java枚举详解

一、简介

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、7种常见的用法

用法一:常量
  在JDK1.5 之前,我们定义常量都是: public static fianl.... 。现在好了,有了枚举,可以把相关的常量分组到一个枚举类型里,而且枚举提供了比常量更多的方法。

1 public enum Color { 
2     RED, GREEN, BLANK, YELLOW 
3 }

用法二:switch

 1 enum Signal {  
 2     GREEN, YELLOW, RED  
 3 }  
 4 public class TrafficLight {  
 5     Signal color = Signal.RED;  
 6     public void change() {  
 7         switch (color) {  
 8         case RED:  
 9             color = Signal.GREEN;  
10             break;  
11         case YELLOW:  
12             color = Signal.RED;  
13             break;  
14         case GREEN:  
15             color = Signal.YELLOW;  
16             break;  
17         }  
18     }  
19 } 

用法三:向枚举中添加新方法

 1 public enum Color {  
 2     RED("红色", 1), GREEN("绿色", 2), BLANK("白色", 3), YELLO("黄色", 4);  
 3     // 成员变量  
 4     private String name;  
 5     private int index;  
 6     // 构造方法  
 7     private Color(String name, int index) {  
 8         this.name = name;  
 9         this.index = index;  
10     }  
11     // 普通方法  
12     public static String getName(int index) {  
13         for (Color c : Color.values()) {  
14             if (c.getIndex() == index) {  
15                 return c.name;  
16             }  
17         }  
18         return null;  
19     }  
20     // get set 方法  
21     public String getName() {  
22         return name;  
23     }  
24     public void setName(String name) {  
25         this.name = name;  
26     }  
27     public int getIndex() {  
28         return index;  
29     }  
30     public void setIndex(int index) {  
31         this.index = index;  
32     }  
33 }

用法四:覆盖枚举的方法

下面给出一个toString()方法覆盖的例子。

 1 public enum Color {  
 2     RED("红色", 1), GREEN("绿色", 2), BLANK("白色", 3), YELLO("黄色", 4);  
 3     // 成员变量  
 4     private String name;  
 5     private int index;  
 6     // 构造方法  
 7     private Color(String name, int index) {  
 8         this.name = name;  
 9         this.index = index;  
10     }  
11     //覆盖方法  
12     @Override  
13     public String toString() {  
14         return this.index+"_"+this.name;  
15     }  
16 } 

用法五:实现接口

  所有的枚举都继承自java.lang.Enum类。由于Java 不支持多继承,所以枚举对象不能再继承其他类。

 1 public interface Behaviour {  
 2     void print();  
 3     String getInfo();  
 4 }  
 5 public enum Color implements Behaviour{  
 6     RED("红色", 1), GREEN("绿色", 2), BLANK("白色", 3), YELLO("黄色", 4);  
 7     // 成员变量  
 8     private String name;  
 9     private int index;  
10     // 构造方法  
11     private Color(String name, int index) {  
12         this.name = name;  
13         this.index = index;  
14     }  
15 //接口方法  
16     @Override  
17     public String getInfo() {  
18         return this.name;  
19     }  
20     //接口方法  
21     @Override  
22     public void print() {  
23         System.out.println(this.index+":"+this.name);  
24     }  
25 }

用法六:使用接口组织枚举

1 public interface Food {  
2     enum Coffee implements Food{  
3         BLACK_COFFEE,DECAF_COFFEE,LATTE,CAPPUCCINO  
4     }  
5     enum Dessert implements Food{  
6         FRUIT, CAKE, GELATO  
7     }  
8 }

用法七:关于枚举集合的使用

java.util.EnumSet和java.util.EnumMap是两个枚举集合。EnumSet保证集合中的元素不重复;EnumMap中的 key是enum类型,而value则可以是任意类型。关于这个两个集合的使用就不在这里赘述。

posted @ 2019-12-21 18:58  跃小云  阅读(4745)  评论(0编辑  收藏  举报