枚举定义

public enum Week {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY;

	//无参构造器，默认private
	Week(){
        System.out.println("hello");
    }
}

public class Test {
    public static void main(String[] args) {
        Week w = Week.FRIDAY;
    }
}

有参构造和枚举类成员

public enum Week {
    SUNDAY(7), MONDAY(1), TUESDAY(2), WEDNESDAY(3), THURSDAY(4), FRIDAY(5), SATURDAY(6);

    private int weekNum;

    Week(int weekNum){
        this.weekNum = weekNum;
    }

    public int getWeekNum() {
        return weekNum;
    }

}

public class Test {
    public static void main(String[] args) {
        Week w = Week.FRIDAY;
        System.out.println(w.getWeekNum()); //5
    }
}

枚举中的抽象方法

public enum Week {
    SUNDAY(){
        @Override
        public void getWeekNum() {
            System.out.println(7);
        }
    },
    MONDAY() {
        @Override
        public void getWeekNum() {
            System.out.println("星期一");
        }
    },

    TUESDAY(){
        @Override
        public void getWeekNum() {
            System.out.println("礼拜二");
        }
    },
    WEDNESDAY(){
        @Override
        public void getWeekNum() {
            System.out.println("周三");
        }
    };

    public abstract void getWeekNum();
}

public class Test {
    public static void main(String[] args) {
        Week w = Week.TUESDAY;
        w.getWeekNum(); //礼拜二
    }
}

每个枚举类都有两个 static 方法：

• static Direction[] values()：返回本类所有枚举常量；
• static Direction valueOf(String name)：通过枚举常量的名字返回Direction常量，注意，这个方法与Enum类中的valueOf()方法的参数个数不同。

public class Test {
    public static void main(String[] args) {
        for (Week w : Week.values()) {
            System.out.println(w);
        }
        System.out.println("星期天：" + Week.valueOf("SUNDAY"));
    }
}

=====================================================================

枚举应用

一、传入商品折扣计算价格

public enum Discount {
    EIGHT_DISCOUNT(0.8), SIX_DISCOUNT(0.6), FIVE_DISCOUNT(0.5);

    private double discountNum;

    Discount(double discountNum) {
        this.discountNum = discountNum;
    }

    double getDiscountNum(){
        return discountNum;
    }
}

public class Test {
    public static void main(String[] args) {
        calculatePrice(Discount.EIGHT_DISCOUNT);//0.8
    }

    static BigDecimal calculatePrice(Discount discount){
        System.out.println(discount.getDiscountNum());
        //...
        return null;
    }
}

二、switch使用

public class Test {
    public static void main(String[] args) {
        Week w = Week.MONDAY; //周一
        switch (w) {
            case MONDAY:
                System.out.println("周一"); break;
            case TUESDAY:
                System.out.println("周二"); break;
        }
    }
}

三、实现接口，消除if/else

原案例：

if ("dog".equals(animalType)){
    System.out.println("吃骨头");
} else if ("cat".equals(animalType)) {
    System.out.println("吃鱼干");
} else if ("sheep") {
    System.out.println("吃草");
}

①、先定义一个接口

public interface Eat {
    //吃
    String eat();
}

②、创建枚举类实现这个接口

public enum AnimalEnum implements Eat {
    Dog(){
        @Override
        public void eat() {
            System.out.println("吃骨头");
        }
    },

    Cat() {
        @Override
        public void eat() {
            System.out.println("吃鱼干");
        }
    },

    Sheep() {
        @Override
        public void eat() {
            System.out.println("吃草");
        }
    }
}

public class Test {
    public static void main(String[] args) {
        AnimalEnum.valueOf("Cat").eat();
    }
}

Java 中其实还有专门用于枚举的集合类，EnumSet 和 EnumMap