使用普通类模拟枚举

public class Weekday {
    private Weekday() {} // 私有化
    
    public static final Weekday MONDAY = new Weekday();　　// 每个枚举都是一个对象
    public static final Weekday TUESDAY = new Weekday();

    public String toString() {
        if(this == MONDAY)
            return "Monday";
        else if(...) ...
    }
}

使用抽象类模拟枚举

public abstract class Weekday {
    private Weekday() {}

    public final static Weekday MONDAY = new Weekday() {
        public Weekday nextDay() {
            return TUESDAY;
        }
    };

    public final static Weekday TUESDAY = new Weekday() {
        public Weekday nextDay() {
            return WEDNESDAY;
        }
    };

    ...

    public abstract Weekday nextDay();
}

使用抽象类模拟可以免除toString()写大量的if-else语句，不需要对每个对象进行判断，只需要有具体实现类完成即可

枚举类

public enum Weekday {
  SUN, MON, TUE, WED, THU, FRI, SAT
}

普通的枚举类末尾不需要写分号

带构造参数枚举

public enum Weekday {    
    SUN(5), MON(), TUE(7), WED(), THU(9), FRI, SAT;    // 必须第一行，且以分号结尾

    private Weekday() {}
    private Weekday(int number) {
        System.out.println(number);
    }
}

枚举中每个变量都是static final的

带抽象函数的枚举

public enum Weekday {    // enum不用abstract修饰
    SUN(5) {
        public Weekday nextDay() {
            return MON;
        }
    }, 
    MON() {
        public Weekday nextDay() {
            return TUE;
        }
    }, 
    TUE() {
        public Weekday nextDay() {
            return WED;
        }
    }, 
    WED() {
        public Weekday nextDay() {
            return THU;
        }
    }, 
    THU() {
        public Weekday nextDay() {
            return FRI;
        }
    }, 
    FRI() {
        public Weekday nextDay() {
            return SAT;
        }
    }, 
    SAT() {
        public Weekday nextDay() {
            return SUN;
        }
    };

    private Weekday() {}
    private Weekday(int number) {
        System.out.println(number);
    }

    public abstract Weekday nextDay();
}

枚举类中的方法

toString() - String
name() - String
ordinal() - int
valueOf(String name) - Weekday
values() - Weekday[]
... ...