Comprehensive Guide to Java Enums: Constants, Switch Statements, Custom Methods, Overriding, Interfaces, and Collections

Usage 1: Constants

Before JDK 1.5 constants were defined with public static final; enums now allow related constants to be grouped in a single type, offering additional built‑in methods.

public enum Color { RED, GREEN, BLANK, YELLOW }

Usage 2: Switch

Enums can be used in switch statements, improving readability compared to primitive types.

enum Signal { GREEN, YELLOW, RED }
public class TrafficLight {
    Signal color = Signal.RED;
    public void change() {
        switch (color) {
            case RED:    color = Signal.GREEN; break;
            case YELLOW: color = Signal.RED;   break;
            case GREEN:  color = Signal.YELLOW; break;
        }
    }
}

Usage 3: Adding Custom Methods

Enums can contain fields, constructors, and regular methods; a semicolon must separate the enum constants from the rest of the definition.

public enum Color {
    RED("红色", 1), GREEN("绿色", 2), BLANK("白色", 3), YELLO("黄色", 4);
    private String name;
    private int index;
    private Color(String name, int index) { this.name = name; this.index = index; }
    public static String getName(int index) {
        for (Color c : Color.values()) {
            if (c.getIndex() == index) return c.name;
        }
        return null;
    }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public int getIndex() { return index; }
    public void setIndex(int index) { this.index = index; }
}

Usage 4: Overriding Methods

Enums can override methods such as toString() to provide custom string representations.

public enum Color {
    RED("红色", 1), GREEN("绿色", 2), BLANK("白色", 3), YELLO("黄色", 4);
    private String name;
    private int index;
    private Color(String name, int index) { this.name = name; this.index = index; }
    @Override
    public String toString() {
        return this.index + "_" + this.name;
    }
}

Usage 5: Implementing Interfaces

All enums extend java.lang.Enum; they can also implement interfaces to add behavior.

public interface Behaviour {
    void print();
    String getInfo();
}
public enum Color implements Behaviour {
    RED("红色", 1), GREEN("绿色", 2), BLANK("白色", 3), YELLO("黄色", 4);
    private String name;
    private int index;
    private Color(String name, int index) { this.name = name; this.index = index; }
    @Override
    public String getInfo() { return this.name; }
    @Override
    public void print() { System.out.println(this.index + ":" + this.name); }
}

Usage 6: Organizing Enums with Interfaces

An interface can serve as a common type for multiple enums.

public interface Food {
    enum Coffee implements Food { BLACK_COFFEE, DECAF_COFFEE, LATTE, CAPPUCCINO }
    enum Dessert implements Food { FRUIT, CAKE, GELATO }
}

Usage 7: Enum Collections

Java provides EnumSet (a high‑performance Set) and EnumMap (a Map with enum keys) for efficient handling of enum collections.

For deeper implementation details, refer to the fourth edition of "Thinking in Java".