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。
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!
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.
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; }
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; } }
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(){ } }