Java 枚举：理解枚举本质

参考链接： 使用Java枚举

C 语言中可以这样来定义枚举 

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 

#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;

}

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关键字 enum 

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();

} 

 到这里你就会觉得迷茫（如果你是初学者的话），为什么这样子也可以？ 

其实，枚举的成员就是枚举对象，只不过他们是静态常量而已。 

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。 

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();

} 

 对比一下，你就明白了。 

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，你就可以明白，枚举和普通类基本一致（但是不完全一样）。 

对于类来讲，最好将其成员变量私有化，然后，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. 

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 enum 的目的是提供一组常量，方便用户设计。 

如果我们冒然的提供 set 方法（外界可以改变其成员属性），好像是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;

        }

} 

 普通类，我们可以将其实例化，那么，能否实例化枚举呢？ 

在回答这个问题之前，先来看看，编译之后的 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）。 

但是，可以重写 toString 方法。 

关于 Enum 源码，详见附录！ 

那么，使用 Java 的不同类来模拟一下枚举，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 

/*

 * %W% %E%

 *

 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.

 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

 */

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() { }

}

时间可以改变一切，但你总得做点什么！