java enum用法

時間 2019-11-13

標籤 java enum 用法欄目 Java 简体版

原文原文鏈接

基本用法java

enum Day {
    SUNDAY, MONDAY, TUESDAY, WENDSDAY, THURSDAY, FRIDAY, SATURDAY;
}

枚舉是常量，因此應該用大寫。this

枚舉是對象code

枚舉隱含地繼承了java.lang.Enum，因此它具備java.lang.Enum的屬性和方法。遍歷枚舉：對象

public class Main {
    public static void main(String[] args) {
        for(Day day:Day.values()) {
            System.out.println(day);
        }
    }
}

枚舉能夠帶字段和方法，如下示例來自官方The Java™ Tutorials繼承

public enum EnumDemo {
    AOBJECT("field one", "field two");

    private String field1;
    private String field2;

    EnumDemo(String val1, String val2){
        this.field1 = val1;
        this.field2 = val2;
    }

    public void printFields(){
        System.out.println(this.field1);
        System.out.println(this.field2);
    }

    public static void main(String[] args) {
        EnumDemo.AOBJECT.printFields();
    }
}

如下現實中的示例來自官方的Java Tutorial:get

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),
    JUPITER (1.9e+27,   7.1492e7),
    SATURN  (5.688e+26, 6.0268e7),
    URANUS  (8.686e+25, 2.5559e7),
    NEPTUNE (1.024e+26, 2.4746e7);

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

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

    private double mass() { return mass; }
    private double radius() { return radius; }

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

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

    double surfaceWeight(double otherMass) {
        return otherMass * surfaceGravity();
    }

    public static void main(String[] args) {
        Double earthWeight = 120;
        for(Planet p: Planet.values()){
            System.out.println(p.surfaceGravity());
            System.out.println(p.surfaceWeight(earthWeight/EARTH.surfaceGravity()));
        }
    }
}

枚舉是singleton，能夠用枚舉來構建一個Singletonit

public enum Singleton {
    INSTANCE(new String[]{"arg1", "arg2"});

    String[] myArgs;
    Singleton(String[] args){
        this.myArgs = args;
    }

    public static Singleton getInstance(){
        return INSTANCE;
    }

    public static void main(String[] args) {
        for(String arg : INSTANCE.myArgs)
            System.out.println(arg);
    }
}