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Java 枚举的本质

本文跟大家一起探讨一下 Java 枚举的本质，这篇文章的内容是我在 2012年09月05日 发布到 CSDN 上面的一篇博文 Java 枚举：理解枚举本质，虽然已经不在 CSDN 上面耕耘了，但偶尔也会去看看朋友们的留言，毕竟感情在那里！今天偶然看到有小伙伴评论这篇文章，一时兴起就想再次分享给大家。

Java 枚举的本质

学习编程语言，会用只是最基本的要求，了解和熟悉其实现、运行机制才使得你有别于常人！

C 枚举

在 C 语言中，可以这样来定义枚举，如下示例：

enum color {
    RED=0, GREEN, BLUE, YELLOW
} col;

关键字 enum 定义枚举，在定义枚举的同时，声明该枚举变量 col.

注意：C 语言中枚举成员的值是根据上下文自动加 1 的（GREEN = 1，BLUE = 2 等）。

C 语言中 switch 语句支持枚举类型，如下示例：

#include<stdio.h>

int main()
{
    enum color {
        RED=0, GREEN, BLUE, YELLOW
    } col;
    
    int cl;
    
    printf("0=red, 1=green, 2=blue, 3=yellow. seclect:/n");
        
    scanf("%d",&cl);
    
    col=(enum color) cl;
    
    switch(col) {
        case RED:
            printf("the color is red/n");
            break;
        case GREEN:
            printf("the color is green/n");
            break;
        case BLUE:
             printf("the color is blue/n");
            break;
        case YELLOW:
            printf("the color is yellow/n");
            break;
        defalut:
            printf("no this color/n");
            break;
    }
    
    return 0;
}

Java 枚举

那么，Java 里面的枚举与其类似，但是又不是完全一样。Java 语言中定义枚举也是使用 enum 关键字，如下示例是 Java 语言的枚举：

public enum Color {
    RED, GREEN, BLUE, YELLOW;
}

上述定义了一个枚举类型 Color （可以说是类，编译之后是 Color.class）.

上面的定义，还可以改成下面的这种形式:

public enum Color {
    RED(), GREEN(), BLUE(), YELLOW();
}

到这里你可能会觉得迷茫（如果你是初学者的话），为什么这样子也可以，why？

其实，枚举的成员就是枚举对象，只不过它们是静态常量而已。

使用 javap 命令（ javap 文件名<没有后缀.class> ）可以反编译 class 文件，如下：

我们可以使用普通类来模拟枚举，下面定义一个 Color 类，如下：

public class Color{
    private static final Color RED = new Color();
    private static final Color GREEN = new Color();
    private static final Color BLUE = new Color();
    private static final Color YELLOW = new Color();
}

结合上图反编译的结果，做一下对比，你是否看出了一点端倪（坏笑），如果没有看出来，那就接着往下看吧。

如果按照这个逻辑，是否还可以为其添加另外的构造方法？答案是肯定的！

public enum Color {
    RED("red color", 0), GREEN("green color", 1), 
    BLUE("blue color", 2), YELLOW("yellow color", 3);
    
    Color(String name, int id) {
        _name = name;
        _id = id;
    }
    
    String _name;
    int _id;
}

为 Color 声明了两个成员变量，并为其构造带参数的构造器。

如果你这样创建一个枚举:

public enum Color {
        RED("red color", 0), GREEN("green color", 1), 
        BLUE("blue color", 2), YELLOW("yellow color", 3);
}

编译器就会报错:

The constructor EnumDemo.Color(String, int) is undefined

到此，你应该看明白了，枚举和普通的 Java 类很像。

对于类来讲，最好将其成员变量私有化，并且为成员变量提供 get 、 set 方法。

按照这个原则，可以进一步写好 enum Color ，如下示例：

public enum Color {
    RED("red color", 0), GREEN("green color", 1),
    BLUE("blue color", 2), YELLOW("yellow color", 3);
    
    Color(String name, int id) {
        _name = name;
        _id = id;
    }
    
    private String _name;
    private int _id;
        
    public void setName(String name){
        _name = name;
    }
        
    public void setId(int id){
        _id = id;
    }
    
    public String getName(){
        return _name;
    }
    
    public int getId(){
        return _id;
    }
}

但是 Java 设计枚举的目的是提供一组常量，方便开发者使用。如果我们冒然的提供 set 方法（外界可以改变其成员属性），好像有点违背了设计的初衷。

那么，我们应该舍弃 set 方法，保留 get 方法。

public enum Color {
    RED("red color", 0), GREEN("green color", 1),
    BLUE("blue color", 2), YELLOW("yellow color", 3);
    
    Color(String name, int id) {
        _name = name;
        _id = id;
    }
    
    private String _name;
    private int _id;
        
    public String getName(){
        return _name;
    }
    
    public int getId(){
        return _id;
    }
}

对于普通的基本类可以将其实例化，那么，能否实例化枚举呢？

在回答这个问题之前，先来看看 Color.class 文件:

public static enum Color {
    RED("red color", 0), GREEN("green color", 1),
    BLUE("blue color", 2), YELLOW("yellow color", 3);
    
    private String _name;
    private int _id;
    
    private Color(String name,int id){
        this._name = name;
        this._id = id;
    }
    
    public String getName(){
        return this._name;
    }
    
    public int getId(){
        return this._id;
    }
}

可以看出，编译器淘气的为其构造方法加上了 private ，那么也就是说，我们无法实例化枚举。

所有枚举类都继承了 Enum 类的方法，包括 toString 、 equals 、 hashcode 等方法。因为 equals 、 hashcode 方法是 final 的，所以不可以被枚举重写（只可以继承），但可以重写 toString 方法。

文末的附录中提供了 Enum 的源码，有兴趣可以查看阅读！

那么，使用 Java 的类来模拟一下枚举，大概是这个样子：

package mark.demo;

import java.util.ArrayList;
import java.util.List;

public class Color{
    private static final Color RED = new Color("red color", 0);
    private static final Color GREEN = new Color("green color", 1);
    private static final Color BLUE = new Color("blue color", 2);
    private static final Color YELLOW = new Color("yellow color", 3);

    private final String _name;
    private final int _id;

    private Color(String name,int id){
        _name = name;
        _id = id;
    }

    public String getName(){
        return _name;
    }

    public int getId(){
        return _id;
    }

    public static List<Color> values(){
        List<Color> list = new ArrayList<Color>();
        list.add(RED);
        list.add(GREEN);
        list.add(BLUE);
        list.add(YELLOW);
        return list;
    }

    @Override
    public String toString(){
        return "the color _name=" + _name + ", _id=" + _id;
    }
}

附录

Enum.java

package java.lang;

import java.io.Serializable;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;

/**
* This is the common base class of all Java language enumeration types.
*
*@authorJosh Bloch
*@authorNeal Gafter
*@version%I%, %G%
*@since1.5
*/
public abstract class Enum<Eextends Enum<E>>
implements Comparable<E>,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.
*
* <b>Most programmers should use the {@link#toString} method in
* preference to this one, as the toString method may return
* a more user-friendly name.</b> 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.
*
*@returnthe 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
* {@linkjava.util.EnumSet} and {@linkjava.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 {@linkjava.util.EnumSet} and {@linkjava.util.EnumMap}.
*
*@returnthe 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.
*
*@paramname - The name of this enum constant, which is the identifier
* used to declare it.
*@paramordinal - 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.
*
*@returnthe name of this enum constant
*/
    public String toString(){
        return name;
    }

    /**
* Returns true if the specified object is equal to this
* enum constant.
*
*@paramother the object to be compared for equality with this object.
*@returntrue 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.
*
*@returna 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 {@linkObject#getClass} method for enum constants with
* constant-specific class bodies.)
*
*@returnthe Class object corresponding to this enum constant's
* enum type
*/
    public final Class<E> 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.)
*
*@paramenumType the <tt>Class</tt> object of the enum type from which
* to return a constant
*@paramname the name of the constant to return
*@returnthe enum constant of the specified enum type with the
* specified name
*@throwsIllegalArgumentException if the specified enum type has
* no constant with the specified name, or the specified
* class object does not represent an enum type
*@throwsNullPointerException if <tt>enumType</tt> or <tt>name</tt>
* is null
*@since1.5
*/
    public static <T extends Enum<T>> TvalueOf(Class<T> 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);
    }

    /**
* prevent default deserialization
*/
    private void readObject(ObjectInputStream in)throws IOException,
ClassNotFoundException{
            throw new InvalidObjectException("can't deserialize enum");
    }

    private void readObjectNoData()throws ObjectStreamException {
        throw new InvalidObjectException("can't deserialize enum");
    }

    /**
* enum classes cannot have finalize methods.
*/
    protected final void finalize(){ }
}