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;
}

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 getset 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 toStringequalshashcode 等方法。因为 equalshashcode 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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