【深入Java枚举类：不仅仅是常量的容器】

前言：

Java 枚举类（enum）是一种特殊的数据类型，用来定义一组预定义的常量。枚举类不仅可以包含常量，还能定义方法、字段和构造器，使其功能更加强大和灵活。

引入

【1】数学：枚举法：
1<x<4
2<y<5
求x+y=6

枚举法：一枚一枚的列举出来。前提：有限，确定

【2】在java中，类的对象是有限个，确定的。这个类我们可以定义为枚举类。
举例：
星期：一二三四五六日  
性别：男女
季节：春夏秋冬

【3】自定义枚举类：

（JDK1.5之前自定义枚举类） 

/**
 * @Auther: themyth
 * 定义枚举类：季节
 */
public class Season {
    //属性：                         属性的值不能变
    private final String seasonName ;//季节名字
    private final String seasonDesc ;//季节描述
    //利用构造器对属性进行赋值操作：
    //构造器私有化，外界不能调用这个构造器，只能Season内部自己调用
    private Season(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }
    //提供枚举类的有限的 确定的对象：   对象的值不能变
    public static final Season SPRING = new Season("春天","春暖花开");
    public static final Season SUMMER = new Season("夏天","烈日炎炎");
    public static final Season AUTUMN = new Season("秋天","硕果累累");
    public static final Season WINTER = new Season("冬天","冰天雪地");
    //额外因素：
    public String getSeasonName() {
        return seasonName;
    }
    public String getSeasonDesc() {
        return seasonDesc;
    }
    //toString():
    @Override
    public String toString() {
        return "Season{" +
                "seasonName='" + seasonName + '\'' +
                ", seasonDesc='" + seasonDesc + '\'' +
                '}';
    }
}

测试类：

/**
 * @Auther: themyth
 */
public class TestSeason {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        Season summer = Season.SUMMER;
        System.out.println(summer/*.toString()*/);
        System.out.println(summer.getSeasonName());
    }
}

结果：

（JDK1.5以后使用enum关键字创建枚举类）： 

变为下面的枚举类： 

/**
 * @Auther: themyth
 * 定义枚举类：季节
 */
public enum Season {
    //提供枚举类的有限的 确定的对象：   对象的值不能变--->enum枚举要求对象（常量）必须放在最开始的位置
    //多个对象之间用，进行连接，最后一个对象后面用；结束
    SPRING("春天","春暖花开"),
    SUMMER("夏天","烈日炎炎"),
    AUTUMN("秋天","硕果累累"),
    WINTER("冬天","冰天雪地");
    //属性：                         属性的值不能变
    private final String seasonName ;//季节名字
    private final String seasonDesc ;//季节描述
    //利用构造器对属性进行赋值操作：
    //构造器私有化，外界不能调用这个构造器，只能Season内部自己调用
    private Season(String seasonName, String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }
    //额外因素：
    public String getSeasonName() {
        return seasonName;
    }
    public String getSeasonDesc() {
        return seasonDesc;
    }
    //toString():
    @Override
    public String toString() {
        return "Season{" +
                "seasonName='" + seasonName + '\'' +
                ", seasonDesc='" + seasonDesc + '\'' +
                '}';
    }
}

使用枚举类：
注意：先将Season枚举类里面的toString()方法注释掉 

/**
 * @Auther: themyth
 */
public class TestSeason {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        Season winter = Season.WINTER;
        System.out.println(winter);
        //enum关键字对应的枚举类的上层父类是：java.lang.Enum
        //但是我们自定义的枚举类的上层父类：Object
        System.out.println(Season.class.getSuperclass().getName());
    }
}

在源码中经常看到别人定义的枚举类形态：

public enum Season {//相当于Season下面有四个对象(常量)，并且它们前面的修饰符都是public static final，后面相当于在调用空构造器，只是这儿可以省略
    SPRING,
    SUMMER(),//括号可以省略。
    AUTUMN(),
    WINTER;
}

为什么这么简单：因为这个枚举类底层没有属性，属性，构造器，toString，get方法都删掉不写了，然后按理来说应该写为：SPRING()   现在连（）可以省略 就变成  SPRING
看到的形态就剩：常量名（对象名）  

案例：
Thread中的枚举类：State  

public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,
        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,
        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
        BLOCKED,
        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * <ul>
         *   <li>{@link Object#wait() Object.wait} with no timeout</li>
         *   <li>{@link #join() Thread.join} with no timeout</li>
         *   <li>{@link LockSupport#park() LockSupport.park}</li>
         * </ul>
         *
         * <p>A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * is waiting for a specified thread to terminate.
         */
        WAITING,
        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * <ul>
         *   <li>{@link #sleep Thread.sleep}</li>
         *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
         *   <li>{@link #join(long) Thread.join} with timeout</li>
         *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
         *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
         * </ul>
         */
        TIMED_WAITING,
        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }

Enum类中的常用方法

/**
 * @Auther: themyth
 */
public class TestSeason {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        //用enum关键字创建的Season枚举类上面的父类是：java.lang.Enum，常用方法子类Season可以直接拿来使用
        //toString():--->获取对象的名字
        Season autumn = Season.AUTUMN;
        System.out.println(autumn/*.toString()*/);
        System.out.println("-----------------------");
        //values():--->返回枚举类对象的数组
        Season[] values = Season.values();
        for(Season s : values){
            System.out.println(s/*.toString()*/);
        }
        System.out.println("-----------------------");
        //valueOf(String name):通过对象名字获取这个枚举对象
        //注意：对象的名字必须传正确，否则抛出异常
        Season autumn1 = Season.valueOf("AUTUMN");
        System.out.println(autumn1);
    }
}

枚举类实现接口

定义一个接口：

/**
 * @Auther: themyth
 */
public interface TestInterface {
    void show();
}

枚举类实现接口，并且重写show方法：

public enum Season implements TestInterface{
    SPRING,
    SUMMER,
    AUTUMN,
    WINTER;
    @Override
    public void show() {
        System.out.println("这是Season...");
    }
}

测试类： 

/**
 * @Auther: themyth
 */
public class Test {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        Season autumn = Season.AUTUMN;
        autumn.show();
        Season summer = Season.SUMMER;
        summer.show();
    }
}

上面发现所有的枚举对象，调用这个show方法的时候走的都是同一个方法，结果都一样：

但是现在我想要：不同的对象  调用的show方法也不同：
 

public enum Season implements TestInterface{
    SPRING{
        @Override
        public void show() {
            System.out.println("这是春天");
        }
    },
    SUMMER{
        @Override
        public void show() {
            System.out.println("这是夏天");
        }
    },
    AUTUMN{
        @Override
        public void show() {
            System.out.println("这是秋天");
        }
    },
    WINTER{
        @Override
        public void show() {
            System.out.println("这是冬天");
        }
    };
   /* @Override
    public void show() {
        System.out.println("这是Season...");
    }*/
}

测试类：

/**
 * @Auther: themyth
 */
public class Test {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        Season autumn = Season.AUTUMN;
        autumn.show();
        Season summer = Season.SUMMER;
        summer.show();
    }
}

实际应用

/**
 * @Auther: themyth
 */
public class Person {
    //属性：
    private int age;
    private String name;
    private Gender sex;//将枚举类作为一个属性
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Gender getSex() {
        return sex;
    }
    public void setSex(Gender sex) {
        this.sex = sex;
    }
    @Override
    public String toString() {
        return "Person{" +
                "age=" + age +
                ", name='" + name + '\'' +
                ", sex='" + sex + '\'' +
                '}';
    }
}

/**
 * @Auther: themyth
 */
public enum Gender {
    男,
    女;
}

/**
 * @Auther: themyth
 */
public class Test {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        Person p = new Person();
        p.setAge(19);
        p.setName("lili");
        p.setSex(Gender.男);//传入枚举类Gender的对象:--->在入口处对参数进行了限制
        System.out.println(p);
    }
}

还可以通过枚举结合switch处理：

/**
 * @Auther: themyth
 */
public class Test02 {
    //这是一个main方法，是程序的入口：
    public static void main(String[] args) {
        Gender sex = Gender.男;
        //switch后面的()中可以传入枚举类型：
        //switch后面的()：byte，short，int，char，string，枚举
        switch (sex){
            case 女:
                System.out.println("是个女孩");
                break;
            case 男:
                System.out.println("是个男孩");
                break;
        }
    }
}