Enum介绍及高级使用-阿里云开发者社区

Enum 的全称为 enumeration，是我们较为常用的类，存放在 java.lang 包中。

public enum EnumTest {
    WALK(1, "慢走"),
    QUICK(2, "疾步"),
    RUN(3, "跑步");

    private Integer code;
    private String desc;

    EnumTest(Integer code, String desc) {
        this.code = code;
        this.desc = desc;
    }

    public Integer getCode() {
        return code;
    }

    public void setCode(Integer code) {
        this.code = code;
    }
}

可以用if/else、switch来遍历

public static EnumTest getByCode(int code) {
    for(EnumTest et : EnumTest.values()) {
        if (et.getCode().intValue() == code) {
            System.out.println(et.getCode() + "_" + et.ordinal());
            return et;
        }
    }
    return null;
}

遍历所有定义的value。
ordinal() 方法来得到定义枚举值常量的位置顺序，初始化第一个常量的顺序为0.

enum 的语法结构尽管和 class 的语法不一样，但是经过编译器编译之后产生的是一个class文件。该class文件经过反编译可以看到实际上是生成了一个类，该类继承了java.lang.Enum。
enum 不能使用 extends 关键字继承其他类，因为 enum 已经继承了 java.lang.Enum（java是单一继承）。

但使用枚举，我们需要了解一些注意事项，且可以用枚举来实现怎样的功效？

枚举类型带参数，必须要对应匹配的构造方法。
可以创建一个enum类，把它看做一个普通的类。除了它不能继承其他类了。
switch()参数可以使用enum了。
values()方法是编译器插入到enum定义中的static方法，所以，当你将enum实例向上转型为父类Enum是，values()就不可访问了。解决办法：在Class中有一个getEnumConstants()方法，所以即便Enum接口中没有values()方法，我们仍然可以通过Class对象取得所有的enum实例。
无法从enum继承子类，如果需要扩展enum中的元素，在一个接口的内部，创建实现该接口的枚举，以此将元素进行分组。达到将枚举元素进行分组。
enum允许程序员为eunm实例编写方法。所以可以为每个enum实例赋予各自不同的行为。
以职责链(Chain of Responsibility) 的方式来使用枚举，把所有任务定义为枚举值，遍历该枚举依次进行任务处理。
使用enum的状态机，然后来动态处理任务。
枚举在初始化时，实际会对每个枚举都进行构造实例化。

    
    EnumTest(Integer code, String desc) {
        System.out.println(code + "_" + desc);
        this.code = code;
        this.desc = desc;
    }

1_慢走
2_疾步
3_跑步
WALK

我们来进行编译以及反编译，看下具体是怎样的内容：

D:\App\src\main\java\com\test>javac EnumTest.java
D:\App\src\main\java\com\test>javap EnumTest.class
Compiled from "EnumTest.java"
public final class com.test.EnumTest extends java.lang.Enum<com.test.EnumTest> {
  public static final com.test.EnumTest WALK;
  public static final com.test.EnumTest QUICK;
  public static final com.test.EnumTest RUN;
  public static com.test.EnumTest[] values();
  public static com.test.EnumTest valueOf(java.lang.String);
  public static com.test.EnumTest getByCode(int);
  public java.lang.Integer getCode();
  public void setCode(java.lang.Integer);
  static {};
}

枚举类是继承了Enum，所以枚举类不能再继承其他类。定义的枚举值，在编译之后都会生成静态不可变的枚举对象。

最后来个完整的实例：

public class soundModeDemo
{
    /**
     * 将用enum枚举去除以下方法中的switch/case
     */
    public void processAlarmEvents(AlarmEvent[] events)
    {
        AlarmEvent lastEvent = events[events.length - 1];
        byte severity = lastEvent.getAlarmData().getPerceivedSeverity();
 
        switch (lastEvent.getEventType())
        {
            case AlarmEvent.NEW_ALARM:
                if (severity != PerceivedSeverity.SEVERITY_CLEARED)
                {
                    System.out.println("playAlarmSound:"+severity);
                }
                break;
            case AlarmEvent.DELETE_ALARM:
                if (severity == PerceivedSeverity.SEVERITY_CLEARED)
                {
                    return;
                }
                System.out.println("playClearSound:"+severity);
                break;
            case AlarmEvent.SEVERITY_CHANGED:
                byte oldSeverity = lastEvent.getOriginalAlarmData().getPerceivedSeverity();
                if ((oldSeverity == PerceivedSeverity.SEVERITY_CLEARED))
                {
                    System.out.println("playAlarmSound:"+severity);
                }
                else if (severity == PerceivedSeverity.SEVERITY_CLEARED)
                {
                    System.out.println("playClearSound:"+severity);
                }
                break;
        }
    }

//带有构造方法的枚举实现

public enum SoundPlay{
    NEW_ALARM(AlarmEvent.NEW_ALARM, PerceivedSeverity.SEVERITY_CLEARED, (byte)0),
    DELETE_ALARM(AlarmEvent.DELETE_ALARM, PerceivedSeverity.SEVERITY_CLEARED, (byte)0),
    SEVERITY_CHANGED(AlarmEvent.SEVERITY_CHANGED, PerceivedSeverity.SEVERITY_CLEARED, (byte)0);
   
    private final byte perceivedSeverity = PerceivedSeverity.SEVERITY_CLEARED;

    private byte eventType;
    private byte severity;
    private byte oldSeverity;

    public void setSeverity(byte severity)
    {
        this.severity = severity;
    }
    public void setOldSeverity(byte oldSeverity)
    {
        this.oldSeverity = oldSeverity;
    }
    Private SoundPlay(byte eventType, byte severity, byte oldSeverity){
        this.eventType = eventType;
        this.severity = severity;
        this.oldSeverity = oldSeverity;
    }
   
    public void printAllValues() {
        System.out.println("eventType:"+this.eventType+
                           "  severity:"+this.severity+
                           "  oldSeverity:"+this.oldSeverity);
    }  
    public void doPlay(){
        if(this.eventType == AlarmEvent.NEW_ALARM)playNewAlarm();
        if(this.eventType == AlarmEvent.DELETE_ALARM)playDeleteAlarm();
        if(this.eventType == AlarmEvent.SEVERITY_CHANGED)playSeverityChange();
    } 
    private void playSeverityChange()
    {
        if (this.oldSeverity == perceivedSeverity){
            System.out.println("playAlarmSound:" + this.oldSeverity);
        }else if (this.severity == perceivedSeverity){
            System.out.println("playClearSound:" + this.severity);
        }
    }
    private void playDeleteAlarm()
    {
        if (this.severity == perceivedSeverity){
            return;
        }else{
            System.out.println("playClearSound:" + this.severity);
        }
    }
    private void playNewAlarm()
    {
        if (this.severity != perceivedSeverity){
            System.out.println("playAlarmSound:" + this.severity);
        }
    } 
}
public static void main(String[] args)
{
    //每次调用，都需要设置severity或oldSeverity
    SoundPlay soundPlay1 = SoundPlay.NEW_ALARM;
    soundPlay1.setSeverity((byte)4);
    soundPlay1.printAllValues();
    soundPlay1.doPlay();
}

}

//最常用标准的枚举方式

public enum SoundPlay{
    NEW_ALARM{
        @Override
        public void play(AlarmEvent event)
        {
            if (event.getAlarmData().getPerceivedSeverity() !=  PerceivedSeverity.SEVERITY_CLEARED){
                System.out.println("playAlarmSound:" + event.getAlarmData().getPerceivedSeverity());
            }
        }

        @Override
        protected byte getType()
        {
            return AlarmEvent.NEW_ALARM;
        }
    },
    DELETE_ALARM{
        @Override
        public void play(AlarmEvent event)
        {
            if (event.getAlarmData().getPerceivedSeverity() == PerceivedSeverity.SEVERITY_CLEARED){
                return;
            }else{
                System.out.println("playClearSound:" + event.getAlarmData().getPerceivedSeverity());
            }
        }
        
        @Override
        protected byte getType()
        {
            return AlarmEvent.DELETE_ALARM;
        }
    },
    SEVERITY_CHANGED {
        @Override
        public void play(AlarmEvent event)
        {
            if (event.getAlarmData().getPerceivedSeverity() == PerceivedSeverity.SEVERITY_CLEARED){
                System.out.println("playAlarmSound:" + event.getOriginalAlarmData().getPerceivedSeverity());
            }else if (event.getAlarmData().getPerceivedSeverity() == PerceivedSeverity.SEVERITY_CLEARED){
                System.out.println("playClearSound:" + event.getAlarmData().getPerceivedSeverity());
            }
        }
        
        @Override
        protected byte getType()
        {
            return AlarmEvent.DELETE_ALARM;
        }
    },
    DEFAULT{
        @Override
        public void play(AlarmEvent event)
        {
           System.out.println("没有匹配的枚举值内容，调用的默认方法！");
        }
        @Override
        protected byte getType()
        {
            return -1;
        }
    };

    public abstract void play(AlarmEvent event);
    protected abstract byte getType();
    
    public static SoundPlay valueOf(byte eventType){
        for(SoundPlay soundPlay:SoundPlay.values()){
            if(eventType==soundPlay.getType()){
                return soundPlay;
            }
        }
        return DEFAULT;
    }
}

Enum介绍及高级使用

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