Java技术教程 – Java中的枚举（Enum）详解与案例

Java 技术教程

Java中的枚举（Enum）详解与案例

枚举（Enum）是Java中的一种特殊数据类型，它允许程序员定义变量为一组预定义的常量。枚举在Java中被广泛使用，特别是在需要表示一组固定常量时，如星期几、月份、颜色等。

1. 枚举的基本用法

定义一个简单的枚举类型：

public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

使用枚举类型：

public class EnumExample {
    public static void main(String[] args) {
        Day today = Day.MONDAY;
        System.out.println("Today is: " + today);
    }
}

2. 枚举中的方法和构造器

枚举可以包含方法、构造器和变量。每个枚举实例都是枚举类的一个实例。

public enum Planet {
    MERCURY(3.303e+23, 2.4397e6),
    VENUS(4.869e+24, 6.0518e6),
    EARTH(5.976e+24, 6.37814e6),
    MARS(6.421e+23, 3.3972e6);

    private final double mass;   // in kilograms
    private final double radius; // in meters

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

    public double mass() { return mass; }

    public double radius() { return radius; }

    // Universal gravitational constant  (m3 kg-1 s-2)
    public static final double G = 6.67300E-11;

    // Calculate surface gravity
    double surfaceGravity() {
        return G * mass / (radius * radius);
    }

    double surfaceWeight(double otherMass) {
        // F = ma
        return otherMass * surfaceGravity();
    }

    public static void main(String[] args) {
        double earthWeight = 175;
        double mass = earthWeight / EARTH.surfaceGravity();

        for (Planet p : Planet.values()) {
            System.out.printf("Your weight on %s is %f%n",
                p, p.surfaceWeight(mass));
        }
    }
}

3. 枚举中的构造器必须是私有的

枚举的构造器默认是私有的，这意味着你不能从枚举类外部创建枚举实例。这也是枚举的一种保护机制，防止枚举常量被外部恶意修改。

4. 枚举中的方法重载和覆写

枚举可以覆盖`toString`方法，用于返回枚举常量的字符串表示。

public enum Operation {
    PLUS {
        public double apply(double x, double y) { return x + y; }
    },
    MINUS {
        public double apply(double x, double y) { return x - y; }
    },
    TIMES {
        public double apply(double x, double y) { return x * y; }
    },
    DIVIDE {
        public double apply(double x, double y) { return x / y; }
    };

    public abstract double apply(double x, double y);
}

5. 枚举在switch语句中的应用

枚举非常适合在switch语句中使用，因为它们的常量是有限的，这使得代码更加清晰和易于维护。

public class EnumSwitchExample {
    public static void main(String[] args) {
        Day day = Day.WEDNESDAY;

        switch (day) {
            case MONDAY:
                System.out.println("Today is Monday");
                break;
            case TUESDAY:
                System.out.println("Today is Tuesday");
                break;
            case WEDNESDAY:
                System.out.println("Today is Wednesday");
                break;
            case THURSDAY:
                System.out.println("Today is Thursday");
                break;
            case FRIDAY:
                System.out.println("Today is Friday");
                break;
            case SATURDAY:
                System.out.println("Today is Saturday");
                break;
            case SUNDAY:
                System.out.println("Today is Sunday");
                break;
        }
    }
}