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:
");
    scanf("%d",&cl);

    col=(enum color) cl;

    switch(col) {
        case RED:
            printf("the color is red
");
            break;
        case GREEN:
            printf("the color is green
");
            break;
        case BLUE:
             printf("the color is blue
");
            break;
        case YELLOW:
            printf("the color is yellow
");
            break;
        defalut:
            printf("no this color
");
            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() { }
}