枚举类型

枚举类

枚举类是Java5的新特性，是一种特殊的类，用于表述有限且固定的对象类型。如季节类，只有四个对象；星期类，只有七个对象。其用关键字enum定义，可以有自己的成员变量、方法、构造器。一个Java源文件只能定义一个public访问权限的枚举类，且与该源文件同名。

1. 与类的区别

• 默认继承java.lang.Enum类，而非Object类。Enum类实现了java.lang.Serializable与java.lang.Comparable接口。
• 非抽象枚举类默认使用final修饰，抽象枚举类默认abstract修饰
• 枚举类构造器只能由private修饰，不添加则默认。
• 枚举类的所有实例必须在类的首行显式写出，默认 punlic static final修饰。无需显示添加

2. 一些方法

• values() 静态方法，返回枚举类所有实例
• int ordinal() 返回枚举值在枚举类中的索引值，首个索引为0
• String toString() 返回枚举常量(实例)的名称
• public static<T extends Enum>T valueOf(ClassenumType,String name) 静态方法
  返回返回指定枚举类中指定名称的枚举值

【案例】

public enum WeekEnum {
    MONDSAY,TUESDAY,WEDNESDAT,THURSDAY,FRIDAY,SATURDAY,SUNDAY;
}

public class EnumTest1 {
    public void judge(WeekEnum weekEnum){
        //switch里的表达式可以是枚举类型
        //注：当前对象是EnumTest的对象，而非枚举实例
        switch (weekEnum ){
            case SUNDAY:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
            case MONDSAY:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
            case TUESDAY:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
            case WEDNESDAT:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
            case THURSDAY:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
            case FRIDAY:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
            case SATURDAY:
                System.out.println("today is "+weekEnum.toString()+" the index is +"+weekEnum.ordinal());
                break;
        }
    }

    public static void main(String[] args) {
        //枚举类型有一个默认的values()方法，返回该枚举类的所有实例
        for(WeekEnum weekEnum:WeekEnum.values()){

            //调用了toString
            System.out.println(weekEnum+" ");
        }
        //使用枚举类型时，可通过EnumClass.variable进行访问
        new EnumTest1().judge(WeekEnum.TUESDAY);
        new EnumTest1().judge(WeekEnum.FRIDAY);
        new EnumTest1().judge(WeekEnum.WEDNESDAT);
        new EnumTest1().judge(WeekEnum.MONDSAY);
        new EnumTest1().judge(WeekEnum.SATURDAY);
        new EnumTest1().judge(WeekEnum.THURSDAY);
        new EnumTest1().judge(WeekEnum.SUNDAY);
        
        //枚举类的实例只能是枚举值，不能随意地通过new来创建枚举对象
        WeekEnum weekEnum = Enum.valueOf(WeekEnum.class, "FRIDAY");
        System.out.println(weekEnum);

    }


}

3. 成员变量与构造器

写在首行的枚举实例，其本质是通过构造器初始化对象。默认public static final 修饰。
如果枚举类中有final修饰的成员变量，则通常要求其在构造器中初始化，此时在列举枚举实例时就需要显示的传入参数，如：

public enum WeekEnum {
    MONDSAY("星期一"),
    TUESDAY("星期二"),
    WEDNESDAT("星期三"),
    THURSDAY("星期四"),
    FRIDAY("星期五"),
    SATURDAY("星期六"),
    SUNDAY("星期天");
    
    private final String name;
    private WeekEnum(String name){
        this.name=name;
    }
}

相当于:
public static final WeekEnum weekEnum=new WeekEnum("星期一")

4. 实现接口的枚举类

【案例】

public interface Work {
    void work();
}

public enum WeekEnum implements Work{
    MONDSAY("星期一"),
    TUESDAY("星期二"),
    WEDNESDAT("星期三"),
    THURSDAY("星期四"),
    FRIDAY("星期五"),
    SATURDAY("星期六"),
    SUNDAY("星期天");

    private final String name;
    private WeekEnum(String name){
        this.name=name;
    }
    public String getName() {
        return name;
    }
    
    @Override
    public void work() {
        System.out.println("I always work in"+this.toString());
    }
}

public class EnumTest2 {
    public static void main(String[] args) {
        for(WeekEnum weekEnum:WeekEnum.values()){
            weekEnum.work();
        }
    }
}   
//结果:
I always work in.MONDSAY
I always work in.TUESDAY
I always work in.WEDNESDAT
I always work in.THURSDAY
I always work in.FRIDAY
I always work in.SATURDAY
I always work in.SUNDAY

由上面案例可知，如果由枚举类来实现接口里的方法，则每个枚举值在调用该方法时都有相同的行为方式，
若想每个实例不同，则需分别实现抽象方法：

public enum WeekEnum implements Work{
    MONDSAY("星期一"){
        public void work(){
            System.out.println("playing...");
        }
    },
    TUESDAY("星期二"){
        public void work(){
            System.out.println("listening..");
        }
    },
    WEDNESDAT("星期三"){
        public void work(){
            System.out.println("studying...");
        }
    },
    THURSDAY("星期四"){
        public void work(){
            System.out.println("reading...");
        }
    },
    FRIDAY("星期五"){
        public void work(){
            System.out.println("watching...");
        }
    },
    SATURDAY("星期六"){
        public void work(){
            System.out.println("eating...");
        }
    },
    SUNDAY("星期天"){
        public void work(){
            System.out.println("sleeping...");
        }
    };

    private final String name;
    private WeekEnum(String name){
        this.name=name;
    }
    public String getName() {
        return name;
    }
}

public class EnumTest2 {
    public static void main(String[] args) {
        for(WeekEnum weekEnum:WeekEnum.values()){
            weekEnum.work();
        }
    }
}

//结果：
listening..
studying...
reading...
watching...
eating...
sleeping...

4. 总结

枚举类，实际是将其可能产生的实例对象，都放在了类中。
有点像是多类型的单例模式。