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() { }
}







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