JAVA enum枚举类型的使用

1. 枚举类型的定义范围：

1）枚举类型可以是单独的一个class文件

/**
 * This enum is declared in the Season.java file.
*/
public enum Season {
    WINTER,
    SPRING,
    SUMMER,
    FALL
}

2）同样可以定义在其他类里面

public class Day {

    private Season season;

    public String getSeason() {
        return season.name();
    }

    public void setSeason(String season) {
        this.season = Season.valueOf(season);
    }

    /**
     * This enum is declared inside the Day.java file and 
     * cannot be accessed outside because it's declared as private.
     */
    private enum Season {
        WINTER,
        SPRING,
        SUMMER,
        FALL
    }

}

3）但是不能定义在构造方法或普通方法体类，会报编译错误

public class Day {

    /**
     * Constructor
    */
    public Day() {
        // Illegal. Compilation error
        enum Season {
            WINTER,
            SPRING,
            SUMMER,
            FALL
        }
    }

    public void aSimpleMethod() {
        // Legal. You can declare a primitive (or an Object) inside a method. Compile!
        int primitiveInt = 42;

        // Illegal. Compilation error.
        enum Season {
            WINTER,
            SPRING,
            SUMMER,
            FALL
        }

        Season season = Season.SPRING;
    }


}

2.枚举类型注意事项

1）枚举中的元素不能重复

public enum Season {
    WINTER,
    WINTER, //Compile Time Error : Duplicate Constants
    SPRING,
    SUMMER,
    FALL
}

2)枚举中定义的常量默认都是public static final 修饰

3) jdk1.6及以下switch参数不能直接传递字符串，但可以可以使用enum类型

public static void enumSwitchExample(Season s) {
    switch(s) {
        case WINTER:
            System.out.println("It's pretty cold");
            break;
        case SPRING:
            System.out.println("It's warming up");
            break;
        case SUMMER:
            System.out.println("It's pretty hot");
            break;
        case FALL:
            System.out.println("It's cooling down");
            break;
    }
}

4）枚举类型中不能有public构造方法，但可以用私有的构造方法（默认为私有的）

public enum Coin {
    PENNY(1), NICKEL(5), DIME(10), QUARTER(25); // usual names for US coins
    // note that the above parentheses and the constructor arguments match
    private int value;

    Coin(int value) { 
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

5）枚举中可以有抽象方法，其中每一个类型可以单独实现该抽象方法

enum Action {
    DODGE {
        public boolean execute(Player player) {
            return player.isAttacking();
        }
    },
    ATTACK {
        public boolean execute(Player player) {
            return player.hasWeapon();
        }
    },
    JUMP {
        public boolean execute(Player player) {
            return player.getCoordinates().equals(new Coordinates(0, 0));
        }
    };

    public abstract boolean execute(Player player);
}

还有一些其他的用法，没有一一列举，可以参考地址：https://stackoverflow.com/documentation/java/155/enums#t=201708250625516795921；

JAVA enum枚举类型的使用

推荐阅读