java 枚举

枚举

注意：枚举是单例的，单例要保证私有化构造器。

1. 如何自定义枚举类

1. 提供类的属性，声明为 private final
2. 声明为 final 的属性，在构造器中初始化
3. 通过公共方法（get）方法来调用属性
4. 创建枚举类的对象：将类的对象声明 public static final

举例如下：

class Season {
    //1. 提供类的属性，声明为 private final
    private final String seasonName;
    private final String seasonDesc;

    //2. 声明为 final 的属性，在构造器中初始化
    private Season (String seasonName,String seasonDesc) {
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    //3. 通过公共的方法来调用属性
    public String getSeasonName() {
        return seasonName;
    }
    public String getSeasonDesc() {
        return seasonDesc;
    }

    //4. 创建枚举类的对象：将类的对象声明 public static final
    public static final Season SPRING = new Season("spring","春暖花开");
    public static final Season SUMMER = new Season("summer","夏日炎炎");
    public static final Season AUTUMN = new Season("autumn","秋高气爽");
    public static final Season WINTER = new Season("winter","白雪皑皑");

    @Override
    public String toString() {
        return "Season [seasonName=" + seasonName + ",seasonDesc=" + seasonDesc + "]";
    }

    public void show() {
        System.out.println("这是一个季节");
    }

}

public class TestSeason {
    public static void main(String[] args) {
        Season spring = Season.SPRING;
        System.out.println(spring);
        spring.show();
        System.out.println(spring.getSeasonName());
    }
}

2. 如何使用 enum 关键字来定义枚举类

将对象放到最前面，并且不需要定义对象的其他属性。
使用 enum 类来定义枚举类，可以使用下述两种方法来返回值。

1. values() 方法：返回所有值
2. valueOf() 方法：返回指定对象的值（要求传入的形参name是枚举类对象的名字）
   否则，报 java.lang.IllegalArgumentException 异常

改造一下上述代码：

enum Season {

    SPRING("spring","春暖花开"),SUMMER("summer","夏日炎炎"),AUTUMN("autumn","秋高气爽"),WINTER("winter","白雪皑皑");


    private final String seasonName;
    private final String seasonDesc;

    private Season (String seasonName,String seasonDesc) {
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    public String getSeasonName() {
        return seasonName;
    }
    public String getSeasonDesc() {
        return seasonDesc;
    }



    @Override
    public String toString() {
        return "Season [seasonName=" + seasonName + ",seasonDesc=" + seasonDesc + "]";
    }

    public void show() {
        System.out.println("这是一个季节");
    }

}

public class TestSeason {
    public static void main(String[] args) {
        Season spring = Season.SPRING;
        System.out.println(spring);
        spring.show();
        System.out.println(spring.getSeasonName());

        // 1. values() 方法：返回所有值
        Season[] seasons = Season.values();
        for (int i=0; i<seasons.length;i++) {
            System.out.println(seasons[i]);
        }

        // 2. valueOf() 方法：返回指定对象的值（要求传入的形参name是枚举类对象的名字）
        // 否则，报 java.lang.IllegalArgumentException 异常
        String str = "SPRING";
        Season sea = Season.valueOf(str);
        System.out.println(sea);
    }
}

3. 枚举类实现接口

可以让不同的枚举类的对象调用被重写的抽象方法，执行的效果不同。（相当于让每个对象重写抽象方法）

改造如下：

interface Info {
    void show();
}

enum Season implements Info {

    SPRING("spring","春暖花开"){
        public void show() {
            System.out.println("春天在哪里?");
        }
    },"夏日炎炎"){
        public void show() {
            System.out.println("夏天在哪里?");
        }
    },"秋高气爽"){
        public void show() {
            System.out.println("秋天在哪里?");
        }
    },"白雪皑皑"){
        public void show() {
            System.out.println("冬天在哪里?");
        }
    };


    private final String seasonName;
    private final String seasonDesc;

    private Season (String seasonName,seasonDesc=" + seasonDesc + "]";
    }

    public void show() {
        System.out.println("这是一个季节");
    }

}

public class TestSeason {
    public static void main(String[] args) {
        Season spring = Season.SPRING;
        System.out.println(spring);
        spring.show();
        System.out.println(spring.getSeasonName());

        // 1. values() 方法：返回所有值
        Season[] seasons = Season.values();
        for (int i=0; i<seasons.length;i++) {
            System.out.println(seasons[i]);
        }

        // 2. valueOf() 方法：返回指定对象的值（要求传入的形参name是枚举类对象的名字）
        // 否则，报 java.lang.IllegalArgumentException 异常
        String str = "WINTER";
        Season sea = Season.valueOf(str);
        System.out.println(sea);
        sea.show();
    }
}

完。