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Java 枚举的本质

概要

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。

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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：

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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：

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#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：

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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:

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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：
​​​​1

我们可以使用普通类来模拟枚举，下面定义一个 Color 类，如下：

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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！

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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:

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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:

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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：

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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。

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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:

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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：

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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

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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.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @version %I%, %G%
 * @since   1.5
 */
public abstract class Enum<E extends 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.
     *
     * @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<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.) 
     *
     * @param enumType the <tt>Class</tt> 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 <tt>enumType</tt> or <tt>name</tt>
     *         is null
     * @since 1.5
     */
    public static <T extends Enum<T>> T valueOf(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() { }
}

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