Enum介绍及高级使用

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？

1. 枚举类型带参数，必须要对应匹配的构造方法。
2. 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。
3. switch()参数可以使用enum了。
4. 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。
5. 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。
6. 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。
7. 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。
8. 使用enum的状态机，然后来动态处理任务。
9. 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; } }