java 枚举

Posted 2022-06-29 weixuqin

枚举

注意：枚举是单例的，单例要保证私有化构造器。

1. 如何自定义枚举类

1. 提供类的属性，声明为 private final
2. 声明为 final 的属性，在构造器中初始化
3. 通过公共方法（get）方法来调用属性
4. 创建枚举类的对象：将类的对象声明 public static final

举例如下：

class Season 
    //1. 提供类的属性，声明为 private final
    private final String seasonName;
    private final String seasonDesc;

    //2. 声明为 final 的属性，在构造器中初始化
    private Season (String seasonName, String seasonDesc) 
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    

    //3. 通过公共的方法来调用属性
    public String getSeasonName() 
        return seasonName;
    
    public String getSeasonDesc() 
        return seasonDesc;
    

    //4. 创建枚举类的对象：将类的对象声明 public static final
    public static final Season SPRING = new Season("spring", "春暖花开");
    public static final Season SUMMER = new Season("summer", "夏日炎炎");
    public static final Season AUTUMN = new Season("autumn", "秋高气爽");
    public static final Season WINTER = new Season("winter", "白雪皑皑");

    @Override
    public String toString() 
        return "Season [seasonName=" + seasonName + ", seasonDesc=" + seasonDesc + "]";
    

    public void show() 
        System.out.println("这是一个季节");
    



public class TestSeason 
    public static void main(String[] args) 
        Season spring = Season.SPRING;
        System.out.println(spring);
        spring.show();
        System.out.println(spring.getSeasonName());
    

2. 如何使用 enum 关键字来定义枚举类

将对象放到最前面，并且不需要定义对象的其他属性。
使用 enum 类来定义枚举类，可以使用下述两种方法来返回值。

1. values() 方法：返回所有值
2. valueOf() 方法：返回指定对象的值（要求传入的形参name是枚举类对象的名字）
   否则，报 java.lang.IllegalArgumentException 异常

改造一下上述代码：

enum Season 

    SPRING("spring", "春暖花开"),
    SUMMER("summer", "夏日炎炎"),
    AUTUMN("autumn", "秋高气爽"),
    WINTER("winter", "白雪皑皑");


    private final String seasonName;
    private final String seasonDesc;

    private Season (String seasonName, String seasonDesc) 
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    

    public String getSeasonName() 
        return seasonName;
    
    public String getSeasonDesc() 
        return seasonDesc;
    



    @Override
    public String toString() 
        return "Season [seasonName=" + seasonName + ", seasonDesc=" + seasonDesc + "]";
    

    public void show() 
        System.out.println("这是一个季节");
    



public class TestSeason 
    public static void main(String[] args) 
        Season spring = Season.SPRING;
        System.out.println(spring);
        spring.show();
        System.out.println(spring.getSeasonName());

        // 1. values() 方法：返回所有值
        Season[] seasons = Season.values();
        for (int i=0; i<seasons.length;i++) 
            System.out.println(seasons[i]);
        

        // 2. valueOf() 方法：返回指定对象的值（要求传入的形参name是枚举类对象的名字）
        // 否则，报 java.lang.IllegalArgumentException 异常
        String str = "SPRING";
        Season sea = Season.valueOf(str);
        System.out.println(sea);
    

3. 枚举类实现接口

可以让不同的枚举类的对象调用被重写的抽象方法，执行的效果不同。（相当于让每个对象重写抽象方法）

改造如下：

interface Info 
    void show();


enum Season implements Info 

    SPRING("spring", "春暖花开")
        public void show() 
            System.out.println("春天在哪里?");
        
    ,
    SUMMER("summer", "夏日炎炎")
        public void show() 
            System.out.println("夏天在哪里?");
        
    ,
    AUTUMN("autumn", "秋高气爽")
        public void show() 
            System.out.println("秋天在哪里?");
        
    ,
    WINTER("winter", "白雪皑皑")
        public void show() 
            System.out.println("冬天在哪里?");
        
    ;


    private final String seasonName;
    private final String seasonDesc;

    private Season (String seasonName, String seasonDesc) 
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    

    public String getSeasonName() 
        return seasonName;
    
    public String getSeasonDesc() 
        return seasonDesc;
    



    @Override
    public String toString() 
        return "Season [seasonName=" + seasonName + ", seasonDesc=" + seasonDesc + "]";
    

    public void show() 
        System.out.println("这是一个季节");
    



public class TestSeason 
    public static void main(String[] args) 
        Season spring = Season.SPRING;
        System.out.println(spring);
        spring.show();
        System.out.println(spring.getSeasonName());

        // 1. values() 方法：返回所有值
        Season[] seasons = Season.values();
        for (int i=0; i<seasons.length;i++) 
            System.out.println(seasons[i]);
        

        // 2. valueOf() 方法：返回指定对象的值（要求传入的形参name是枚举类对象的名字）
        // 否则，报 java.lang.IllegalArgumentException 异常
        String str = "WINTER";
        Season sea = Season.valueOf(str);
        System.out.println(sea);
        sea.show();
    

---

完。