Java 枚舉:理解枚舉本質 – JAVA編程語言程序開發技術文章

C 語言中可以這樣來定義枚舉

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

關鍵字 enum 定義枚舉,在定義枚舉的同時,聲明該枚舉變量 col.

註意:C 語言中枚舉成員的值是根據上下文自動加 1 的(GREEN = 1, BLUE = 2 等)。

 

C 語言中 switch 語句支持枚舉類型

[cpp] 
#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; 

那麼,Java 裡面的枚舉與其類似,但是又不是完全一樣。

Java 語言中定義枚舉也是使用關鍵字 enum

[java]
public enum Color { 
    RED, GREEN, BLUE, YELLOW; 

上述定義瞭一個枚舉類型 Color(可以說是類,編譯之後是 Color.class).

 

上面的定義,還可以改成下面的這種形式

[java] 
public enum Color { 
    RED(), GREEN(), BLUE(), YELLOW(); 

到這裡你就會覺得迷茫(如果你是初學者的話),為什麼這樣子也可以?

其實,枚舉的成員就是枚舉對象,隻不過他們是靜態常量而已。

 

我們可以使用普通類來模擬枚舉,下面定義一個 Color 類。

[java] 
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(); 

對比一下,你就明白瞭。

如果按照這個邏輯,是否還可以為其添加另外的構造方法?答案是肯定的!

[html] 
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; 

為 Color 聲明瞭兩個成員變量,並為其構造帶參數的構造器。

如果你這樣創建一個枚舉

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

編譯器就會報錯

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

到此,你就可以明白,枚舉和普通類基本一致(但是不完全一樣)。

 

對於類來講,最好將其成員變量私有化,然後,為成員變量提供 get、set 方法。

按照這個原則,可以進一步寫好 enum Color.

[java] 
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 設計 enum 的目的是提供一組常量,方便用戶設計。
如果我們冒然的提供 set 方法(外界可以改變其成員屬性),好像是有點違背瞭設計的初衷。

那麼,我們應該舍棄 set 方法,保留 get 方法。

[java] 
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; 
        } 

普通類,我們可以將其實例化,那麼,能否實例化枚舉呢?

在回答這個問題之前,先來看看,編譯之後的 Color.class 文件

[java] 
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,編譯器淘氣的為其構造方法加上瞭 private,那麼也就是說,我們無法實例化枚舉。

所有枚舉類都繼承瞭 Enum 類的方法,包括 toString 、equals、hashcode 等方法。

因為 equals、hashcode 方法是 final 的,所以不可以被枚舉重寫(隻可以繼承)。

但是,可以重寫 equals 方法。

 

關於 Enum 源碼,詳見附錄!

 

那麼,使用 Java 的不同類來模擬一下枚舉,大概是這個樣子

[java] 
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; 
    } 
 

附錄

 

Enum.java

[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() { } 

 

 

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