLengthRule⟨Validator$2 B⟩);139System.out.println("\"hello world\": " + combined.validate("hello world"));⋯output70: ✗ Invalid: Value must be between 18 and 65

--- Combined validator (notEmpty AND minLength) ---
30static <T> Validator<T> and(Validator<T> v1, Validator<T> v2)⋯59// Static combinator methods60static <T> Validator<T> and(Validator<T> v1⟨Validator$1 A⟩, Validator<T> v2⟨Validator$2 B⟩) {61    return new Validator<>() {62        @Override63        public boolean isValid(T value) {64            return v1.isValid(value) && v2.isValid(value);65        }66        67        @Override68        public String getErrorMessage() {69            return v1.getErrorMessage() + " AND " + v2.getErrorMessage();70        }71    };72}⋯
31combined ← ⟨Validator$4 D⟩⋯137System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");138Validator<String> combined→ ⟨Validator$4 D⟩ = Validator.and(notEmpty⟨Validator$1 A⟩, minLengthRule⟨Validator$2 B⟩);139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));⋯
32@Override
            public boolean isValid(T value)pass 1 of 3⋯61return new Validator<>() {62    @Override63    public boolean isValid(T valuehello world) {64        return v1.isValid(valuehello world) && v2.isValid(value);65    }⋯All 3 passes — pass 1 is the card abovepassvalue
1hello world
2hi
3(empty)
33System.out.println("\"hello world\": " + combined.validate("hello worl…⋯138Validator<String> combined = Validator.and(notEmpty, minLengthRule);139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));⋯output"hello world": ✓ Valid
34return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
35System.out.println("\"hi\": " + combined.validate("hi"));⋯139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));⋯output"hi": ✗ Invalid: Value cannot be empty AND Value must be at least 3 characters
36return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
37email ← ⟨EmailValidator E⟩⋯140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));142143// Custom validator uses default method144System.out.println("\n--- Custom EmailValidator ---");145Validator<String> email→ ⟨EmailValidator E⟩ = new EmailValidator();146System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147System.out.println("\"invalid-email\": " + email.validate("invalid-email"));⋯output"": ✗ Invalid: Value cannot be empty AND Value must be at least 3 characters

--- Custom EmailValidator ---
38@Override
    public boolean isValid(String value)pass 1 of 2⋯100class EmailValidator implements Validator<String> {101    @Override102    public boolean isValid(String valuetest@example.com) {103        return valuetest@example.com != null && value.contains("@") && value.contains(".");104    }⋯
39System.out.println("\"test@example.com\": " + email.validate("test@exa…⋯145Validator<String> email = new EmailValidator();146System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147System.out.println("\"invalid-email\": " + email.validate("invalid-email"));⋯output"test@example.com": ✓ Valid
40@Override
    public boolean isValid(String value)pass 2 of 2⋯100class EmailValidator implements Validator<String> {101    @Override102    public boolean isValid(String valueinvalid-email) {103        return valueinvalid-email != null && value.contains("@") && value.contains(".");104    }⋯
41return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
42System.out.println("\"invalid-email\": " + email.validate("invalid-ema…⋯146    System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147    System.out.println("\"invalid-email\": " + email.validate("invalid-email"));148    149    System.out.println("\n=== Design Summary ===");150    System.out.println("""151        Static methods:152        - Validator.notEmpty() - factory for common validator153        - Validator.minLength(n) - configurable factory154        - Validator.range(min, max) - another factory155        - Validator.and(v1, v2) - combinator156        157        Default method:158        - validate(value) - uses isValid() and getErrorMessage()159        160        Abstract methods:161        - isValid() - must implement162        - getErrorMessage() - must implement163        164        This pattern:165        - Factories create pre-built validators166        - Custom validators implement interface167        - All get validate() for free168        """);169}⋯output"invalid-email": ✗ Invalid: Invalid email format

=== Design Summary ===
Static methods:
- Validator.notEmpty() - factory for common validator
- Validator.minLength(n) - configurable factory
- Validator.range(min, max) - another factory
- Validator.and(v1, v2) - combinator

Default method:
- validate(value) - uses isValid() and getErrorMessage()

Abstract methods:
- isValid() - must implement
- getErrorMessage() - must implement

This pattern:
- Factories create pre-built validators
- Custom validators implement interface
- All get validate() for free
1public static void main(String[] args)⋯112public class StaticWithDefault {113    public static void main(String[] args) {114        System.out.println("=== Validator Interface ===\n");115        116        // Using static factory methods117        Validator<String> notEmpty = Validator.notEmpty();118        int minChars = 8;⋯output=== Validator Interface ===
2notEmpty ← ⟨Validator$1 A⟩, minChars ← 8⋯116// Using static factory methods117Validator<String> notEmpty→ ⟨Validator$1 A⟩ = Validator.notEmpty();118int minChars→ 8 = 8;119Validator<String> minLengthRule = Validator.minLength(minChars8);120Validator<Integer> ageRange = Validator.range(18, 65);⋯
3static Validator<String> minLength(int min)⋯31static Validator<String> minLength(int min8) {32    return new Validator<>() {33        @Override34        public boolean isValid(String value) {35            return value != null && value.length() >= min;36        }37        38        @Override39        public String getErrorMessage() {40            return "Value must be at least " + min + " characters";41        }42    };43}⋯
4minLengthRule ← ⟨Validator$2 B⟩⋯118int minChars = 8;119Validator<String> minLengthRule→ ⟨Validator$2 B⟩ = Validator.minLength(minChars8);120Validator<Integer> ageRange = Validator.range(18, 65);⋯
5static Validator<Integer> range(int min, int max)⋯45static Validator<Integer> range(int min18, int max65) {46    return new Validator<>() {47        @Override48        public boolean isValid(Integer value) {49            return value != null && value >= min && value <= max;50        }51        52        @Override53        public String getErrorMessage() {54            return "Value must be between " + min + " and " + max;55        }56    };57}⋯
6ageRange ← ⟨Validator$3 C⟩⋯119Validator<String> minLengthRule = Validator.minLength(minChars);120Validator<Integer> ageRange→ ⟨Validator$3 C⟩ = Validator.range(18, 65);121122System.out.println("--- notEmpty validator ---");123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));⋯output--- notEmpty validator ---
7default ValidationResult validate(T value)pass 1 of 13⋯8// Default - common behavior9default ValidationResult validate(T valuehello) {10    if (isValid(value)) {⋯13 passes — pass 1 is the card abovepassvalue
1hello
2(empty)
3null
4hello
5hi
625
715
870
9hello world
⋯ 2 more passes ⋯
12test@example.com
13invalid-email
8@Override
            public boolean isValid(String value)pass 1 of 6⋯18return new Validator<>() {19    @Override20    public boolean isValid(String valuehello) {21        return valuehello != null && !value.isEmpty();22    }⋯All 6 passes — pass 1 is the card abovepassvalue
1hello
2(empty)
3null
4hello world
5hi
6(empty)
9if (isValid(value))pass 1 of 4⋯9default ValidationResult validate(T value) {10    if (isValid(valuehello)) {11        return ValidationResult.success();12    }⋯All 4 passes — pass 1 is the card abovepassvalue
1hello
225
3hello world
4test@example.com
10this.valid ← true, this.message ← OKpass 1 of 13⋯80private ValidationResult(boolean validtrue, String messageOK) {81    this.valid→ true = validtrue;82    this.message→ OK = messageOK;83}⋯13 passes — pass 1 is the card abovepassvalidmessagethis.validthis.message
1trueOKtrueOK
2falseValue cannot be emptyfalseValue cannot be empty
3falseValue cannot be emptyfalseValue cannot be empty
4falseValue must be at least 8 charactersfalseValue must be at least 8 characters
5falseValue must be at least 8 charactersfalseValue must be at least 8 characters
6trueOKtrueOK
7falseValue must be between 18 and 65falseValue must be between 18 and 65
8falseValue must be between 18 and 65falseValue must be between 18 and 65
9trueOKtrueOK
⋯ 2 more passes ⋯
12trueOKtrueOK
13falseInvalid email formatfalseInvalid email format
11System.out.println("\"hello\": " + notEmpty.validate("hello"));⋯122System.out.println("--- notEmpty validator ---");123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));⋯output"hello": ✓ Valid
12return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
13static ValidationResult failure(String message)pass 1 of 9⋯89static ValidationResult failure(String messageValue cannot be empty) {90    return new ValidationResult(false, message);91}⋯All 9 passes — pass 1 is the card abovepassmessage
1Value cannot be empty
2Value cannot be empty
3Value must be at least 8 characters
4Value must be at least 8 characters
5Value must be between 18 and 65
6Value must be between 18 and 65
7Value cannot be empty AND Value must be at least 8 characters
8Value cannot be empty AND Value must be at least 8 characters
9Invalid email format
14System.out.println("\"\": " + notEmpty.validate(""));⋯123System.out.println("\"hello\": " + notEmpty.validate("hello"));124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));⋯output"": ✗ Invalid: Value cannot be empty
15return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
16System.out.println(" --- minLength(" + minChars + ") validator ---");⋯124System.out.println("\"\": " + notEmpty.validate(""));125System.out.println("null: " + notEmpty.validate(null));126127System.out.println("\n--- minLength(" + minChars8 + ") validator ---");128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯outputnull: ✗ Invalid: Value cannot be empty

--- minLength(8) validator ---
17@Override
            public boolean isValid(String value)pass 1 of 4⋯32return new Validator<>() {33    @Override34    public boolean isValid(String valuehello) {35        return valuehello != null && value.length() >= min8;36    }⋯All 4 passes — pass 1 is the card abovepassvalue
1hello
2hi
3hello world
4hi
18return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
19@Override
            public String getErrorMessage()pass 1 of 4⋯38@Override39public String getErrorMessage() {40    return "Value must be at least " + min8 + " characters";41}⋯
20System.out.println("\"hello\": " + minLengthRule.validate("hello"));⋯127System.out.println("\n--- minLength(" + minChars + ") validator ---");128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯output"hello": ✗ Invalid: Value must be at least 8 characters
21return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
22System.out.println("\"hi\": " + minLengthRule.validate("hi"));⋯128System.out.println("\"hello\": " + minLengthRule.validate("hello"));129System.out.println("\"hi\": " + minLengthRule.validate("hi"));130131System.out.println("\n--- range(18, 65) validator ---");132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));⋯output"hi": ✗ Invalid: Value must be at least 8 characters

--- range(18, 65) validator ---
23@Override
            public boolean isValid(Integer value)pass 1 of 3⋯46return new Validator<>() {47    @Override48    public boolean isValid(Integer value25) {49        return value25 != null && value >= min18 && value <= max65;50    }⋯All 3 passes — pass 1 is the card abovepassvalue
125
215
370
24System.out.println("25: " + ageRange.validate(25));⋯131System.out.println("\n--- range(18, 65) validator ---");132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));⋯output25: ✓ Valid
25return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
26@Override
            public String getErrorMessage()pass 1 of 2⋯52@Override53public String getErrorMessage() {54    return "Value must be between " + min18 + " and " + max65;55}⋯
27System.out.println("15: " + ageRange.validate(15));⋯132System.out.println("25: " + ageRange.validate(25));133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));⋯output15: ✗ Invalid: Value must be between 18 and 65
28return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
29@Override
            public String getErrorMessage()pass 2 of 2⋯52@Override53public String getErrorMessage() {54    return "Value must be between " + min18 + " and " + max65;55}⋯
30Validator<String> combined = Validator.and(notEmpty, minLengthRule);⋯133System.out.println("15: " + ageRange.validate(15));134System.out.println("70: " + ageRange.validate(70));135136// Using combinator137System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");138Validator<String> combined = Validator.and(notEmpty⟨Validator$1 A⟩, minLengthRule⟨Validator$2 B⟩);139System.out.println("\"hello world\": " + combined.validate("hello world"));⋯output70: ✗ Invalid: Value must be between 18 and 65

--- Combined validator (notEmpty AND minLength) ---
31static <T> Validator<T> and(Validator<T> v1, Validator<T> v2)⋯59// Static combinator methods60static <T> Validator<T> and(Validator<T> v1⟨Validator$1 A⟩, Validator<T> v2⟨Validator$2 B⟩) {61    return new Validator<>() {62        @Override63        public boolean isValid(T value) {64            return v1.isValid(value) && v2.isValid(value);65        }66        67        @Override68        public String getErrorMessage() {69            return v1.getErrorMessage() + " AND " + v2.getErrorMessage();70        }71    };72}⋯
32combined ← ⟨Validator$4 D⟩⋯137System.out.println("\n--- Combined validator (notEmpty AND minLength) ---");138Validator<String> combined→ ⟨Validator$4 D⟩ = Validator.and(notEmpty⟨Validator$1 A⟩, minLengthRule⟨Validator$2 B⟩);139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));⋯
33@Override
            public boolean isValid(T value)pass 1 of 3⋯61return new Validator<>() {62    @Override63    public boolean isValid(T valuehello world) {64        return v1.isValid(valuehello world) && v2.isValid(value);65    }⋯All 3 passes — pass 1 is the card abovepassvalue
1hello world
2hi
3(empty)
34System.out.println("\"hello world\": " + combined.validate("hello worl…⋯138Validator<String> combined = Validator.and(notEmpty, minLengthRule);139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));⋯output"hello world": ✓ Valid
35return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
36System.out.println("\"hi\": " + combined.validate("hi"));⋯139System.out.println("\"hello world\": " + combined.validate("hello world"));140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));⋯output"hi": ✗ Invalid: Value cannot be empty AND Value must be at least 8 characters
37return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
38email ← ⟨EmailValidator E⟩⋯140System.out.println("\"hi\": " + combined.validate("hi"));141System.out.println("\"\": " + combined.validate(""));142143// Custom validator uses default method144System.out.println("\n--- Custom EmailValidator ---");145Validator<String> email→ ⟨EmailValidator E⟩ = new EmailValidator();146System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147System.out.println("\"invalid-email\": " + email.validate("invalid-email"));⋯output"": ✗ Invalid: Value cannot be empty AND Value must be at least 8 characters

--- Custom EmailValidator ---
39@Override
    public boolean isValid(String value)pass 1 of 2⋯100class EmailValidator implements Validator<String> {101    @Override102    public boolean isValid(String valuetest@example.com) {103        return valuetest@example.com != null && value.contains("@") && value.contains(".");104    }⋯
40System.out.println("\"test@example.com\": " + email.validate("test@exa…⋯145Validator<String> email = new EmailValidator();146System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147System.out.println("\"invalid-email\": " + email.validate("invalid-email"));⋯output"test@example.com": ✓ Valid
41@Override
    public boolean isValid(String value)pass 2 of 2⋯100class EmailValidator implements Validator<String> {101    @Override102    public boolean isValid(String valueinvalid-email) {103        return valueinvalid-email != null && value.contains("@") && value.contains(".");104    }⋯
42return ValidationResult.failure(getErrorMessage());⋯12    }13    return ValidationResult.failure(getErrorMessage());14}⋯
43System.out.println("\"invalid-email\": " + email.validate("invalid-ema…⋯146    System.out.println("\"test@example.com\": " + email.validate("test@example.com"));147    System.out.println("\"invalid-email\": " + email.validate("invalid-email"));148    149    System.out.println("\n=== Design Summary ===");150    System.out.println("""151        Static methods:152        - Validator.notEmpty() - factory for common validator153        - Validator.minLength(n) - configurable factory154        - Validator.range(min, max) - another factory155        - Validator.and(v1, v2) - combinator156        157        Default method:158        - validate(value) - uses isValid() and getErrorMessage()159        160        Abstract methods:161        - isValid() - must implement162        - getErrorMessage() - must implement163        164        This pattern:165        - Factories create pre-built validators166        - Custom validators implement interface167        - All get validate() for free168        """);169}⋯output"invalid-email": ✗ Invalid: Invalid email format

=== Design Summary ===
Static methods:
- Validator.notEmpty() - factory for common validator
- Validator.minLength(n) - configurable factory
- Validator.range(min, max) - another factory
- Validator.and(v1, v2) - combinator

Default method:
- validate(value) - uses isValid() and getErrorMessage()

Abstract methods:
- isValid() - must implement
- getErrorMessage() - must implement

This pattern:
- Factories create pre-built validators
- Custom validators implement interface
- All get validate() for free

13 passes — pass 1 is the card above
pass	`value`
1	hello
2	(empty)
3	null
4	hello
5	hi
6	25
7	15
8	70
9	hello world
⋯ 2 more passes ⋯
12	test@example.com
13	invalid-email

All 6 passes — pass 1 is the card above
pass	`value`
1	hello
2	(empty)
3	null
4	hello world
5	hi
6	(empty)

All 5 passes — pass 1 is the card above
pass	`value`
1	hello
2	hello
3	25
4	hello world
5	test@example.com

13 passes — pass 1 is the card above
pass	`valid`	`message`	`this.valid`	`this.message`
1	true	OK	true	OK
2	false	Value cannot be empty	false	Value cannot be empty
3	false	Value cannot be empty	false	Value cannot be empty
4	true	OK	true	OK
5	false	Value must be at least 5 characters	false	Value must be at least 5 characters
6	true	OK	true	OK
7	false	Value must be between 18 and 65	false	Value must be between 18 and 65
8	false	Value must be between 18 and 65	false	Value must be between 18 and 65
9	true	OK	true	OK
⋯ 2 more passes ⋯
12	true	OK	true	OK
13	false	Invalid email format	false	Invalid email format

All 8 passes — pass 1 is the card above
pass	`message`
1	Value cannot be empty
2	Value cannot be empty
3	Value must be at least 5 characters
4	Value must be between 18 and 65
5	Value must be between 18 and 65
6	Value cannot be empty AND Value must be at least 5 characters
7	Value cannot be empty AND Value must be at least 5 characters
8	Invalid email format

All 4 passes — pass 1 is the card above
pass	`value`
1	hello
2	hi
3	hello world
4	hi

All 3 passes — pass 1 is the card above
pass	`value`
1	25
2	15
3	70

All 3 passes — pass 1 is the card above
pass	`value`
1	hello world
2	hi
3	(empty)

13 passes — pass 1 is the card above
pass	`value`
1	hello
2	(empty)
3	null
4	hello
5	hi
6	25
7	15
8	70
9	hello world
⋯ 2 more passes ⋯
12	test@example.com
13	invalid-email

All 6 passes — pass 1 is the card above
pass	`value`
1	hello
2	(empty)
3	null
4	hello world
5	hi
6	(empty)

All 5 passes — pass 1 is the card above
pass	`value`
1	hello
2	hello
3	25
4	hello world
5	test@example.com

13 passes — pass 1 is the card above
pass	`valid`	`message`	`this.valid`	`this.message`
1	true	OK	true	OK
2	false	Value cannot be empty	false	Value cannot be empty
3	false	Value cannot be empty	false	Value cannot be empty
4	true	OK	true	OK
5	false	Value must be at least 3 characters	false	Value must be at least 3 characters
6	true	OK	true	OK
7	false	Value must be between 18 and 65	false	Value must be between 18 and 65
8	false	Value must be between 18 and 65	false	Value must be between 18 and 65
9	true	OK	true	OK
⋯ 2 more passes ⋯
12	true	OK	true	OK
13	false	Invalid email format	false	Invalid email format

All 8 passes — pass 1 is the card above
pass	`message`
1	Value cannot be empty
2	Value cannot be empty
3	Value must be at least 3 characters
4	Value must be between 18 and 65
5	Value must be between 18 and 65
6	Value cannot be empty AND Value must be at least 3 characters
7	Value cannot be empty AND Value must be at least 3 characters
8	Invalid email format

All 4 passes — pass 1 is the card above
pass	`value`
1	hello
2	hi
3	hello world
4	hi

All 3 passes — pass 1 is the card above
pass	`value`
1	25
2	15
3	70

All 3 passes — pass 1 is the card above
pass	`value`
1	hello world
2	hi
3	(empty)

13 passes — pass 1 is the card above
pass	`value`
1	hello
2	(empty)
3	null
4	hello
5	hi
6	25
7	15
8	70
9	hello world
⋯ 2 more passes ⋯
12	test@example.com
13	invalid-email

All 6 passes — pass 1 is the card above
pass	`value`
1	hello
2	(empty)
3	null
4	hello world
5	hi
6	(empty)

All 4 passes — pass 1 is the card above
pass	`value`
1	hello
2	25
3	hello world
4	test@example.com

13 passes — pass 1 is the card above
pass	`valid`	`message`	`this.valid`	`this.message`
1	true	OK	true	OK
2	false	Value cannot be empty	false	Value cannot be empty
3	false	Value cannot be empty	false	Value cannot be empty
4	false	Value must be at least 8 characters	false	Value must be at least 8 characters
5	false	Value must be at least 8 characters	false	Value must be at least 8 characters
6	true	OK	true	OK
7	false	Value must be between 18 and 65	false	Value must be between 18 and 65
8	false	Value must be between 18 and 65	false	Value must be between 18 and 65
9	true	OK	true	OK
⋯ 2 more passes ⋯
12	true	OK	true	OK
13	false	Invalid email format	false	Invalid email format

All 9 passes — pass 1 is the card above
pass	`message`
1	Value cannot be empty
2	Value cannot be empty
3	Value must be at least 8 characters
4	Value must be at least 8 characters
5	Value must be between 18 and 65
6	Value must be between 18 and 65
7	Value cannot be empty AND Value must be at least 8 characters
8	Value cannot be empty AND Value must be at least 8 characters
9	Invalid email format

All 4 passes — pass 1 is the card above
pass	`value`
1	hello
2	hi
3	hello world
4	hi

All 3 passes — pass 1 is the card above
pass	`value`
1	25
2	15
3	70

All 3 passes — pass 1 is the card above
pass	`value`
1	hello world
2	hi
3	(empty)

Static for utilities, default for shared implementation, abstract for required behavior.

Not inherited

Static methods don't become part of implementing classes.

                            NotInherited.java
                        
Replay: real traced execution (multi-file project)

                            // Static Methods Are NOT Inherited

interface Counter {
    // Static method
    static int defaultStart() {
        return 0;
    }

    // Default method
    default void increment() {
        System.out.println("Incrementing...");
    }

    // Abstract method
    int getCount();
}

class SimpleCounter implements Counter {
    private int count = Counter.defaultStart();

    @Override
    public int getCount() {
        return count;
    }

    // Note: we DON'T have defaultStart() method!
    // Static methods are NOT inherited
}

// Let's prove it
class InheritanceDemo {

    // This would work with default method
    static void testDefault(Counter counter) {
        counter.increment();  // Works! Default is inherited
    }

    // Cannot call static via instance
    static void showStaticDifference() {
        SimpleCounter sc = new SimpleCounter();

        // WORKS - default method via instance
        sc.increment();  // inherited!

        // DOES NOT WORK - static via instance
        // sc.defaultStart();  // COMPILE ERROR!

        // Must use interface name
        int start = Counter.defaultStart();
        System.out.println("Start value: " + start);
    }
}

// What if class has same-named static method?
interface Vehicle {
    static String getType() {
        return "Generic Vehicle";
    }
}

class Car implements Vehicle {
    // This is NOT overriding!
    static String getType() {
        return "Car";
    }

    // It's a completely separate method
    // Car.getType() and Vehicle.getType() are different
}

// Contrast with default methods
interface Animal {
    default String speak() {
        return "Some sound";
    }
}

class Dog implements Animal {
    @Override  // This IS overriding
    public String speak() {
        return "Woof!";
    }
}

public class NotInherited {
    public static void main(String[] args) {
        System.out.println("=== Static vs Default Inheritance ===\n");

        // Default method IS inherited
        System.out.println("--- Default Method (inherited) ---");
        Dog dog = new Dog();
        System.out.println("dog.speak() = " + dog.speak());  // Overridden

        Animal genericAnimal = new Animal() {
            // Uses default
        };
        System.out.println("genericAnimal.speak() = " + genericAnimal.speak());  // Default

        // Static method NOT inherited
        System.out.println("\n--- Static Method (NOT inherited) ---");
        System.out.println("Vehicle.getType() = " + Vehicle.getType());
        System.out.println("Car.getType() = " + Car.getType());  // Different method!

        // They are completely separate
        System.out.println("\n--- Proving Separation ---");
        Vehicle v = new Car();
        // v.getType();  // COMPILE ERROR - no instance method
        System.out.println("Vehicle.getType() on Car instance: " + Vehicle.getType());
        System.out.println("Car.getType() directly: " + Car.getType());

        System.out.println("\n--- Counter Example ---");
        SimpleCounter counter = new SimpleCounter();
        counter.increment();  // Default - inherited
        // counter.defaultStart();  // Static - NOT available!
        System.out.println("Counter.defaultStart() = " + Counter.defaultStart());
        System.out.println("counter.getCount() = " + counter.getCount());

        System.out.println("\n=== Summary ===");
        System.out.println("""
            +-------------------+------------+--------------+
            | Feature           | Default    | Static       |
            +-------------------+------------+--------------+
            | Belongs to        | Instance   | Interface    |
            | Inherited         | YES        | NO           |
            | Can override      | YES        | NO           |
            | Call via instance | YES        | NO           |
            | Call via interface| NO         | YES          |
            +-------------------+------------+--------------+

            Key insight:
            - Default methods behave like instance methods
            - Static methods belong ONLY to the interface
            - Same-named static in class is completely separate
            """);
    }
}

                        
1dog ← ⟨Dog A⟩⋯85public class NotInherited {86    public static void main(String[] args) {87        System.out.println("=== Static vs Default Inheritance ===\n");88        89        // Default method IS inherited //?testdefault90        System.out.println("--- Default Method (inherited) ---");91        Dog dog→ ⟨Dog A⟩ = new Dog();92        System.out.println("dog.speak() = " + dog.speak());  // Overridden⋯output=== Static vs Default Inheritance ===
--- Default Method (inherited) ---
2genericAnimal ← ⟨NotInherited$1 B⟩⋯91Dog dog = new Dog();92System.out.println("dog.speak() = " + dog.speak());  // Overridden9394Animal genericAnimal→ ⟨NotInherited$1 B⟩ = new Animal() {95    // Uses default96};97System.out.println("genericAnimal.speak() = " + genericAnimal.speak());  // Default⋯outputdog.speak() = Woof!
3System.out.println("genericAnimal.speak() = " + genericAnimal.speak())…⋯96};97System.out.println("genericAnimal.speak() = " + genericAnimal.speak());  // Default9899// Static method NOT inherited //?teststatic100System.out.println("\n--- Static Method (NOT inherited) ---");101System.out.println("Vehicle.getType() = " + Vehicle.getType());102System.out.println("Car.getType() = " + Car.getType());  // Different method!⋯outputgenericAnimal.speak() = Some sound

--- Static Method (NOT inherited) ---
4System.out.println("Vehicle.getType() = " + Vehicle.getType());⋯100System.out.println("\n--- Static Method (NOT inherited) ---");101System.out.println("Vehicle.getType() = " + Vehicle.getType());102System.out.println("Car.getType() = " + Car.getType());  // Different method!⋯outputVehicle.getType() = Generic Vehicle
5v ← ⟨Car C⟩⋯101System.out.println("Vehicle.getType() = " + Vehicle.getType());102System.out.println("Car.getType() = " + Car.getType());  // Different method!103104// They are completely separate105System.out.println("\n--- Proving Separation ---");106Vehicle v→ ⟨Car C⟩ = new Car();107// v.getType();  // COMPILE ERROR - no instance method108System.out.println("Vehicle.getType() on Car instance: " + Vehicle.getType());109System.out.println("Car.getType() directly: " + Car.getType());⋯outputCar.getType() = Car

--- Proving Separation ---
6System.out.println("Vehicle.getType() on Car instance: " + Vehicle.get…⋯107// v.getType();  // COMPILE ERROR - no instance method108System.out.println("Vehicle.getType() on Car instance: " + Vehicle.getType());109System.out.println("Car.getType() directly: " + Car.getType());⋯outputVehicle.getType() on Car instance: Generic Vehicle
7System.out.println("Car.getType() directly: " + Car.getType());⋯108System.out.println("Vehicle.getType() on Car instance: " + Vehicle.getType());109System.out.println("Car.getType() directly: " + Car.getType());110111System.out.println("\n--- Counter Example ---");112SimpleCounter counter = new SimpleCounter();113counter.increment();  // Default - inherited⋯outputCar.getType() directly: Car

--- Counter Example ---
8counter ← ⟨SimpleCounter D⟩⋯111System.out.println("\n--- Counter Example ---");112SimpleCounter counter→ ⟨SimpleCounter D⟩ = new SimpleCounter();113counter.increment();  // Default - inherited114// counter.defaultStart();  // Static - NOT available!⋯
9default void increment()⋯9// Default method //?defaultmethod10default void increment() {11    System.out.println("Incrementing...");12}⋯outputIncrementing...
10counter.increment();  // Default - inherited⋯112SimpleCounter counter = new SimpleCounter();113counter.increment();  // Default - inherited114// counter.defaultStart();  // Static - NOT available!115System.out.println("Counter.defaultStart() = " + Counter.defaultStart());116System.out.println("counter.getCount() = " + counter.getCount());⋯
11System.out.println("Counter.defaultStart() = " + Counter.defaultStart(…⋯114// counter.defaultStart();  // Static - NOT available!115System.out.println("Counter.defaultStart() = " + Counter.defaultStart());116System.out.println("counter.getCount() = " + counter.getCount());⋯outputCounter.defaultStart() = 0
12@Override
    public int getCount()⋯21@Override22public int getCount() {23    return count0;24}⋯
13System.out.println("counter.getCount() = " + counter.getCount());⋯115    System.out.println("Counter.defaultStart() = " + Counter.defaultStart());116    System.out.println("counter.getCount() = " + counter.getCount());117    118    System.out.println("\n=== Summary ===");119    System.out.println("""120        +-------------------+------------+--------------+121        | Feature           | Default    | Static       |122        +-------------------+------------+--------------+123        | Belongs to        | Instance   | Interface    |124        | Inherited         | YES        | NO           |125        | Can override      | YES        | NO           |126        | Call via instance | YES        | NO           |127        | Call via interface| NO         | YES          |128        +-------------------+------------+--------------+129        130        Key insight:131        - Default methods behave like instance methods132        - Static methods belong ONLY to the interface133        - Same-named static in class is completely separate134        """);135}⋯outputcounter.getCount() = 0

=== Summary ===
+-------------------+------------+--------------+
| Feature           | Default    | Static       |
+-------------------+------------+--------------+
| Belongs to        | Instance   | Interface    |
| Inherited         | YES        | NO           |
| Can override      | YES        | NO           |
| Call via instance | YES        | NO           |
| Call via interface| NO         | YES          |
+-------------------+------------+--------------+

Key insight:
- Default methods behave like instance methods
- Static methods belong ONLY to the interface
- Same-named static in class is completely separate

Call via interface name only. Implementing class doesn't get the method.

Exercise: Practical.java

Build a complete interface with static factories and utilities

pass	`arr.length`	`item`	`value`
1	6	—	—
2	0	—	—
3	6	8	8
4	6	—	—
5	6	—	—
6	6	—	—
7	6	—	—
8	6	—	—
9	6	—	—
10	6	—	—

pass	`arr.length`	`item`	`value`
1	6	—	—
2	0	—	—
3	6	8	8
4	6	—	—
5	6	—	—
6	6	—	—
7	6	—	—
8	6	—	—
9	6	—	—
10	6	—	—

pass	`arr.length`	`item`	`value`
1	6	—	—
2	0	—	—
3	6	8	8
4	6	—	—
5	6	—	—
6	6	—	—
7	6	—	—
8	6	—	—
9	6	—	—
10	6	—	—