枚举

提示

• Enums

枚举

• 枚举是类
• 在编译时，实例是已知的
• 隐式扩展 java.lang.Enum

public enum Direction {
    NORTH, SOUTH, EAST, WEST
}

class DirectionTest {
    public test() {
        const Direction direction = Direction.EAST;
        if (direction == Direction.EAST) {
            // ..
        }
    }
}

提示

使用 switch 表达式时，编译器会检查是否枚举了所有值，阅读 Accessing, evaluating, and comparing enums。

一些方法

• 枚举可以有构造函数、方法和字段
• values()
• valueOf()
• name()
• ordinal()
• compareTo()

public class Test {

    public enum DayOfWeek {
        MONDAY("MON"), TUESDAY("TUE"), WEDNESDAY("WED"), THURSDAY("THU"), FRIDAY("FRI"), SATURDAY("SAT"), SUNDAY("SUN");

        private final String abbreviation;

        DayOfWeek(String abbreviation) {
            this.abbreviation = abbreviation;
        }

        public String getAbbreviation() {
            return abbreviation;
        }
    }

    public static void main(String[] args) {

        for (DayOfWeek day : DayOfWeek.values()) {
            System.out.print(day.getAbbreviation() + " ");
        }
        // MON TUE WED THU FRI SAT SUN

        System.out.println("\n\ngetAbbreviation() 返回值");
        System.out.println(DayOfWeek.MONDAY.getAbbreviation());
        // MON

        System.out.println("\nvalueOf() 返回值");
        System.out.println(DayOfWeek.valueOf("MONDAY").getAbbreviation());
        // MON

        System.out.println("\nname() 返回名称");
        System.out.println(DayOfWeek.MONDAY.name());
        // MONDAY

        System.out.println("\nordinal() 返回索引（出现的顺序）");
        System.out.println(DayOfWeek.MONDAY.ordinal());
        // 0

        System.out.println("\ncompareTo() 返回比较结果");
        System.out.println(DayOfWeek.MONDAY.compareTo(DayOfWeek.TUESDAY));   // -1
        System.out.println(DayOfWeek.WEDNESDAY.compareTo(DayOfWeek.MONDAY)); // 2
    }
}

枚举的单例

利用枚举的特性，确保一个类只有一个实例。

public enum Singleton {
    INSTANCE;

   // some fields, methods, etc.

    public void doSomething() {
        // ...
    }
}

抽象方法

enum MyEnum {
    A() {
        @Override
        void doSomething() {
            System.out.println("a");
        }
    },
    B() {
        @Override
        void doSomething() {
            System.out.println("b");
        }
    };

    abstract void doSomething();
}