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12.2 Enums (Enumerations)

An Enum (Enumeration) is a special Java type that groups a set of related constants together into a single class-like entity. It was introduced in Java 5.

1. Why Do We Need Enums?​

Without enums, "seasons" in a program used to be represented as plain integer constants.

// The old way (problematic!)
final int SPRING = 1;
final int SUMMER = 2;
final int FALL = 3;
final int WINTER = 4;

int season = 5; // Invalid value — and NO ONE can stop it!

This approach has a critical flaw: assigning an invalid value like 5 is perfectly acceptable to the compiler, causing bugs that only surface silently at runtime.

2. Defining an Enum​

// Defining a Season enum
public enum Season {
    SPRING, SUMMER, FALL, WINTER
}
// Defining a Direction enum
public enum Direction {
    NORTH, SOUTH, EAST, WEST
}

3. Using an Enum​

public class EnumExample {
    public static void main(String[] args) {
        Season today = Season.SUMMER;
        System.out.println(today);           // SUMMER
        System.out.println(today.name());    // "SUMMER" (as a String)
        System.out.println(today.ordinal()); // 1 (declaration index, starts from 0)

        // Works seamlessly with switch statements
        switch (today) {
            case SPRING -> System.out.println("Warm Spring!");
            case SUMMER -> System.out.println("Blazing Summer!");
            case FALL   -> System.out.println("Cool Autumn!");
            case WINTER -> System.out.println("Freezing Winter!");
        }
        // Output: Blazing Summer!
    }
}

4. Enums with Data and Methods​

Enums can go far beyond simple named constants — each constant can carry its own unique data and methods.

public enum Planet {
    MERCURY(3.303e+23, 2.4397e6),
    EARTH  (5.976e+24, 6.37814e6),
    MARS   (6.421e+23, 3.3972e6);

    private final double mass;   // Mass in kg
    private final double radius; // Radius in meters

    // Enum constructor
    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

    // Enum method (calculates surface gravity)
    public double surfaceGravity() {
        final double G = 6.67300E-11;
        return G * mass / (radius * radius);
    }
}

public class PlanetTest {
    public static void main(String[] args) {
        for (Planet p : Planet.values()) {
            System.out.printf("%-8s: Gravity = %.2f%n", p, p.surfaceGravity());
        }
    }
}

Using Enums provides guaranteed type safety and dramatically improves code readability and maintainability.

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