Control Flow
Switch Statement
Multiple Choices
Your app has a menu: 1=New, 2=Open, 3=Save, 4=Quit. With if-else you'd write four conditions. With switch, you list each option clearly - the code mirrors the menu structure perfectly.
Day number to name
Convert a number (1-7) to the day name.
DayName.java
Replay: real traced execution (multi-file project)
public class DayName {
public static void main(String[] args) {
int day = 3;
String name;
switch (day) {
case 1:
name = "Monday";
break;
case 2:
name = "Tuesday";
break;
case 3:
name = "Wednesday";
break;
case 4:
name = "Thursday";
break;
case 5:
name = "Friday";
break;
case 6:
name = "Saturday";
break;
case 7:
name = "Sunday";
break;
default:
name = "Invalid day";
}
System.out.println("Day " + day + " is " + name);
}
}
public class DayName {
public static void main(String[] args) {
int day = 1;
String name;
switch (day) {
case 1:
name = "Monday";
break;
case 2:
name = "Tuesday";
break;
case 3:
name = "Wednesday";
break;
case 4:
name = "Thursday";
break;
case 5:
name = "Friday";
break;
case 6:
name = "Saturday";
break;
case 7:
name = "Sunday";
break;
default:
name = "Invalid day";
}
System.out.println("Day " + day + " is " + name);
}
}
public class DayName {
public static void main(String[] args) {
int day = 5;
String name;
switch (day) {
case 1:
name = "Monday";
break;
case 2:
name = "Tuesday";
break;
case 3:
name = "Wednesday";
break;
case 4:
name = "Thursday";
break;
case 5:
name = "Friday";
break;
case 6:
name = "Saturday";
break;
case 7:
name = "Sunday";
break;
default:
name = "Invalid day";
}
System.out.println("Day " + day + " is " + name);
}
}
public class DayName {
public static void main(String[] args) {
int day = 7;
String name;
switch (day) {
case 1:
name = "Monday";
break;
case 2:
name = "Tuesday";
break;
case 3:
name = "Wednesday";
break;
case 4:
name = "Thursday";
break;
case 5:
name = "Friday";
break;
case 6:
name = "Saturday";
break;
case 7:
name = "Sunday";
break;
default:
name = "Invalid day";
}
System.out.println("Day " + day + " is " + name);
}
}
day ← 3
1public class DayName {2 public static void main(String[] args) {3 int day→ 3 = 3; //@day=1, 5, 74 String name;switch (day)
6switch (day3) {7 case 1:8 name = "Monday";name ← Wednesday
12 break;13case 3:14 name→ Wednesday = "Wednesday";15 break;16case 4:System.out.println("Day " + day + " is " + name);
32 System.out.println("Day " + day3 + " is " + nameWednesday);33}outputDay 3 is Wednesday
day ← 1
1public class DayName {2 public static void main(String[] args) {3 int day→ 1 = 1;4 String name;switch (day)
6switch (day1) {7 case 1:8 name = "Monday";name ← Monday
6switch (day) {7 case 1:8 name→ Monday = "Monday";9 break;10 case 2:System.out.println("Day " + day + " is " + name);
32 System.out.println("Day " + day1 + " is " + nameMonday);33}outputDay 1 is Monday
day ← 5
1public class DayName {2 public static void main(String[] args) {3 int day→ 5 = 5;4 String name;switch (day)
6switch (day5) {7 case 1:8 name = "Monday";name ← Friday
18 break;19case 5:20 name→ Friday = "Friday";21 break;22case 6:System.out.println("Day " + day + " is " + name);
32 System.out.println("Day " + day5 + " is " + nameFriday);33}outputDay 5 is Friday
day ← 7
1public class DayName {2 public static void main(String[] args) {3 int day→ 7 = 7;4 String name;switch (day)
6switch (day7) {7 case 1:8 name = "Monday";name ← Sunday
24 break;25case 7:26 name→ Sunday = "Sunday";27 break;28default:System.out.println("Day " + day + " is " + name);
32 System.out.println("Day " + day7 + " is " + nameSunday);33}outputDay 7 is Sunday
Each case handles one value. Don't forget break or execution falls through!
switch
Multi-way branch based on a value. Cleaner than long if-else chains.
case
A specific value to match: `case 1:`, `case 'A':`, `case "yes":`
break
Exit the switch. Without it, execution "falls through" to next case.
Menu option handler
Handle user menu selections with switch.
Menu.java
Replay: real traced execution (multi-file project)
public class Menu {
public static void main(String[] args) {
int choice = 2;
System.out.println("=== Main Menu ===");
System.out.println("1. New Game");
System.out.println("2. Load Game");
System.out.println("3. Settings");
System.out.println("4. Exit");
System.out.println("Choice: " + choice);
System.out.println();
switch (choice) {
case 1:
System.out.println("Starting new game...");
break;
case 2:
System.out.println("Loading saved game...");
break;
case 3:
System.out.println("Opening settings...");
break;
case 4:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid option. Please try again.");
}
}
}
public class Menu {
public static void main(String[] args) {
int choice = 1;
System.out.println("=== Main Menu ===");
System.out.println("1. New Game");
System.out.println("2. Load Game");
System.out.println("3. Settings");
System.out.println("4. Exit");
System.out.println("Choice: " + choice);
System.out.println();
switch (choice) {
case 1:
System.out.println("Starting new game...");
break;
case 2:
System.out.println("Loading saved game...");
break;
case 3:
System.out.println("Opening settings...");
break;
case 4:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid option. Please try again.");
}
}
}
public class Menu {
public static void main(String[] args) {
int choice = 3;
System.out.println("=== Main Menu ===");
System.out.println("1. New Game");
System.out.println("2. Load Game");
System.out.println("3. Settings");
System.out.println("4. Exit");
System.out.println("Choice: " + choice);
System.out.println();
switch (choice) {
case 1:
System.out.println("Starting new game...");
break;
case 2:
System.out.println("Loading saved game...");
break;
case 3:
System.out.println("Opening settings...");
break;
case 4:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid option. Please try again.");
}
}
}
public class Menu {
public static void main(String[] args) {
int choice = 4;
System.out.println("=== Main Menu ===");
System.out.println("1. New Game");
System.out.println("2. Load Game");
System.out.println("3. Settings");
System.out.println("4. Exit");
System.out.println("Choice: " + choice);
System.out.println();
switch (choice) {
case 1:
System.out.println("Starting new game...");
break;
case 2:
System.out.println("Loading saved game...");
break;
case 3:
System.out.println("Opening settings...");
break;
case 4:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid option. Please try again.");
}
}
}
public class Menu {
public static void main(String[] args) {
int choice = 9;
System.out.println("=== Main Menu ===");
System.out.println("1. New Game");
System.out.println("2. Load Game");
System.out.println("3. Settings");
System.out.println("4. Exit");
System.out.println("Choice: " + choice);
System.out.println();
switch (choice) {
case 1:
System.out.println("Starting new game...");
break;
case 2:
System.out.println("Loading saved game...");
break;
case 3:
System.out.println("Opening settings...");
break;
case 4:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid option. Please try again.");
}
}
}
choice ← 2
1public class Menu {2 public static void main(String[] args) {3 int choice→ 2 = 2; //@choice=1, 3, 4, 94 5 System.out.println("=== Main Menu ===");6 System.out.println("1. New Game");7 System.out.println("2. Load Game");8 System.out.println("3. Settings");9 System.out.println("4. Exit");10 System.out.println("Choice: " + choice2);11 System.out.println();output=== Main Menu === 1. New Game 2. Load Game 3. Settings 4. Exit Choice: 2switch (choice)
13switch (choice2) {14 case 1:15 System.out.println("Starting new game...");case 2:
16 break;17case 2:18 System.out.println("Loading saved game...");19 break;20case 3:outputLoading saved game...
choice ← 1
1public class Menu {2 public static void main(String[] args) {3 int choice→ 1 = 1;4 5 System.out.println("=== Main Menu ===");6 System.out.println("1. New Game");7 System.out.println("2. Load Game");8 System.out.println("3. Settings");9 System.out.println("4. Exit");10 System.out.println("Choice: " + choice1);11 System.out.println();output=== Main Menu === 1. New Game 2. Load Game 3. Settings 4. Exit Choice: 1switch (choice)
13switch (choice1) {14 case 1:15 System.out.println("Starting new game...");case 1:
13switch (choice) {14 case 1:15 System.out.println("Starting new game...");16 break;17 case 2:outputStarting new game...
choice ← 3
1public class Menu {2 public static void main(String[] args) {3 int choice→ 3 = 3;4 5 System.out.println("=== Main Menu ===");6 System.out.println("1. New Game");7 System.out.println("2. Load Game");8 System.out.println("3. Settings");9 System.out.println("4. Exit");10 System.out.println("Choice: " + choice3);11 System.out.println();output=== Main Menu === 1. New Game 2. Load Game 3. Settings 4. Exit Choice: 3switch (choice)
13switch (choice3) {14 case 1:15 System.out.println("Starting new game...");case 3:
19 break;20case 3:21 System.out.println("Opening settings...");22 break;23case 4:outputOpening settings...
choice ← 4
1public class Menu {2 public static void main(String[] args) {3 int choice→ 4 = 4;4 5 System.out.println("=== Main Menu ===");6 System.out.println("1. New Game");7 System.out.println("2. Load Game");8 System.out.println("3. Settings");9 System.out.println("4. Exit");10 System.out.println("Choice: " + choice4);11 System.out.println();output=== Main Menu === 1. New Game 2. Load Game 3. Settings 4. Exit Choice: 4switch (choice)
13switch (choice4) {14 case 1:15 System.out.println("Starting new game...");case 4:
22 break;23case 4:24 System.out.println("Goodbye!");25 break;26default:outputGoodbye!
choice ← 9
1public class Menu {2 public static void main(String[] args) {3 int choice→ 9 = 9;4 5 System.out.println("=== Main Menu ===");6 System.out.println("1. New Game");7 System.out.println("2. Load Game");8 System.out.println("3. Settings");9 System.out.println("4. Exit");10 System.out.println("Choice: " + choice9);11 System.out.println();output=== Main Menu === 1. New Game 2. Load Game 3. Settings 4. Exit Choice: 9switch (choice)
13switch (choice9) {14 case 1:15 System.out.println("Starting new game...");default:
25 break;26 default:27 System.out.println("Invalid option. Please try again.");28}outputInvalid option. Please try again.
Switch is perfect for menu systems - each option maps to an action.
Grade to description
Convert letter grades to descriptions.
Grade.java
Replay: real traced execution (multi-file project)
public class Grade {
public static void main(String[] args) {
char grade = 'B';
String description;
switch (grade) {
case 'A':
description = "Excellent work!";
break;
case 'B':
description = "Good job!";
break;
case 'C':
description = "Satisfactory";
break;
case 'D':
description = "Needs improvement";
break;
case 'F':
description = "Please see instructor";
break;
default:
description = "Invalid grade";
}
System.out.println("Grade: " + grade);
System.out.println("Feedback: " + description);
}
}
public class Grade {
public static void main(String[] args) {
char grade = 'A';
String description;
switch (grade) {
case 'A':
description = "Excellent work!";
break;
case 'B':
description = "Good job!";
break;
case 'C':
description = "Satisfactory";
break;
case 'D':
description = "Needs improvement";
break;
case 'F':
description = "Please see instructor";
break;
default:
description = "Invalid grade";
}
System.out.println("Grade: " + grade);
System.out.println("Feedback: " + description);
}
}
public class Grade {
public static void main(String[] args) {
char grade = 'C';
String description;
switch (grade) {
case 'A':
description = "Excellent work!";
break;
case 'B':
description = "Good job!";
break;
case 'C':
description = "Satisfactory";
break;
case 'D':
description = "Needs improvement";
break;
case 'F':
description = "Please see instructor";
break;
default:
description = "Invalid grade";
}
System.out.println("Grade: " + grade);
System.out.println("Feedback: " + description);
}
}
public class Grade {
public static void main(String[] args) {
char grade = 'F';
String description;
switch (grade) {
case 'A':
description = "Excellent work!";
break;
case 'B':
description = "Good job!";
break;
case 'C':
description = "Satisfactory";
break;
case 'D':
description = "Needs improvement";
break;
case 'F':
description = "Please see instructor";
break;
default:
description = "Invalid grade";
}
System.out.println("Grade: " + grade);
System.out.println("Feedback: " + description);
}
}
public class Grade {
public static void main(String[] args) {
char grade = 'X';
String description;
switch (grade) {
case 'A':
description = "Excellent work!";
break;
case 'B':
description = "Good job!";
break;
case 'C':
description = "Satisfactory";
break;
case 'D':
description = "Needs improvement";
break;
case 'F':
description = "Please see instructor";
break;
default:
description = "Invalid grade";
}
System.out.println("Grade: " + grade);
System.out.println("Feedback: " + description);
}
}
grade ← B
1public class Grade {2 public static void main(String[] args) {3 char grade→ B = 'B'; //@grade='A', 'C', 'F', 'X'4 String description;switch (grade)
6switch (gradeB) {7 case 'A':8 description = "Excellent work!";description ← Good job!
9 break;10case 'B':11 description→ Good job! = "Good job!";12 break;13case 'C':System.out.println("Grade: " + grade);
26 System.out.println("Grade: " + gradeB);27 System.out.println("Feedback: " + descriptionGood job!);28}outputGrade: B Feedback: Good job!
grade ← A
1public class Grade {2 public static void main(String[] args) {3 char grade→ A = 'A';4 String description;switch (grade)
6switch (gradeA) {7 case 'A':8 description = "Excellent work!";description ← Excellent work!
6switch (grade) {7 case 'A':8 description→ Excellent work! = "Excellent work!";9 break;10 case 'B':System.out.println("Grade: " + grade);
26 System.out.println("Grade: " + gradeA);27 System.out.println("Feedback: " + descriptionExcellent work!);28}outputGrade: A Feedback: Excellent work!
grade ← C
1public class Grade {2 public static void main(String[] args) {3 char grade→ C = 'C';4 String description;switch (grade)
6switch (gradeC) {7 case 'A':8 description = "Excellent work!";description ← Satisfactory
12 break;13case 'C':14 description→ Satisfactory = "Satisfactory";15 break;16case 'D':System.out.println("Grade: " + grade);
26 System.out.println("Grade: " + gradeC);27 System.out.println("Feedback: " + descriptionSatisfactory);28}outputGrade: C Feedback: Satisfactory
grade ← F
1public class Grade {2 public static void main(String[] args) {3 char grade→ F = 'F';4 String description;switch (grade)
6switch (gradeF) {7 case 'A':8 description = "Excellent work!";description ← Please see instructor
18 break;19case 'F':20 description→ Please see instructor = "Please see instructor";21 break;22default:System.out.println("Grade: " + grade);
26 System.out.println("Grade: " + gradeF);27 System.out.println("Feedback: " + descriptionPlease see instructor);28}outputGrade: F Feedback: Please see instructor
grade ← X
1public class Grade {2 public static void main(String[] args) {3 char grade→ X = 'X';4 String description;switch (grade)
6switch (gradeX) {7 case 'A':8 description = "Excellent work!";description ← Invalid grade
21 break;22 default:23 description→ Invalid grade = "Invalid grade";24}System.out.println("Grade: " + grade);
26 System.out.println("Grade: " + gradeX);27 System.out.println("Feedback: " + descriptionInvalid grade);28}outputGrade: X Feedback: Invalid grade
default handles any value not matched by a case.
default
Handles any value not matched by a case. Like the final `else`.
Calculator operator
Implement a basic calculator using switch.
Calculator.java
Replay: real traced execution (multi-file project)
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 3;
char op = '/';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 20;
double b = 3;
char op = '/';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 15.5;
double b = 3;
char op = '/';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 2;
char op = '/';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 4;
char op = '/';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 3;
char op = '+';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 3;
char op = '-';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 3;
char op = '*';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
public class Calculator {
public static void main(String[] args) {
double a = 10;
double b = 3;
char op = '%';
double result;
boolean valid = true;
switch (op) {
case '+':
result = a + b;
break;
case '-':
result = a - b;
break;
case '*':
result = a * b;
break;
case '/':
if (b != 0) {
result = a / b;
} else {
System.out.println("Error: Division by zero");
result = 0;
valid = false;
}
break;
case '%':
result = a % b;
break;
default:
System.out.println("Unknown operator: " + op);
result = 0;
valid = false;
}
if (valid) {
System.out.println(a + " " + op + " " + b + " = " + result);
}
}
}
a ← 10.0, b ← 3.0, op ← /, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10; //@a=20, 15.54 double b→ 3.0 = 3; //@b=4, 25 char op→ / = '/'; //@op='+', '-', '*', '%'6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op/) {11 case '+':12 result = a + b;result ← 3.3333333333333335
20case '/':21 if (b3.0 != 0) {22 result→ 3.3333333333333335 = a10.0 / b3.0;23 } else {break;
27 }28 break;29case '%':if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op/ + " " + b3.0 + " = " + result3.3333333333333335);40}output10.0 / 3.0 = 3.3333333333333335
a ← 20.0, b ← 3.0, op ← /, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 20.0 = 20;4 double b→ 3.0 = 3;5 char op→ / = '/';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op/) {11 case '+':12 result = a + b;result ← 6.666666666666667
20case '/':21 if (b3.0 != 0) {22 result→ 6.666666666666667 = a20.0 / b3.0;23 } else {break;
27 }28 break;29case '%':if (valid)
38if (validtrue) {39 System.out.println(a20.0 + " " + op/ + " " + b3.0 + " = " + result6.666666666666667);40}output20.0 / 3.0 = 6.666666666666667
a ← 15.5, b ← 3.0, op ← /, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 15.5 = 15.5;4 double b→ 3.0 = 3;5 char op→ / = '/';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op/) {11 case '+':12 result = a + b;result ← 5.166666666666667
20case '/':21 if (b3.0 != 0) {22 result→ 5.166666666666667 = a15.5 / b3.0;23 } else {break;
27 }28 break;29case '%':if (valid)
38if (validtrue) {39 System.out.println(a15.5 + " " + op/ + " " + b3.0 + " = " + result5.166666666666667);40}output15.5 / 3.0 = 5.166666666666667
a ← 10.0, b ← 2.0, op ← /, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10;4 double b→ 2.0 = 2;5 char op→ / = '/';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op/) {11 case '+':12 result = a + b;result ← 5.0
20case '/':21 if (b2.0 != 0) {22 result→ 5.0 = a10.0 / b2.0;23 } else {break;
27 }28 break;29case '%':if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op/ + " " + b2.0 + " = " + result5.0);40}output10.0 / 2.0 = 5.0
a ← 10.0, b ← 4.0, op ← /, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10;4 double b→ 4.0 = 4;5 char op→ / = '/';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op/) {11 case '+':12 result = a + b;result ← 2.5
20case '/':21 if (b4.0 != 0) {22 result→ 2.5 = a10.0 / b4.0;23 } else {break;
27 }28 break;29case '%':if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op/ + " " + b4.0 + " = " + result2.5);40}output10.0 / 4.0 = 2.5
a ← 10.0, b ← 3.0, op ← +, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10;4 double b→ 3.0 = 3;5 char op→ + = '+';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op+) {11 case '+':12 result = a + b;result ← 13.0
10switch (op) {11 case '+':12 result→ 13.0 = a10.0 + b3.0;13 break;14 case '-':if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op+ + " " + b3.0 + " = " + result13.0);40}output10.0 + 3.0 = 13.0
a ← 10.0, b ← 3.0, op ← -, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10;4 double b→ 3.0 = 3;5 char op→ - = '-';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op-) {11 case '+':12 result = a + b;result ← 7.0
13 break;14case '-':15 result→ 7.0 = a10.0 - b3.0;16 break;17case '*':if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op- + " " + b3.0 + " = " + result7.0);40}output10.0 - 3.0 = 7.0
a ← 10.0, b ← 3.0, op ← *, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10;4 double b→ 3.0 = 3;5 char op→ * = '*';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op*) {11 case '+':12 result = a + b;result ← 30.0
16 break;17case '*':18 result→ 30.0 = a10.0 * b3.0;19 break;20case '/':if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op* + " " + b3.0 + " = " + result30.0);40}output10.0 * 3.0 = 30.0
a ← 10.0, b ← 3.0, op ← %, valid ← true
1public class Calculator {2 public static void main(String[] args) {3 double a→ 10.0 = 10;4 double b→ 3.0 = 3;5 char op→ % = '%';6 7 double result;8 boolean valid→ true = true;switch (op)
10switch (op%) {11 case '+':12 result = a + b;result ← 1.0
28 break;29case '%':30 result→ 1.0 = a10.0 % b3.0;31 break;32default:if (valid)
38if (validtrue) {39 System.out.println(a10.0 + " " + op% + " " + b3.0 + " = " + result1.0);40}output10.0 % 3.0 = 1.0
Switch on char works just like switching on int.
Multiple cases, same action
Group cases that should do the same thing.
Grouped.java
Replay: real traced execution (multi-file project)
public class Grouped {
public static void main(String[] args) {
int month = 4;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 6;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
public class Grouped {
public static void main(String[] args) {
int month = 1;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 6;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
public class Grouped {
public static void main(String[] args) {
int month = 6;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 6;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
public class Grouped {
public static void main(String[] args) {
int month = 11;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 6;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
public class Grouped {
public static void main(String[] args) {
int month = 4;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 1;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
public class Grouped {
public static void main(String[] args) {
int month = 4;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 3;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
public class Grouped {
public static void main(String[] args) {
int month = 4;
String season;
switch (month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6:
case 7:
case 8:
season = "Summer";
break;
case 9:
case 10:
case 11:
season = "Autumn";
break;
default:
season = "Invalid month";
}
System.out.println("Month " + month + " is in " + season);
// Another example: weekday vs weekend
int day = 7;
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5:
System.out.println("Day " + day + ": Weekday");
break;
case 6:
case 7:
System.out.println("Day " + day + ": Weekend!");
break;
}
}
}
month ← 4
1public class Grouped {2 public static void main(String[] args) {3 int month→ 4 = 4; //@month=1, 6, 114 String season;switch (month)
6switch (month4) {7 case 12:8 case 1:season ← Spring
13case 4:14case 5:15 season→ Spring = "Spring";16 break;17case 6:day ← 6
31System.out.println("Month " + month4 + " is in " + seasonSpring);3233// Another example: weekday vs weekend34int day→ 6 = 6; //@day=1, 3, 735switch (day) {outputMonth 4 is in Springswitch (day)
34int day = 6; //@day=1, 3, 735switch (day6) {36 case 1:37 case 2:case 7:
43 case 6:44 case 7:45 System.out.println("Day " + day6 + ": Weekend!");46 break;47}outputDay 6: Weekend!
month ← 1
1public class Grouped {2 public static void main(String[] args) {3 int month→ 1 = 1;4 String season;switch (month)
6switch (month1) {7 case 12:8 case 1:season ← Winter
8case 1:9case 2:10 season→ Winter = "Winter";11 break;12case 3:day ← 6
31System.out.println("Month " + month1 + " is in " + seasonWinter);3233// Another example: weekday vs weekend34int day→ 6 = 6;35switch (day) {outputMonth 1 is in Winterswitch (day)
34int day = 6;35switch (day6) {36 case 1:37 case 2:case 7:
43 case 6:44 case 7:45 System.out.println("Day " + day6 + ": Weekend!");46 break;47}outputDay 6: Weekend!
month ← 6
1public class Grouped {2 public static void main(String[] args) {3 int month→ 6 = 6;4 String season;switch (month)
6switch (month6) {7 case 12:8 case 1:season ← Summer
18case 7:19case 8:20 season→ Summer = "Summer";21 break;22case 9:day ← 6
31System.out.println("Month " + month6 + " is in " + seasonSummer);3233// Another example: weekday vs weekend34int day→ 6 = 6;35switch (day) {outputMonth 6 is in Summerswitch (day)
34int day = 6;35switch (day6) {36 case 1:37 case 2:case 7:
43 case 6:44 case 7:45 System.out.println("Day " + day6 + ": Weekend!");46 break;47}outputDay 6: Weekend!
month ← 11
1public class Grouped {2 public static void main(String[] args) {3 int month→ 11 = 11;4 String season;switch (month)
6switch (month11) {7 case 12:8 case 1:season ← Autumn
23case 10:24case 11:25 season→ Autumn = "Autumn";26 break;27default:day ← 6
31System.out.println("Month " + month11 + " is in " + seasonAutumn);3233// Another example: weekday vs weekend34int day→ 6 = 6;35switch (day) {outputMonth 11 is in Autumnswitch (day)
34int day = 6;35switch (day6) {36 case 1:37 case 2:case 7:
43 case 6:44 case 7:45 System.out.println("Day " + day6 + ": Weekend!");46 break;47}outputDay 6: Weekend!
month ← 4
1public class Grouped {2 public static void main(String[] args) {3 int month→ 4 = 4;4 String season;switch (month)
6switch (month4) {7 case 12:8 case 1:season ← Spring
13case 4:14case 5:15 season→ Spring = "Spring";16 break;17case 6:day ← 1
31System.out.println("Month " + month4 + " is in " + seasonSpring);3233// Another example: weekday vs weekend34int day→ 1 = 1;35switch (day) {outputMonth 4 is in Springswitch (day)
34int day = 1;35switch (day1) {36 case 1:37 case 2:case 5:
39case 4:40case 5:41 System.out.println("Day " + day1 + ": Weekday");42 break;43case 6:outputDay 1: Weekday
month ← 4
1public class Grouped {2 public static void main(String[] args) {3 int month→ 4 = 4;4 String season;switch (month)
6switch (month4) {7 case 12:8 case 1:season ← Spring
13case 4:14case 5:15 season→ Spring = "Spring";16 break;17case 6:day ← 3
31System.out.println("Month " + month4 + " is in " + seasonSpring);3233// Another example: weekday vs weekend34int day→ 3 = 3;35switch (day) {outputMonth 4 is in Springswitch (day)
34int day = 3;35switch (day3) {36 case 1:37 case 2:case 5:
39case 4:40case 5:41 System.out.println("Day " + day3 + ": Weekday");42 break;43case 6:outputDay 3: Weekday
month ← 4
1public class Grouped {2 public static void main(String[] args) {3 int month→ 4 = 4;4 String season;switch (month)
6switch (month4) {7 case 12:8 case 1:season ← Spring
13case 4:14case 5:15 season→ Spring = "Spring";16 break;17case 6:day ← 7
31System.out.println("Month " + month4 + " is in " + seasonSpring);3233// Another example: weekday vs weekend34int day→ 7 = 7;35switch (day) {outputMonth 4 is in Springswitch (day)
34int day = 7;35switch (day7) {36 case 1:37 case 2:case 7:
43 case 6:44 case 7:45 System.out.println("Day " + day7 + ": Weekend!");46 break;47}outputDay 7: Weekend!
Fall-through (no break) lets multiple cases share code.
Exercise: EnhancedSwitch.java
Explore enhanced switch (Java 14+): arrow syntax, no fall-through, returns values