Enums - Java Programming

Your app uses day-of-week values. Using integers (1=Monday, 2=Tuesday) is error-prone - what if someone passes 8? Enums define a fixed set of valid values. The compiler catches invalid values at compile time.

Define an enum

Create a type with fixed set of values.

                            example
                            today = DayOfWeek.MONDAY, weekend = DayOfWeek.SATURDAY (default)today = DayOfWeek.FRIDAYtoday = DayOfWeek.SUNDAYweekend = DayOfWeek.SUNDAY
                        

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

                            // Basic enum definition and usage
// Concept: enum - type-safe constants

enum DayOfWeek {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

public class DaysOfWeek {
    public static void main(String[] args) {
        // Basic enum usage
        DayOfWeek today = DayOfWeek.MONDAY;
        System.out.println("Today is: " + today);

        // Compare enums with ==
        if (today == DayOfWeek.MONDAY) {
            System.out.println("Start of work week");
        }

        // Get enum name as string
        String dayName = today.name();
        System.out.println("Day name: " + dayName);

        // Get ordinal position (0-based index)
        int position = today.ordinal();
        System.out.println("Position in week: " + position);


        // Try different day
        DayOfWeek weekend = DayOfWeek.SATURDAY;
        System.out.println("\nWeekend day: " + weekend);
        System.out.println("Ordinal: " + weekend.ordinal());

    }
}
// Basic enum definition and usage
// Concept: enum - type-safe constants

enum DayOfWeek {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

public class DaysOfWeek {
    public static void main(String[] args) {
        // Basic enum usage
        DayOfWeek today = DayOfWeek.FRIDAY;
        System.out.println("Today is: " + today);

        // Compare enums with ==
        if (today == DayOfWeek.MONDAY) {
            System.out.println("Start of work week");
        }

        // Get enum name as string
        String dayName = today.name();
        System.out.println("Day name: " + dayName);

        // Get ordinal position (0-based index)
        int position = today.ordinal();
        System.out.println("Position in week: " + position);


        // Try different day
        DayOfWeek weekend = DayOfWeek.SATURDAY;
        System.out.println("\nWeekend day: " + weekend);
        System.out.println("Ordinal: " + weekend.ordinal());

    }
}
// Basic enum definition and usage
// Concept: enum - type-safe constants

enum DayOfWeek {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

public class DaysOfWeek {
    public static void main(String[] args) {
        // Basic enum usage
        DayOfWeek today = DayOfWeek.SUNDAY;
        System.out.println("Today is: " + today);

        // Compare enums with ==
        if (today == DayOfWeek.MONDAY) {
            System.out.println("Start of work week");
        }

        // Get enum name as string
        String dayName = today.name();
        System.out.println("Day name: " + dayName);

        // Get ordinal position (0-based index)
        int position = today.ordinal();
        System.out.println("Position in week: " + position);


        // Try different day
        DayOfWeek weekend = DayOfWeek.SATURDAY;
        System.out.println("\nWeekend day: " + weekend);
        System.out.println("Ordinal: " + weekend.ordinal());

    }
}
// Basic enum definition and usage
// Concept: enum - type-safe constants

enum DayOfWeek {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

public class DaysOfWeek {
    public static void main(String[] args) {
        // Basic enum usage
        DayOfWeek today = DayOfWeek.MONDAY;
        System.out.println("Today is: " + today);

        // Compare enums with ==
        if (today == DayOfWeek.MONDAY) {
            System.out.println("Start of work week");
        }

        // Get enum name as string
        String dayName = today.name();
        System.out.println("Day name: " + dayName);

        // Get ordinal position (0-based index)
        int position = today.ordinal();
        System.out.println("Position in week: " + position);


        // Try different day
        DayOfWeek weekend = DayOfWeek.SUNDAY;
        System.out.println("\nWeekend day: " + weekend);
        System.out.println("Ordinal: " + weekend.ordinal());

    }
}

                        
1today ← MONDAY⋯8public class DaysOfWeek {9    public static void main(String[] args) {10        // Basic enum usage11        DayOfWeek today→ MONDAY = DayOfWeek.MONDAY;12        //@today=DayOfWeek.MONDAY, DayOfWeek.FRIDAY, DayOfWeek.SUNDAY13        System.out.println("Today is: " + todayMONDAY);⋯outputToday is: MONDAY
2if (today == DayOfWeek.MONDAY)⋯15// Compare enums with ==16if (todayMONDAY == DayOfWeek.MONDAY) {17    System.out.println("Start of work week");18}⋯outputStart of work week
3dayName ← MONDAY, position ← 0, weekend ← SATURDAY⋯20// Get enum name as string21String dayName→ MONDAY = today.name();22System.out.println("Day name: " + dayNameMONDAY);2324// Get ordinal position (0-based index)25int position→ 0 = today.ordinal();26System.out.println("Position in week: " + position0);2728//@help h129// Why use enum instead of String?30// 1. Type safety - compiler catches typos31// 2. Can't accidentally assign "Mondy" or "MONNDAY"32// 3. IDE autocomplete shows all valid options33//@end3435// Try different day36DayOfWeek weekend→ SATURDAY = DayOfWeek.SATURDAY;37//@weekend=DayOfWeek.SATURDAY, DayOfWeek.SUNDAY38System.out.println("\nWeekend day: " + weekendSATURDAY);39System.out.println("Ordinal: " + weekend.ordinal());⋯outputDay name: MONDAY
Position in week: 0

Weekend day: SATURDAY
Ordinal: 5
1today ← FRIDAY, dayName ← FRIDAY, position ← 4, weekend ← SATURDAY⋯8public class DaysOfWeek {9    public static void main(String[] args) {10        // Basic enum usage11        DayOfWeek today→ FRIDAY = DayOfWeek.FRIDAY;12        System.out.println("Today is: " + todayFRIDAY);13        14        // Compare enums with ==15        if (today == DayOfWeek.MONDAY) {16            System.out.println("Start of work week");17        }18        19        // Get enum name as string20        String dayName→ FRIDAY = today.name();21        System.out.println("Day name: " + dayNameFRIDAY);22        23        // Get ordinal position (0-based index)24        int position→ 4 = today.ordinal();25        System.out.println("Position in week: " + position4);26        27        28        // Try different day29        DayOfWeek weekend→ SATURDAY = DayOfWeek.SATURDAY;30        System.out.println("\nWeekend day: " + weekendSATURDAY);31        System.out.println("Ordinal: " + weekend.ordinal());⋯outputToday is: FRIDAY
Day name: FRIDAY
Position in week: 4

Weekend day: SATURDAY
Ordinal: 5
1today ← SUNDAY, dayName ← SUNDAY, position ← 6, weekend ← SATURDAY⋯8public class DaysOfWeek {9    public static void main(String[] args) {10        // Basic enum usage11        DayOfWeek today→ SUNDAY = DayOfWeek.SUNDAY;12        System.out.println("Today is: " + todaySUNDAY);13        14        // Compare enums with ==15        if (today == DayOfWeek.MONDAY) {16            System.out.println("Start of work week");17        }18        19        // Get enum name as string20        String dayName→ SUNDAY = today.name();21        System.out.println("Day name: " + dayNameSUNDAY);22        23        // Get ordinal position (0-based index)24        int position→ 6 = today.ordinal();25        System.out.println("Position in week: " + position6);26        27        28        // Try different day29        DayOfWeek weekend→ SATURDAY = DayOfWeek.SATURDAY;30        System.out.println("\nWeekend day: " + weekendSATURDAY);31        System.out.println("Ordinal: " + weekend.ordinal());⋯outputToday is: SUNDAY
Day name: SUNDAY
Position in week: 6

Weekend day: SATURDAY
Ordinal: 5
1today ← MONDAY⋯8public class DaysOfWeek {9    public static void main(String[] args) {10        // Basic enum usage11        DayOfWeek today→ MONDAY = DayOfWeek.MONDAY;12        System.out.println("Today is: " + todayMONDAY);⋯outputToday is: MONDAY
2if (today == DayOfWeek.MONDAY)⋯14// Compare enums with ==15if (todayMONDAY == DayOfWeek.MONDAY) {16    System.out.println("Start of work week");17}⋯outputStart of work week
3dayName ← MONDAY, position ← 0, weekend ← SUNDAY⋯19// Get enum name as string20String dayName→ MONDAY = today.name();21System.out.println("Day name: " + dayNameMONDAY);2223// Get ordinal position (0-based index)24int position→ 0 = today.ordinal();25System.out.println("Position in week: " + position0);262728// Try different day29DayOfWeek weekend→ SUNDAY = DayOfWeek.SUNDAY;30System.out.println("\nWeekend day: " + weekendSUNDAY);31System.out.println("Ordinal: " + weekend.ordinal());⋯outputDay name: MONDAY
Position in week: 0

Weekend day: SUNDAY
Ordinal: 6

enum Name { VALUE1, VALUE2 } defines the allowed values.

enum Type-safe named constants. Fixed set of values known at compile time.

Access enum values

Use enum values in your code.

                            input
                            input = "HIGH" (default)input = "LOW"input = "URGENT"
                        

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

                            // Accessing all enum values and converting from string
// Concept: values() - get all enum constants
// Concept: valueOf() - string to enum conversion

enum Priority {
    LOW, MEDIUM, HIGH, URGENT
}

public class AccessValues {
    public static void main(String[] args) {
        // Get all enum values using values()
        Priority[] allPriorities = Priority.values();

        System.out.println("All priority levels:");
        for (Priority p : allPriorities) {
            System.out.println("  " + p);
        }

        // Count total values
        System.out.println("\nTotal priorities: " + allPriorities.length);

        // Convert string to enum using valueOf()
        String input = "HIGH";
        Priority taskPriority = Priority.valueOf(input);
        System.out.println("\nTask priority: " + taskPriority);


        // Check if specific value exists
        Priority current = Priority.MEDIUM;
        boolean isUrgent = (current == Priority.URGENT);
        System.out.println("Is urgent? " + isUrgent);

        // Print with ordinal
        System.out.println("\nPriorities with ordinal:");
        for (Priority p : Priority.values()) {
            System.out.println(p.ordinal() + ": " + p);
        }

    }
}
// Accessing all enum values and converting from string
// Concept: values() - get all enum constants
// Concept: valueOf() - string to enum conversion

enum Priority {
    LOW, MEDIUM, HIGH, URGENT
}

public class AccessValues {
    public static void main(String[] args) {
        // Get all enum values using values()
        Priority[] allPriorities = Priority.values();

        System.out.println("All priority levels:");
        for (Priority p : allPriorities) {
            System.out.println("  " + p);
        }

        // Count total values
        System.out.println("\nTotal priorities: " + allPriorities.length);

        // Convert string to enum using valueOf()
        String input = "LOW";
        Priority taskPriority = Priority.valueOf(input);
        System.out.println("\nTask priority: " + taskPriority);


        // Check if specific value exists
        Priority current = Priority.MEDIUM;
        boolean isUrgent = (current == Priority.URGENT);
        System.out.println("Is urgent? " + isUrgent);

        // Print with ordinal
        System.out.println("\nPriorities with ordinal:");
        for (Priority p : Priority.values()) {
            System.out.println(p.ordinal() + ": " + p);
        }

    }
}
// Accessing all enum values and converting from string
// Concept: values() - get all enum constants
// Concept: valueOf() - string to enum conversion

enum Priority {
    LOW, MEDIUM, HIGH, URGENT
}

public class AccessValues {
    public static void main(String[] args) {
        // Get all enum values using values()
        Priority[] allPriorities = Priority.values();

        System.out.println("All priority levels:");
        for (Priority p : allPriorities) {
            System.out.println("  " + p);
        }

        // Count total values
        System.out.println("\nTotal priorities: " + allPriorities.length);

        // Convert string to enum using valueOf()
        String input = "URGENT";
        Priority taskPriority = Priority.valueOf(input);
        System.out.println("\nTask priority: " + taskPriority);


        // Check if specific value exists
        Priority current = Priority.MEDIUM;
        boolean isUrgent = (current == Priority.URGENT);
        System.out.println("Is urgent? " + isUrgent);

        // Print with ordinal
        System.out.println("\nPriorities with ordinal:");
        for (Priority p : Priority.values()) {
            System.out.println(p.ordinal() + ": " + p);
        }

    }
}

                        
1public static void main(String[] args)⋯9public class AccessValues {10    public static void main(String[] args) {11        // Get all enum values using values()12        Priority[] allPriorities = Priority.values();13        14        System.out.println("All priority levels:");15        for (Priority p : allPriorities) {⋯outputAll priority levels:
2for (Priority p : allPriorities)pass 1 of 4⋯14System.out.println("All priority levels:");15for (Priority pLOW : allPriorities) {16    System.out.println("  " + pLOW);17}⋯output  LOWAll 4 passes — pass 1 is the card abovepassp
1LOW
2MEDIUM
3HIGH
4URGENT
3input ← HIGH, taskPriority ← HIGH, current ← MEDIUM, isUrgent ← false⋯19// Count total values20System.out.println("\nTotal priorities: " + allPriorities.length4);2122// Convert string to enum using valueOf()23String input→ HIGH = "HIGH";24//@input="HIGH", "LOW", "URGENT"25Priority taskPriority→ HIGH = Priority.valueOf(inputHIGH);26System.out.println("\nTask priority: " + taskPriorityHIGH);2728//@help h129// valueOf() throws IllegalArgumentException if string doesn't match30// Always match exact case: "HIGH" works, "high" or "High" don't31//@end3233// Check if specific value exists34Priority current→ MEDIUM = Priority.MEDIUM;35boolean isUrgent→ false = (currentMEDIUM == Priority.URGENT);36System.out.println("Is urgent? " + isUrgentfalse);3738// Print with ordinal39System.out.println("\nPriorities with ordinal:");40for (Priority p : Priority.values()) {⋯output
Total priorities: 4

Task priority: HIGH
Is urgent? false

Priorities with ordinal:
4for (Priority p : Priority.values())pass 1 of 4⋯39System.out.println("\nPriorities with ordinal:");40for (Priority pLOW : Priority.values()) {41    System.out.println(p.ordinal() + ": " + pLOW);42}⋯output0: LOWAll 4 passes — pass 1 is the card abovepassp
1LOW
2MEDIUM
3HIGH
4URGENT
1public static void main(String[] args)⋯9public class AccessValues {10    public static void main(String[] args) {11        // Get all enum values using values()12        Priority[] allPriorities = Priority.values();13        14        System.out.println("All priority levels:");15        for (Priority p : allPriorities) {⋯outputAll priority levels:
2for (Priority p : allPriorities)pass 1 of 4⋯14System.out.println("All priority levels:");15for (Priority pLOW : allPriorities) {16    System.out.println("  " + pLOW);17}⋯output  LOWAll 4 passes — pass 1 is the card abovepassp
1LOW
2MEDIUM
3HIGH
4URGENT
3input ← LOW, taskPriority ← LOW, current ← MEDIUM, isUrgent ← false⋯19// Count total values20System.out.println("\nTotal priorities: " + allPriorities.length4);2122// Convert string to enum using valueOf()23String input→ LOW = "LOW";24Priority taskPriority→ LOW = Priority.valueOf(inputLOW);25System.out.println("\nTask priority: " + taskPriorityLOW);262728// Check if specific value exists29Priority current→ MEDIUM = Priority.MEDIUM;30boolean isUrgent→ false = (currentMEDIUM == Priority.URGENT);31System.out.println("Is urgent? " + isUrgentfalse);3233// Print with ordinal34System.out.println("\nPriorities with ordinal:");35for (Priority p : Priority.values()) {⋯output
Total priorities: 4

Task priority: LOW
Is urgent? false

Priorities with ordinal:
4for (Priority p : Priority.values())pass 1 of 4⋯34System.out.println("\nPriorities with ordinal:");35for (Priority pLOW : Priority.values()) {36    System.out.println(p.ordinal() + ": " + pLOW);37}⋯output0: LOWAll 4 passes — pass 1 is the card abovepassp
1LOW
2MEDIUM
3HIGH
4URGENT
1public static void main(String[] args)⋯9public class AccessValues {10    public static void main(String[] args) {11        // Get all enum values using values()12        Priority[] allPriorities = Priority.values();13        14        System.out.println("All priority levels:");15        for (Priority p : allPriorities) {⋯outputAll priority levels:
2for (Priority p : allPriorities)pass 1 of 4⋯14System.out.println("All priority levels:");15for (Priority pLOW : allPriorities) {16    System.out.println("  " + pLOW);17}⋯output  LOWAll 4 passes — pass 1 is the card abovepassp
1LOW
2MEDIUM
3HIGH
4URGENT
3input ← URGENT, taskPriority ← URGENT, current ← MEDIUM, isUrgent ← false⋯19// Count total values20System.out.println("\nTotal priorities: " + allPriorities.length4);2122// Convert string to enum using valueOf()23String input→ URGENT = "URGENT";24Priority taskPriority→ URGENT = Priority.valueOf(inputURGENT);25System.out.println("\nTask priority: " + taskPriorityURGENT);262728// Check if specific value exists29Priority current→ MEDIUM = Priority.MEDIUM;30boolean isUrgent→ false = (currentMEDIUM == Priority.URGENT);31System.out.println("Is urgent? " + isUrgentfalse);3233// Print with ordinal34System.out.println("\nPriorities with ordinal:");35for (Priority p : Priority.values()) {⋯output
Total priorities: 4

Task priority: URGENT
Is urgent? false

Priorities with ordinal:
4for (Priority p : Priority.values())pass 1 of 4⋯34System.out.println("\nPriorities with ordinal:");35for (Priority pLOW : Priority.values()) {36    System.out.println(p.ordinal() + ": " + pLOW);37}⋯output0: LOWAll 4 passes — pass 1 is the card abovepassp
1LOW
2MEDIUM
3HIGH
4URGENT

All 4 passes — pass 1 is the card above
pass	`p`
1	LOW
2	MEDIUM
3	HIGH
4	URGENT

All 4 passes — pass 1 is the card above
pass	`p`
1	LOW
2	MEDIUM
3	HIGH
4	URGENT

All 4 passes — pass 1 is the card above
pass	`p`
1	LOW
2	MEDIUM
3	HIGH
4	URGENT

All 4 passes — pass 1 is the card above
pass	`p`
1	LOW
2	MEDIUM
3	HIGH
4	URGENT

All 4 passes — pass 1 is the card above
pass	`p`
1	LOW
2	MEDIUM
3	HIGH
4	URGENT

All 4 passes — pass 1 is the card above
pass	`p`
1	LOW
2	MEDIUM
3	HIGH
4	URGENT

EnumName.VALUE - access enum constant. Type-safe, IDE auto-completes.

Enum in switch

Match on enum values cleanly.

                            light
                            light = TrafficLight.RED (default)light = TrafficLight.YELLOWlight = TrafficLight.GREEN
                        

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

                            // Using enums with switch expressions (Java 14+)
// Concept: switch with enum
// Concept: pattern - state-based behavior

enum TrafficLight {
    RED, YELLOW, GREEN
}

public class EnumInSwitch {
    public static void main(String[] args) {
        // Modern switch expression with enum
        TrafficLight light = TrafficLight.RED;

        String action = switch (light) {
            case RED -> "Stop";
            case YELLOW -> "Slow down";
            case GREEN -> "Go";
        };

        System.out.println("Light: " + light);
        System.out.println("Action: " + action);

        // Classic switch statement
        TrafficLight current = TrafficLight.YELLOW;

        switch (current) {
            case RED:
                System.out.println("\n🔴 RED - Wait for green");
                break;
            case YELLOW:
                System.out.println("\n🟡 YELLOW - Prepare to stop");
                break;
            case GREEN:
                System.out.println("\n🟢 GREEN - Proceed");
                break;
        }


        // Multiple cases same action
        TrafficLight signal = TrafficLight.GREEN;

        boolean shouldWait = switch (signal) {
            case RED, YELLOW -> true;
            case GREEN -> false;
        };

        System.out.println("\nShould wait? " + shouldWait);

    }
}
// Using enums with switch expressions (Java 14+)
// Concept: switch with enum
// Concept: pattern - state-based behavior

enum TrafficLight {
    RED, YELLOW, GREEN
}

public class EnumInSwitch {
    public static void main(String[] args) {
        // Modern switch expression with enum
        TrafficLight light = TrafficLight.YELLOW;

        String action = switch (light) {
            case RED -> "Stop";
            case YELLOW -> "Slow down";
            case GREEN -> "Go";
        };

        System.out.println("Light: " + light);
        System.out.println("Action: " + action);

        // Classic switch statement
        TrafficLight current = TrafficLight.YELLOW;

        switch (current) {
            case RED:
                System.out.println("\n🔴 RED - Wait for green");
                break;
            case YELLOW:
                System.out.println("\n🟡 YELLOW - Prepare to stop");
                break;
            case GREEN:
                System.out.println("\n🟢 GREEN - Proceed");
                break;
        }


        // Multiple cases same action
        TrafficLight signal = TrafficLight.GREEN;

        boolean shouldWait = switch (signal) {
            case RED, YELLOW -> true;
            case GREEN -> false;
        };

        System.out.println("\nShould wait? " + shouldWait);

    }
}
// Using enums with switch expressions (Java 14+)
// Concept: switch with enum
// Concept: pattern - state-based behavior

enum TrafficLight {
    RED, YELLOW, GREEN
}

public class EnumInSwitch {
    public static void main(String[] args) {
        // Modern switch expression with enum
        TrafficLight light = TrafficLight.GREEN;

        String action = switch (light) {
            case RED -> "Stop";
            case YELLOW -> "Slow down";
            case GREEN -> "Go";
        };

        System.out.println("Light: " + light);
        System.out.println("Action: " + action);

        // Classic switch statement
        TrafficLight current = TrafficLight.YELLOW;

        switch (current) {
            case RED:
                System.out.println("\n🔴 RED - Wait for green");
                break;
            case YELLOW:
                System.out.println("\n🟡 YELLOW - Prepare to stop");
                break;
            case GREEN:
                System.out.println("\n🟢 GREEN - Proceed");
                break;
        }


        // Multiple cases same action
        TrafficLight signal = TrafficLight.GREEN;

        boolean shouldWait = switch (signal) {
            case RED, YELLOW -> true;
            case GREEN -> false;
        };

        System.out.println("\nShould wait? " + shouldWait);

    }
}

                        
1light ← RED, action ← Stop, current ← YELLOW⋯9public class EnumInSwitch {10    public static void main(String[] args) {11        // Modern switch expression with enum12        TrafficLight light→ RED = TrafficLight.RED;13        //@light=TrafficLight.RED, TrafficLight.YELLOW, TrafficLight.GREEN14        15        String action→ Stop = switch (light) {16            case RED -> "Stop";17            case YELLOW -> "Slow down";18            case GREEN -> "Go";19        };20        21        System.out.println("Light: " + lightRED);22        System.out.println("Action: " + actionStop);23        24        // Classic switch statement25        TrafficLight current→ YELLOW = TrafficLight.YELLOW;⋯outputLight: RED
Action: Stop
2switch (current)⋯27switch (currentYELLOW) {28    case RED:29        System.out.println("\n🔴 RED - Wait for green");⋯
3case YELLOW:⋯30    break;31case YELLOW:32    System.out.println("\n🟡 YELLOW - Prepare to stop");33    break;34case GREEN:⋯output
🟡 YELLOW - Prepare to stop
4signal ← GREEN, shouldWait ← false⋯45// Multiple cases same action46TrafficLight signal→ GREEN = TrafficLight.GREEN;4748boolean shouldWait→ false = switch (signal) {49    case RED, YELLOW -> true;50    case GREEN -> false;51};5253System.out.println("\nShould wait? " + shouldWaitfalse);⋯output
Should wait? false
1light ← YELLOW, action ← Slow down, current ← YELLOW⋯9public class EnumInSwitch {10    public static void main(String[] args) {11        // Modern switch expression with enum12        TrafficLight light→ YELLOW = TrafficLight.YELLOW;13        14        String action→ Slow down = switch (light) {15            case RED -> "Stop";16            case YELLOW -> "Slow down";17            case GREEN -> "Go";18        };19        20        System.out.println("Light: " + lightYELLOW);21        System.out.println("Action: " + actionSlow down);22        23        // Classic switch statement24        TrafficLight current→ YELLOW = TrafficLight.YELLOW;⋯outputLight: YELLOW
Action: Slow down
2switch (current)⋯26switch (currentYELLOW) {27    case RED:28        System.out.println("\n🔴 RED - Wait for green");⋯
3case YELLOW:⋯29    break;30case YELLOW:31    System.out.println("\n🟡 YELLOW - Prepare to stop");32    break;33case GREEN:⋯output
🟡 YELLOW - Prepare to stop
4signal ← GREEN, shouldWait ← false⋯39// Multiple cases same action40TrafficLight signal→ GREEN = TrafficLight.GREEN;4142boolean shouldWait→ false = switch (signal) {43    case RED, YELLOW -> true;44    case GREEN -> false;45};4647System.out.println("\nShould wait? " + shouldWaitfalse);⋯output
Should wait? false
1light ← GREEN, action ← Go, current ← YELLOW⋯9public class EnumInSwitch {10    public static void main(String[] args) {11        // Modern switch expression with enum12        TrafficLight light→ GREEN = TrafficLight.GREEN;13        14        String action→ Go = switch (light) {15            case RED -> "Stop";16            case YELLOW -> "Slow down";17            case GREEN -> "Go";18        };19        20        System.out.println("Light: " + lightGREEN);21        System.out.println("Action: " + actionGo);22        23        // Classic switch statement24        TrafficLight current→ YELLOW = TrafficLight.YELLOW;⋯outputLight: GREEN
Action: Go
2switch (current)⋯26switch (currentYELLOW) {27    case RED:28        System.out.println("\n🔴 RED - Wait for green");⋯
3case YELLOW:⋯29    break;30case YELLOW:31    System.out.println("\n🟡 YELLOW - Prepare to stop");32    break;33case GREEN:⋯output
🟡 YELLOW - Prepare to stop
4signal ← GREEN, shouldWait ← false⋯39// Multiple cases same action40TrafficLight signal→ GREEN = TrafficLight.GREEN;4142boolean shouldWait→ false = switch (signal) {43    case RED, YELLOW -> true;44    case GREEN -> false;45};4647System.out.println("\nShould wait? " + shouldWaitfalse);⋯output
Should wait? false

Switch on enum is exhaustive. Compiler warns if cases are missing.

Iterate all values

Loop through all enum constants.

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

                            // Iterating through enum values
// Concept: iteration - process all enum constants
// Concept: ordinal - enum position

enum Month {
    JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE,
    JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
}

public class IterateValues {
    public static void main(String[] args) {
        // Iterate all months
        System.out.println("All months:");
        for (Month m : Month.values()) {
            System.out.println(m);
        }

        // Print with month number (ordinal + 1)
        System.out.println("\nMonths with numbers:");
        for (Month m : Month.values()) {
            int monthNumber = m.ordinal() + 1;  // ordinal is 0-based
            System.out.println(monthNumber + ". " + m);
        }


        // Find specific months
        System.out.println("\nSummer months:");
        for (Month m : Month.values()) {
            if (m.ordinal() >= 5 && m.ordinal() <= 7) {  // June-August
                System.out.println("  " + m);
            }
        }

        // Count months in first quarter
        int q1Count = 0;
        for (Month m : Month.values()) {
            if (m.ordinal() < 3) {  // Jan, Feb, Mar
                q1Count++;
            }
        }
        System.out.println("\nQ1 months: " + q1Count);

        // Get specific month by ordinal
        Month current = Month.values()[3];  // April (index 3)
        System.out.println("Month at index 3: " + current);
    }
}

                        
1public static void main(String[] args)⋯10public class IterateValues {11    public static void main(String[] args) {12        // Iterate all months13        System.out.println("All months:");14        for (Month m : Month.values()) {⋯outputAll months:
2for (Month m : Month.values())pass 1 of 12⋯13System.out.println("All months:");14for (Month mJANUARY : Month.values()) {15    System.out.println(mJANUARY);16}⋯outputJANUARYAll 12 passes — pass 1 is the card abovepassm
1JANUARY
2FEBRUARY
3MARCH
4APRIL
5MAY
6JUNE
7JULY
8AUGUST
9SEPTEMBER
10OCTOBER
11NOVEMBER
12DECEMBER
3System.out.println(" Months with numbers:");⋯18// Print with month number (ordinal + 1)19System.out.println("\nMonths with numbers:");20for (Month m : Month.values()) {⋯output
Months with numbers:
4monthNumber ← 1pass 1 of 12⋯19System.out.println("\nMonths with numbers:");20for (Month mJANUARY : Month.values()) {21    int monthNumber→ 1 = m.ordinal() + 1;  // ordinal is 0-based22    System.out.println(monthNumber1 + ". " + mJANUARY);23}⋯output1. JANUARYAll 12 passes — pass 1 is the card abovepassmmonthNumber
1JANUARY1
2FEBRUARY2
3MARCH3
4APRIL4
5MAY5
6JUNE6
7JULY7
8AUGUST8
9SEPTEMBER9
10OCTOBER10
11NOVEMBER11
12DECEMBER12
5System.out.println(" Summer months:");⋯30// Find specific months31System.out.println("\nSummer months:");32for (Month m : Month.values()) {⋯output
Summer months:
6for (Month m : Month.values())pass 1 of 12⋯31System.out.println("\nSummer months:");32for (Month mJANUARY : Month.values()) {33    if (m.ordinal() >= 5 && m.ordinal() <= 7) {  // June-August⋯All 12 passes — pass 1 is the card abovepassm
1JANUARY
2FEBRUARY
3MARCH
4APRIL
5MAY
6JUNE
7JULY
8AUGUST
9SEPTEMBER
10OCTOBER
11NOVEMBER
12DECEMBER
7if (m.ordinal() >= 5 && m.ordinal() <= 7)pass 1 of 3⋯32for (Month m : Month.values()) {33    if (m.ordinal() >= 5 && m.ordinal() <= 7) {  // June-August34        System.out.println("  " + mJUNE);35    }⋯output  JUNEAll 3 passes — pass 1 is the card abovepassm
1JUNE
2JULY
3AUGUST
8q1Count ← 0⋯38// Count months in first quarter39int q1Count→ 0 = 0;40for (Month m : Month.values()) {⋯
9for (Month m : Month.values())pass 1 of 12⋯39int q1Count = 0;40for (Month mJANUARY : Month.values()) {41    if (m.ordinal() < 3) {  // Jan, Feb, Mar⋯All 12 passes — pass 1 is the card abovepassm
1JANUARY
2FEBRUARY
3MARCH
4APRIL
5MAY
6JUNE
7JULY
8AUGUST
9SEPTEMBER
10OCTOBER
11NOVEMBER
12DECEMBER
10q1Count ← 1pass 1 of 3⋯40for (Month m : Month.values()) {41    if (m.ordinal() < 3) {  // Jan, Feb, Mar42        q1Count→ 1++;43    }⋯All 3 passes — pass 1 is the card abovepassq1Count
10 → 1
21 → 2
32 → 3
11current ← APRIL⋯44    }45    System.out.println("\nQ1 months: " + q1Count3);46    47    // Get specific month by ordinal48    Month current→ APRIL = Month.values()[3];  // April (index 3)49    System.out.println("Month at index 3: " + currentAPRIL);50}⋯output
Q1 months: 3
Month at index 3: APRIL

All 12 passes — pass 1 is the card above
pass	`m`
1	JANUARY
2	FEBRUARY
3	MARCH
4	APRIL
5	MAY
6	JUNE
7	JULY
8	AUGUST
9	SEPTEMBER
10	OCTOBER
11	NOVEMBER
12	DECEMBER

All 12 passes — pass 1 is the card above
pass	`m`	`monthNumber`
1	JANUARY	1
2	FEBRUARY	2
3	MARCH	3
4	APRIL	4
5	MAY	5
6	JUNE	6
7	JULY	7
8	AUGUST	8
9	SEPTEMBER	9
10	OCTOBER	10
11	NOVEMBER	11
12	DECEMBER	12

All 12 passes — pass 1 is the card above
pass	`m`
1	JANUARY
2	FEBRUARY
3	MARCH
4	APRIL
5	MAY
6	JUNE
7	JULY
8	AUGUST
9	SEPTEMBER
10	OCTOBER
11	NOVEMBER
12	DECEMBER

All 3 passes — pass 1 is the card above
pass	`m`
1	JUNE
2	JULY
3	AUGUST

All 12 passes — pass 1 is the card above
pass	`m`
1	JANUARY
2	FEBRUARY
3	MARCH
4	APRIL
5	MAY
6	JUNE
7	JULY
8	AUGUST
9	SEPTEMBER
10	OCTOBER
11	NOVEMBER
12	DECEMBER

All 3 passes — pass 1 is the card above
pass	`q1Count`
1	0 → 1
2	1 → 2
3	2 → 3

EnumName.values() returns array of all constants.

values() Built-in method returning all enum constants in declaration order.

Compare enums

Use == for enum comparison (not equals).

                            customerSize
                            customerSize = Size.LARGE (default)customerSize = Size.SMALLcustomerSize = Size.EXTRA_LARGE
                        

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

                            // Comparing enum values
// Concept: comparison - enum equality and ordering
// Concept: natural order - based on declaration order

enum Size {
    SMALL, MEDIUM, LARGE, EXTRA_LARGE
}

public class CompareEnums {
    public static void main(String[] args) {
        // Compare with == (recommended)
        Size shirt = Size.MEDIUM;
        Size pants = Size.MEDIUM;

        if (shirt == pants) {
            System.out.println("Same size");
        }

        // Can use equals() but == is better
        boolean sameSize = shirt.equals(pants);
        System.out.println("Using equals(): " + sameSize);


        // Compare ordinal for ordering
        Size small = Size.SMALL;
        Size large = Size.LARGE;

        if (small.ordinal() < large.ordinal()) {
            System.out.println("\nSMALL comes before LARGE");
        }

        // Check if size is at least MEDIUM
        Size customerSize = Size.LARGE;
        boolean qualifiesForDiscount = customerSize.ordinal() >= Size.MEDIUM.ordinal();
        System.out.println("\nCustomer size: " + customerSize);
        System.out.println("Qualifies for bulk discount: " + qualifiesForDiscount);

        // Find max of two sizes
        Size requested = Size.SMALL;
        Size available = Size.MEDIUM;

        Size actualSize = (requested.ordinal() > available.ordinal())
                          ? requested : available;
        System.out.println("\nRequested: " + requested);
        System.out.println("Available: " + available);
        System.out.println("Actual: " + actualSize);

    }
}
// Comparing enum values
// Concept: comparison - enum equality and ordering
// Concept: natural order - based on declaration order

enum Size {
    SMALL, MEDIUM, LARGE, EXTRA_LARGE
}

public class CompareEnums {
    public static void main(String[] args) {
        // Compare with == (recommended)
        Size shirt = Size.MEDIUM;
        Size pants = Size.MEDIUM;

        if (shirt == pants) {
            System.out.println("Same size");
        }

        // Can use equals() but == is better
        boolean sameSize = shirt.equals(pants);
        System.out.println("Using equals(): " + sameSize);


        // Compare ordinal for ordering
        Size small = Size.SMALL;
        Size large = Size.LARGE;

        if (small.ordinal() < large.ordinal()) {
            System.out.println("\nSMALL comes before LARGE");
        }

        // Check if size is at least MEDIUM
        Size customerSize = Size.SMALL;
        boolean qualifiesForDiscount = customerSize.ordinal() >= Size.MEDIUM.ordinal();
        System.out.println("\nCustomer size: " + customerSize);
        System.out.println("Qualifies for bulk discount: " + qualifiesForDiscount);

        // Find max of two sizes
        Size requested = Size.SMALL;
        Size available = Size.MEDIUM;

        Size actualSize = (requested.ordinal() > available.ordinal())
                          ? requested : available;
        System.out.println("\nRequested: " + requested);
        System.out.println("Available: " + available);
        System.out.println("Actual: " + actualSize);

    }
}
// Comparing enum values
// Concept: comparison - enum equality and ordering
// Concept: natural order - based on declaration order

enum Size {
    SMALL, MEDIUM, LARGE, EXTRA_LARGE
}

public class CompareEnums {
    public static void main(String[] args) {
        // Compare with == (recommended)
        Size shirt = Size.MEDIUM;
        Size pants = Size.MEDIUM;

        if (shirt == pants) {
            System.out.println("Same size");
        }

        // Can use equals() but == is better
        boolean sameSize = shirt.equals(pants);
        System.out.println("Using equals(): " + sameSize);


        // Compare ordinal for ordering
        Size small = Size.SMALL;
        Size large = Size.LARGE;

        if (small.ordinal() < large.ordinal()) {
            System.out.println("\nSMALL comes before LARGE");
        }

        // Check if size is at least MEDIUM
        Size customerSize = Size.EXTRA_LARGE;
        boolean qualifiesForDiscount = customerSize.ordinal() >= Size.MEDIUM.ordinal();
        System.out.println("\nCustomer size: " + customerSize);
        System.out.println("Qualifies for bulk discount: " + qualifiesForDiscount);

        // Find max of two sizes
        Size requested = Size.SMALL;
        Size available = Size.MEDIUM;

        Size actualSize = (requested.ordinal() > available.ordinal())
                          ? requested : available;
        System.out.println("\nRequested: " + requested);
        System.out.println("Available: " + available);
        System.out.println("Actual: " + actualSize);

    }
}

                        
1shirt ← MEDIUM, pants ← MEDIUM⋯9public class CompareEnums {10    public static void main(String[] args) {11        // Compare with == (recommended)12        Size shirt→ MEDIUM = Size.MEDIUM;13        Size pants→ MEDIUM = Size.MEDIUM;⋯
2if (shirt == pants)⋯15if (shirtMEDIUM == pantsMEDIUM) {16    System.out.println("Same size");17}⋯outputSame size
3sameSize ← true, small ← SMALL, large ← LARGE⋯19// Can use equals() but == is better20boolean sameSize→ true = shirt.equals(pantsMEDIUM);21System.out.println("Using equals(): " + sameSizetrue);2223//@help h124// Use == for enum comparison (simpler, safer)25// equals() works but == is idiomatic for enums26// Enums are singleton per constant27//@end2829// Compare ordinal for ordering30Size small→ SMALL = Size.SMALL;31Size large→ LARGE = Size.LARGE;⋯outputUsing equals(): true
4if (small.ordinal() < large.ordinal())⋯33if (small.ordinal() < large.ordinal()) {34    System.out.println("\nSMALL comes before LARGE");35}⋯output
SMALL comes before LARGE
5customerSize ← LARGE, qualifiesForDiscount ← true, requested ← SMALL⋯37// Check if size is at least MEDIUM38Size customerSize→ LARGE = Size.LARGE;39//@customerSize=Size.LARGE, Size.SMALL, Size.EXTRA_LARGE40boolean qualifiesForDiscount→ true = customerSize.ordinal() >= Size.MEDIUM.ordinal();41System.out.println("\nCustomer size: " + customerSizeLARGE);42System.out.println("Qualifies for bulk discount: " + qualifiesForDiscounttrue);4344// Find max of two sizes45Size requested→ SMALL = Size.SMALL;46Size available→ MEDIUM = Size.MEDIUM;4748Size actualSize→ MEDIUM = (requested.ordinal() > available.ordinal()) 49                  ? requestedSMALL : availableMEDIUM;50System.out.println("\nRequested: " + requestedSMALL);51System.out.println("Available: " + availableMEDIUM);52System.out.println("Actual: " + actualSizeMEDIUM);⋯output
Customer size: LARGE
Qualifies for bulk discount: true

Requested: SMALL
Available: MEDIUM
Actual: MEDIUM
1shirt ← MEDIUM, pants ← MEDIUM⋯9public class CompareEnums {10    public static void main(String[] args) {11        // Compare with == (recommended)12        Size shirt→ MEDIUM = Size.MEDIUM;13        Size pants→ MEDIUM = Size.MEDIUM;⋯
2if (shirt == pants)⋯15if (shirtMEDIUM == pantsMEDIUM) {16    System.out.println("Same size");17}⋯outputSame size
3sameSize ← true, small ← SMALL, large ← LARGE⋯19// Can use equals() but == is better20boolean sameSize→ true = shirt.equals(pantsMEDIUM);21System.out.println("Using equals(): " + sameSizetrue);222324// Compare ordinal for ordering25Size small→ SMALL = Size.SMALL;26Size large→ LARGE = Size.LARGE;⋯outputUsing equals(): true
4if (small.ordinal() < large.ordinal())⋯28if (small.ordinal() < large.ordinal()) {29    System.out.println("\nSMALL comes before LARGE");30}⋯output
SMALL comes before LARGE
5customerSize ← SMALL, qualifiesForDiscount ← false, requested ← SMALL⋯32// Check if size is at least MEDIUM33Size customerSize→ SMALL = Size.SMALL;34boolean qualifiesForDiscount→ false = customerSize.ordinal() >= Size.MEDIUM.ordinal();35System.out.println("\nCustomer size: " + customerSizeSMALL);36System.out.println("Qualifies for bulk discount: " + qualifiesForDiscountfalse);3738// Find max of two sizes39Size requested→ SMALL = Size.SMALL;40Size available→ MEDIUM = Size.MEDIUM;4142Size actualSize→ MEDIUM = (requested.ordinal() > available.ordinal()) 43                  ? requestedSMALL : availableMEDIUM;44System.out.println("\nRequested: " + requestedSMALL);45System.out.println("Available: " + availableMEDIUM);46System.out.println("Actual: " + actualSizeMEDIUM);⋯output
Customer size: SMALL
Qualifies for bulk discount: false

Requested: SMALL
Available: MEDIUM
Actual: MEDIUM
1shirt ← MEDIUM, pants ← MEDIUM⋯9public class CompareEnums {10    public static void main(String[] args) {11        // Compare with == (recommended)12        Size shirt→ MEDIUM = Size.MEDIUM;13        Size pants→ MEDIUM = Size.MEDIUM;⋯
2if (shirt == pants)⋯15if (shirtMEDIUM == pantsMEDIUM) {16    System.out.println("Same size");17}⋯outputSame size
3sameSize ← true, small ← SMALL, large ← LARGE⋯19// Can use equals() but == is better20boolean sameSize→ true = shirt.equals(pantsMEDIUM);21System.out.println("Using equals(): " + sameSizetrue);222324// Compare ordinal for ordering25Size small→ SMALL = Size.SMALL;26Size large→ LARGE = Size.LARGE;⋯outputUsing equals(): true
4if (small.ordinal() < large.ordinal())⋯28if (small.ordinal() < large.ordinal()) {29    System.out.println("\nSMALL comes before LARGE");30}⋯output
SMALL comes before LARGE
5customerSize ← EXTRA_LARGE, qualifiesForDiscount ← true, requested ← SMALL⋯32// Check if size is at least MEDIUM33Size customerSize→ EXTRA_LARGE = Size.EXTRA_LARGE;34boolean qualifiesForDiscount→ true = customerSize.ordinal() >= Size.MEDIUM.ordinal();35System.out.println("\nCustomer size: " + customerSizeEXTRA_LARGE);36System.out.println("Qualifies for bulk discount: " + qualifiesForDiscounttrue);3738// Find max of two sizes39Size requested→ SMALL = Size.SMALL;40Size available→ MEDIUM = Size.MEDIUM;4142Size actualSize→ MEDIUM = (requested.ordinal() > available.ordinal()) 43                  ? requestedSMALL : availableMEDIUM;44System.out.println("\nRequested: " + requestedSMALL);45System.out.println("Available: " + availableMEDIUM);46System.out.println("Actual: " + actualSizeMEDIUM);⋯output
Customer size: EXTRA_LARGE
Qualifies for bulk discount: true

Requested: SMALL
Available: MEDIUM
Actual: MEDIUM

== is safe and preferred for enums. They're singletons.

Exercise: Practical.java

Build a state machine using enums