Enum枚举详解和EnumSet、EnumMap使用

一、Enum枚举

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

一个简单的测试类：

public class TestEnum {

    public static void main(String[] args) {
        Day today=Day.FRIDAY;
        switch(today)
        {
        case MONDAY:
            System.out.println("today is monday");
            break;
        case TUESDAY:
            System.out.println("today is tuesday");
            break;
        case WEDNESDAY:
            System.out.println("today is webnesday");
            break;
        case THURSDAY:
            System.out.println("today is thursday");
            break;
        case FRIDAY:
            System.out.println("today is firday");
            break;
        case SATURDAY:
            System.out.println("today is saturday");
            break;
        case SUNDAY:
            System.out.println("today is sunday");
            break;
        }
    }

}

测试结果：

today is firday

Day枚举的本质就是一个类，编译器会自动为我们生成Day类，通过反编译得到该类如下：

final class Day extends Enum
{
    //编译器为我们添加的静态的values()方法
    public static Day[] values()
    {
        return (Day[])$VALUES.clone();
    }
    //编译器为我们添加的静态的valueOf()方法，注意间接调用了Enum也类的valueOf方法
    public static Day valueOf(String s)
    {
        return (Day)Enum.valueOf(com/zejian/enumdemo/Day, s);
    }
    //私有构造函数
    private Day(String s, int i)
    {
        super(s, i);
    }
     //前面定义的7种枚举实例
    public static final Day MONDAY;
    public static final Day TUESDAY;
    public static final Day WEDNESDAY;
    public static final Day THURSDAY;
    public static final Day FRIDAY;
    public static final Day SATURDAY;
    public static final Day SUNDAY;
    private static final Day $VALUES[];

    static 
    {    
        //实例化枚举实例
        MONDAY = new Day("MONDAY", 0);
        TUESDAY = new Day("TUESDAY", 1);
        WEDNESDAY = new Day("WEDNESDAY", 2);
        THURSDAY = new Day("THURSDAY", 3);
        FRIDAY = new Day("FRIDAY", 4);
        SATURDAY = new Day("SATURDAY", 5);
        SUNDAY = new Day("SUNDAY", 6);
        $VALUES = (new Day[] {
            MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
        });
    }
}

可以清楚地看出每个枚举类型即星期数就是该Day类的一个实例对象，该构成方式和单例模式有些类似，故可以用只有一个枚举类型的枚举作为单例模式，而且枚举的构造器由编译器管理安全性十分高，既可以防止反射破解也可以防止反序列破解。

Day类继承了Enum类，下面看下Enum代码，可以更好理解枚举的方法：

public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {

    private final String name; //枚举字符串名称

    public final String name() {
        return name;
    }

    private final int ordinal;//枚举顺序值

    public final int ordinal() {
        return ordinal;
    }

    //枚举的构造方法，只能由编译器调用
    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }

    public String toString() {
        return name;
    }

    public final boolean equals(Object other) {
        return this==other;
    }

    //比较的是ordinal值
    public final int compareTo(E o) {
        Enum<?> other = (Enum<?>)o;
        Enum<E> self = this;
        if (self.getClass() != other.getClass() && // optimization
            self.getDeclaringClass() != other.getDeclaringClass())
            throw new ClassCastException();
        return self.ordinal - other.ordinal;//根据ordinal值比较大小
    }

    @SuppressWarnings("unchecked")
    public final Class<E> getDeclaringClass() {
        //获取class对象引用，getClass()是Object的方法
        Class<?> clazz = getClass();
        //获取父类Class对象引用
        Class<?> zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
    }


    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        //enumType.enumConstantDirectory()获取到的是一个map集合，key值就是name，value则是枚举变量值   
        //enumConstantDirectory是class对象内部的方法，根据class对象获取一个map集合的值       
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum constant " + enumType.getCanonicalName() + "." + name);
    }

    //.....省略其他没用的方法
}

二、EnumMap

public class TestEnumMap {

    public static void main(String[] args) {

        Day today=Day.FRIDAY;

        EnumMap enummap=new EnumMap(Day.class); //对应Day类的EnumMap
        enummap.put(Day.MONDAY, "work work");   //第一个值为key不能为null，第二个值为values可以为null
        enummap.put(Day.TUESDAY, "work work");  //第一个值填入枚举实例，第二个值为该枚举实例的记录信息
        enummap.put(Day.WEDNESDAY, "work work");
        enummap.put(Day.THURSDAY, "work work");
        enummap.put(Day.FRIDAY, "work work");
        enummap.put(Day.SATURDAY, "have fun");
        enummap.put(Day.SUNDAY, "have fun");

        System.out.println(enummap);

        String job=(String) enummap.get(today); //用get方法输入key值获得今天的工作信息
        System.out.println("job is:"+job);

    }

}

输出结果为：

{MONDAY=work work, TUESDAY=work work, WEDNESDAY=work work, THURSDAY=work work, FRIDAY=work work, SATURDAY=have fun, SUNDAY=have fun}
job is:work work

这个例子很清晰展示了EnumMap的基本用法

三、EnumSet

1.noneOf创建空队列

public class TestEnumSet {

    public static void main(String[] args) {
        EnumSet enumset=EnumSet.noneOf(Day.class);//创建一个空的Day枚举类型的队列
        System.out.println("Before:"+enumset);    //添加元素前状态
        enumset.add(Day.MONDAY);                  //添加三个元素
        enumset.add(Day.TUESDAY);
        enumset.add(Day.WEDNESDAY);
        System.out.println("After:"+enumset);     //添加元素后状态
    }

}

运行结果：

Before:[]
After:[MONDAY, TUESDAY, WEDNESDAY]

2.allOf创建满队列

public class TestEnumSet {

    public static void main(String[] args) {
        EnumSet enumset=EnumSet.allOf(Day.class);
        System.out.println(enumset);
    }

}

运行结果：

[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY]

3.range创建指定范围队列

public class TestEnumSet {

    public static void main(String[] args) {
        EnumSet enumset=EnumSet.range(Day.MONDAY, Day.FRIDAY);
        System.out.println(enumset);
    }

}

运行结果：

[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]

4.complementOf补集创建队列

public class TestEnumSet {

    public static void main(String[] args) {
        EnumSet enumset1=EnumSet.range(Day.MONDAY, Day.FRIDAY);
        System.out.println("enumset1:"+enumset1);

        EnumSet enumset2=EnumSet.complementOf(enumset1);//创建enumset1的补集enumset2
        System.out.println("enumset2:"+enumset2);
    }

}

运行结果：

enumset1:[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]
enumset2:[SATURDAY, SUNDAY]

5.copyOf复制创建

public class TestEnumSet {

    public static void main(String[] args) {
        EnumSet enumset1=EnumSet.range(Day.MONDAY, Day.FRIDAY);
        System.out.println("enumset1:"+enumset1);

        EnumSet enumset2=EnumSet.copyOf(enumset1);//复制enumset1创建enumset2
        System.out.println("enumset2:"+enumset2);
    }

}

运行结果：

enumset1:[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]
enumset2:[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]

6.copyOf复制ArrayList创建

public class TestEnumSet {

    public static void main(String[] args) {
        ArrayList<Day> list=new ArrayList<Day>();
        list.add(Day.MONDAY);
        list.add(Day.TUESDAY);
        list.add(Day.WEDNESDAY);
        list.add(Day.WEDNESDAY);
        System.out.println("list:"+list);

        EnumSet enumset=EnumSet.copyOf(list);
        System.out.println("enumset:"+enumset);
    }

}

运行结果：

list:[MONDAY, TUESDAY, WEDNESDAY, WEDNESDAY]
enumset:[MONDAY, TUESDAY, WEDNESDAY]

通过结果可以看出ArrayList内放置的元素可以重复，而EnumSet内放置的元素不重复，毕竟是枚举列嘛

Enum枚举详解和EnumSet、EnumMap使用

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