enum枚举的研究
enum这个关键字在java中既熟悉又陌生，因为很少用它。其实enum的功能完全可以用其他方式实现，不过对它的原理
还是不太清楚。下面开始研究。
写个简单的例子：
package org.test.algo;
import java.util.EnumMap;
import java.util.EnumSet;
import java.util.Enumeration;
public class EnumTest {
enum CellphoneSystem{Android,IOS,WP8,BLACBERRY,Ubuntu}
// enum SEASON{SPRING("googd"),SUMMER("better"),AUTOMN("best"),WINTER("bad")}
/*直接向上面写，会有报错: The constructor EnumTest.SEASON(String) is undefined
即没有这种声明的构造函数。到这里，是不是觉得enum像类？
*/
enum SEASON{
SPRING("googd"),SUMMER("better"),AUTOMN("best"),WINTER("bad");
String feel="";
SEASON(String myfeel){
this.feel=myfeel;
}
}
public static void main(String[] sdfs){
}
}
感觉enum的声明既像声明对象，又像声明类。
对这个类编译后，我发现会多出2个类：
由此我们大胆猜测，enum就是类。反编译后看看源码：
EnumTest
// Decompiled Using: FrontEnd Plus v2.03 and the JAD Engine
// Available From: http://www.reflections.ath.cx
// Decompiler options: packimports(3)
// Source File Name: EnumTest.java
package org.test.algo;
public class EnumTest
{
static final class CellphoneSystem extends Enum
{
public static CellphoneSystem[] values()
{
CellphoneSystem acellphonesystem[];
int i;
CellphoneSystem acellphonesystem1[];
System.arraycopy(acellphonesystem = ENUM$VALUES, 0, acellphonesystem1 = new
CellphoneSystem[i = acellphonesystem.length], 0, i);
return acellphonesystem1;
}
public static CellphoneSystem valueOf(String s)
{
return (CellphoneSystem)Enum.valueOf(org/test/algo/EnumTest$CellphoneSystem, s);
}
public static final CellphoneSystem Android;
public static final CellphoneSystem IOS;
public static final CellphoneSystem WP8;
public static final CellphoneSystem BLACBERRY;
public static final CellphoneSystem Ubuntu;
private static final CellphoneSystem ENUM$VALUES[];
static
{
Android = new CellphoneSystem("Android", 0);
IOS = new CellphoneSystem("IOS", 1);
WP8 = new CellphoneSystem("WP8", 2);
BLACBERRY = new CellphoneSystem("BLACBERRY", 3);
Ubuntu = new CellphoneSystem("Ubuntu", 4);
ENUM$VALUES = (new CellphoneSystem[] {
Android, IOS, WP8, BLACBERRY, Ubuntu
});
}
private CellphoneSystem(String s, int i)
{
super(s, i);
}
}
static final class SEASON extends Enum
{
public static SEASON[] values()
{
SEASON aseason[];
int i;
SEASON aseason1[];
System.arraycopy(aseason = ENUM$VALUES, 0, aseason1 = new SEASON[i = aseason.length],
0, i);
return aseason1;
}
public static SEASON valueOf(String s)
{
return (SEASON)Enum.valueOf(org/test/algo/EnumTest$SEASON, s);
}
public static final SEASON SPRING;
public static final SEASON SUMMER;
public static final SEASON AUTOMN;
public static final SEASON WINTER;
String feel;
private static final SEASON ENUM$VALUES[];
static
{
SPRING = new SEASON("SPRING", 0, "googd");
SUMMER = new SEASON("SUMMER", 1, "better");
AUTOMN = new SEASON("AUTOMN", 2, "best");
WINTER = new SEASON("WINTER", 3, "bad");
ENUM$VALUES = (new SEASON[] {
SPRING, SUMMER, AUTOMN, WINTER
});
}
private SEASON(String s, int i, String myfeel)
{
super(s, i);
feel = "";
feel = myfeel;
}
}
public EnumTest()
{
}
public static void main(String args[])
{
}
}EnumTest$CellphoneSystem
// Decompiled Using: FrontEnd Plus v2.03 and the JAD Engine
// Available From: http://www.reflections.ath.cx
// Decompiler options: packimports(3)
// Source File Name: EnumTest.java
package org.test.algo;
// Referenced classes of package org.test.algo:
// EnumTest
static final class EnumTest$CellphoneSystem extends Enum
{
public static EnumTest$CellphoneSystem[] values()
{
EnumTest$CellphoneSystem aenumtest$cellphonesystem[];
int i;
EnumTest$CellphoneSystem aenumtest$cellphonesystem1[];
System.arraycopy(aenumtest$cellphonesystem = ENUM$VALUES, 0, aenumtest$cellphonesystem1 =
new EnumTest$CellphoneSystem[i = aenumtest$cellphonesystem.length], 0, i);
return aenumtest$cellphonesystem1;
}
public static EnumTest$CellphoneSystem valueOf(String s)
{
return (EnumTest$CellphoneSystem)Enum.valueOf(org/test/algo/EnumTest$CellphoneSystem, s);
}
public static final EnumTest$CellphoneSystem Android;
public static final EnumTest$CellphoneSystem IOS;
public static final EnumTest$CellphoneSystem WP8;
public static final EnumTest$CellphoneSystem BLACBERRY;
public static final EnumTest$CellphoneSystem Ubuntu;
private static final EnumTest$CellphoneSystem ENUM$VALUES[];
static
{
Android = new EnumTest$CellphoneSystem("Android", 0);
IOS = new EnumTest$CellphoneSystem("IOS", 1);
WP8 = new EnumTest$CellphoneSystem("WP8", 2);
BLACBERRY = new EnumTest$CellphoneSystem("BLACBERRY", 3);
Ubuntu = new EnumTest$CellphoneSystem("Ubuntu", 4);
ENUM$VALUES = (new EnumTest$CellphoneSystem[] {
Android, IOS, WP8, BLACBERRY, Ubuntu
});
}
private EnumTest$CellphoneSystem(String s, int i)
{
super(s, i);
}
}EnumTest$SEASON
// Decompiled Using: FrontEnd Plus v2.03 and the JAD Engine
// Available From: http://www.reflections.ath.cx
// Decompiler options: packimports(3)
// Source File Name: EnumTest.java
package org.test.algo;
// Referenced classes of package org.test.algo:
// EnumTest
static final class EnumTest$SEASON extends Enum
{
public static EnumTest$SEASON[] values()
{
EnumTest$SEASON aenumtest$season[];
int i;
EnumTest$SEASON aenumtest$season1[];
System.arraycopy(aenumtest$season = ENUM$VALUES, 0, aenumtest$season1 = new
EnumTest$SEASON[i = aenumtest$season.length], 0, i);
return aenumtest$season1;
}
public static EnumTest$SEASON valueOf(String s)
{
return (EnumTest$SEASON)Enum.valueOf(org/test/algo/EnumTest$SEASON, s);
}
public static final EnumTest$SEASON SPRING;
public static final EnumTest$SEASON SUMMER;
public static final EnumTest$SEASON AUTOMN;
public static final EnumTest$SEASON WINTER;
String feel;
private static final EnumTest$SEASON ENUM$VALUES[];
static
{
SPRING = new EnumTest$SEASON("SPRING", 0, "googd");
SUMMER = new EnumTest$SEASON("SUMMER", 1, "better");
AUTOMN = new EnumTest$SEASON("AUTOMN", 2, "best");
WINTER = new EnumTest$SEASON("WINTER", 3, "bad");
ENUM$VALUES = (new EnumTest$SEASON[] {
SPRING, SUMMER, AUTOMN, WINTER
});
}
private EnumTest$SEASON(String s, int i, String myfeel)
{
super(s, i);
feel = "";
feel = myfeel;
}
}在反编译的EnumTest 中，我们发现2个enum确实声明了2个类，而且是static final类型，都继承了Enum。
里面对应的变量都是public static final类型，都有构造函数，都有values()，valueOf(String s)方法。
在values()，valueOf(String s)方法中会用到和上面类似的类 EnumTest$CellphoneSystem，EnumTest$SEASON
从源码中可以看出，enum在编译的时候就声明对应的enum类final static。每个值都是一个public static final变量，并在static
块中调用构造函数。
所以我们才可以通过类名来直接取值，就像public static 一样。
下面我把上面的EnumTest类改改，研究enum的用法：
package org.test.algo;
import java.util.EnumMap;
import java.util.EnumSet;
import java.util.Enumeration;
import java.util.Iterator;
public class EnumTest {
enum CellphoneSystem{Android,IOS,WP8,BLACBERRY,Ubuntu};
// enum SEASON{SPRING("googd"),SUMMER("better"),AUTOMN("best"),WINTER("bad")}
/*直接向上面写，会有报错: The constructor EnumTest.SEASON(String) is undefined
即没有这种声明的构造函数。到这里，是不是觉得enum像类？
*/
enum SEASON{
SPRING("googd"),SUMMER("better"),AUTOMN("best"),WINTER("bad");//声明
String feel="";
//构造函数
SEASON(String myfeel){
this.feel=myfeel;
}
//配置get,set方法就可以改变参数值
public String getFeel() {
return feel;
}
public void setFeel(String feel) {
this.feel = feel;
}
//和在类中一样，也可以声明方法。
public static SEASON whichlevel(String level){
SEASON res=null;
if(level==null){
}else{
if(level.trim().equals("A")){
res=SPRING;
}else if(level.trim().equals("B")){
res=SUMMER;
}else if(level.trim().equals("C")){
res=AUTOMN;
}else if(level.trim().equals("D")){
res=WINTER;
}
}
return res;
}
};
public static void main(String[] sdfs){
//1遍历
// EnumTest.SEASON[] seasons= EnumTest.SEASON.values();//或者下面的方式也可以
SEASON[] seasons= SEASON.values();
for(SEASON se:seasons){
System.out.println(se.ordinal()+","+se+","+se.toString()+","+se.getFeel());
}
//2
System.out.println("-----------------------------");
System.out.println(EnumTest.SEASON.SUMMER);//SUMMER
System.out.println(EnumTest.SEASON.AUTOMN.getFeel());//best
EnumTest.SEASON.SUMMER.setFeel("awesome");
System.out.println(EnumTest.SEASON.SUMMER.getFeel());//awesome
System.out.println(EnumTest.SEASON.SUMMER.whichlevel("C"));//AUTOMN
System.out.println(EnumTest.SEASON.valueOf("WINTER"));//WINTER
System.out.println(EnumTest.SEASON.WINTER.getDeclaringClass());//class org.test.algo.EnumTest$SEASON
//3单独的enum类声明public enum Pelple
System.out.println("-----------------------------");
System.out.println(Pelple.SEX+"--"+Pelple.SEX.getSexValueOfpeople());
Pelple.SEX.setFlag(false);
System.out.println(Pelple.SEX+"--"+Pelple.SEX.getSexValueOfpeople());
//4,EnumMap的使用
System.out.println("-----------------------------");
EnumMap emap=new EnumMap(EnumTest.SEASON.class);
emap.put(EnumTest.SEASON.SPRING, "春天");
emap.put(EnumTest.SEASON.SUMMER, "夏天");
emap.put(EnumTest.SEASON.AUTOMN, "秋天");
emap.put(EnumTest.SEASON.WINTER, "冬天");
System.out.println(emap.get(EnumTest.SEASON.SUMMER));
System.out.println("-----");
//遍历map
for(Iterator it=emap.keySet().iterator();it.hasNext();){
Object key=it.next();
Object value=emap.get(key);
System.out.println("key:"+key+",value:"+value);
}
}
}
/*
打印：
0,SPRING,SPRING,googd
1,SUMMER,SUMMER,better
2,AUTOMN,AUTOMN,best
3,WINTER,WINTER,bad
-----------------------------
SUMMER
best
awesome
AUTOMN
WINTER
class org.test.algo.EnumTest$SEASON
-----------------------------
SEX--男
SEX--女
-----------------------------
夏天
-----
key:SPRING,value:春天
key:SUMMER,value:夏天
key:AUTOMN,value:秋天
key:WINTER,value:冬天
注意;
一个打印EnumTest.SEASON.WINTER的声明类显示：class org.test.algo.EnumTest$SEASON
而不是EnumTest
声明的时候可以把enum看成变量，也可以看成类。
在大括号的后面可以有分号;也可以不加分号;
例如enum CellphoneSystem{Android,IOS,WP8,BLACBERRY,Ubuntu}
和enum CellphoneSystem{Android,IOS,WP8,BLACBERRY,Ubuntu};都是对的。
我们在这个EnumTest中声明了enum，第一感觉就是变量。
我们也可以单独在外面申请一个enum的类（和class相似 ）
public enum Pelple.
*/
里面用到一个enum类：
Pelple
package org.test.algo;
public enum Pelple {
SEX(true);
boolean flag=false;
Pelple(boolean f){
this.flag=f;
}
public String getSexValueOfpeople(){
String res="男";
if(this.isFlag()){
res="男";
}else{
res="女";
}
return res;
}
public boolean isFlag() {
return flag;
}
public void setFlag(boolean flag) {
this.flag = flag;
}
}
对enum的使用，我们完全可以当做特殊的static final 类来使用，需要有构造函数，需要用构造函数声明对象。
也可以像普通类一样有自己的方法。这样使用enum就会很简单，只要理解了，在程序设计上才会得心应手。
------------下面是Enum类的源码，在java.lang.*包中，有兴趣可以研究研究---------
/*
* @(#)Enum.java 1.15 06/01/27
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang;
import java.io.Serializable;
/**
* This is the common base class of all Java language enumeration types.
*
* @author Josh Bloch
* @author Neal Gafter
* @version 1.15, 01/27/06
* @since 1.5
*/
public abstract class Enum>
implements Comparable, Serializable {
/**
* The name of this enum constant, as declared in the enum declaration.
* Most programmers should use the {@link #toString} method rather than
* accessing this field.
*/
private final String name;
/**
* Returns the name of this enum constant, exactly as declared in its
* enum declaration.
*
* Most programmers should use the {@link #toString} method in
* preference to this one, as the toString method may return
* a more user-friendly name. This method is designed primarily for
* use in specialized situations where correctness depends on getting the
* exact name, which will not vary from release to release.
*
* @return the name of this enum constant
*/
public final String name() {
return name;
}
/**
* The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this field. It is designed
* for use by sophisticated enum-based data structures, such as
* {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*/
private final int ordinal;
/**
* Returns the ordinal of this enumeration constant (its position
* in its enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this method. It is
* designed for use by sophisticated enum-based data structures, such
* as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*
* @return the ordinal of this enumeration constant
*/
public final int ordinal() {
return ordinal;
}
/**
* Sole constructor. Programmers cannot invoke this constructor.
* It is for use by code emitted by the compiler in response to
* enum type declarations.
*
* @param name - The name of this enum constant, which is the identifier
* used to declare it.
* @param ordinal - The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*/
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
}
/**
* Returns the name of this enum constant, as contained in the
* declaration. This method may be overridden, though it typically
* isn't necessary or desirable. An enum type should override this
* method when a more "programmer-friendly" string form exists.
*
* @return the name of this enum constant
*/
public String toString() {
return name;
}
/**
* Returns true if the specified object is equal to this
* enum constant.
*
* @param other the object to be compared for equality with this object.
* @return true if the specified object is equal to this
* enum constant.
*/
public final boolean equals(Object other) {
return this==other;
}
/**
* Returns a hash code for this enum constant.
*
* @return a hash code for this enum constant.
*/
public final int hashCode() {
return super.hashCode();
}
/**
* Throws CloneNotSupportedException. This guarantees that enums
* are never cloned, which is necessary to preserve their "singleton"
* status.
*
* @return (never returns)
*/
protected final Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
/**
* Compares this enum with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* Enum constants are only comparable to other enum constants of the
* same enum type. The natural order implemented by this
* method is the order in which the constants are declared.
*/
public final int compareTo(E o) {
Enum other = (Enum)o;
Enum self = this;
if (self.getClass() != other.getClass() && // optimization
self.getDeclaringClass() != other.getDeclaringClass())
throw new ClassCastException();
return self.ordinal - other.ordinal;
}
/**
* Returns the Class object corresponding to this enum constant's
* enum type. Two enum constants e1 and e2 are of the
* same enum type if and only if
* e1.getDeclaringClass() == e2.getDeclaringClass().
* (The value returned by this method may differ from the one returned
* by the {@link Object#getClass} method for enum constants with
* constant-specific class bodies.)
*
* @return the Class object corresponding to this enum constant's
* enum type
*/
public final Class getDeclaringClass() {
Class clazz = getClass();
Class zuper = clazz.getSuperclass();
return (zuper == Enum.class) ? clazz : zuper;
}
/**
* Returns the enum constant of the specified enum type with the
* specified name. The name must match exactly an identifier used
* to declare an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* @param enumType the Class object of the enum type from which
* to return a constant
* @param name the name of the constant to return
* @return the enum constant of the specified enum type with the
* specified name
* @throws IllegalArgumentException if the specified enum type has
* no constant with the specified name, or the specified
* class object does not represent an enum type
* @throws NullPointerException if enumType or name
* is null
* @since 1.5
*/
public static > T valueOf(Class enumType,
String name) {
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 const " + enumType +"." + name);
}
/**
* enum classes cannot have finalize methods.
*/
protected final void finalize() { }
}