Java基础:枚举的用法与原理
在学习过程中,我们也只是在定义常量的时候,会意识到枚举的存在,而定义常量其实可以在类中实现,这时就会感觉枚举有点鸡肋。但在实际项目开发的过程中,枚举因相当迷人的特性而受到越来越多的关注。
本文将按以下小节点来,一一介绍枚举:
枚举的实现
枚举的用法
枚举的原理
枚举与单例
1. 枚举的实现
枚举是JDK1.5之后的特性,在此之前一般是在类中对常量进行定义。那么为什么需要枚举呢?举个栗子:
使用静态变量定义四季
假如我们需要使用四个变量来代表“春夏秋冬”:
public?class?Season?{
public?final?static?int?SRPING?=?1;
public?final?static?int?SUMMER?=?2;
public?final?static?int?AUTUMN?=?3;
public?final?static?int?WINTER?=?4;
}
这时候只要直接引用Season.SPRING就可以了,我们不需要去操心SPRING在存储时是什么数据。但是如果我们想做更多的事:知道下一个季节是什么,还想把季节打印出来:
public?class?Season?{
private?Season(){}
public?final?static?Season?SPRING?=?new?Season();
public?final?static?Season?SUMMER?=?new?Season();
public?final?static?Season?AUTUMN?=?new?Season();
public?final?static?Season?WINTER?=?new?Season();
public?static?Season?getNextSeason(Season?nowSeason){
if(nowSeason?==?SPRING){
return?SUMMER;
}else?if(nowSeason?==?SUMMER){
return?AUTUMN;
}else?if(nowSeason?==?AUTUMN){
return?WINTER;
}else{
return?SPRING;
}
}
public?static?void?printNowSeason(Season?nowSeason){
if(nowSeason?==?SPRING){
System.out.println("春季");
}else?if(nowSeason?==?SUMMER){
System.out.println("夏季");
}else?if(nowSeason?==?AUTUMN){
System.out.println("秋季");
}else{
System.out.println("冬季");
}
}
public?static?void?main(String[]?args){
Season?nowSeason?=?Season.SUMMER;
Season.printNowSeason(nowSeason);
Season?nextSeason?=?Season.getNextSeason(nowSeason);
Season.printNowSeason(nextSeason);
}
}
因为将Season类的构造方法私有化,外界就不能创建该类的对象了,这就避免了其他奇怪的季节的出现,所有Season对象都在该内部创建。
但是有个问题,用于存储的int值不见了,所以我们还需要设定另一个方法:
public?static?int?toInt(Season?nowSeason){
if(nowSeason?==?SPRING){
return?1;
}else?if(nowSeason?==?SUMMER){
return?2;
}else?if(nowSeason?==?AUTUMN){
return?3;
}else{
return?4;
}
}
这时如果需要一个Season对象对应的int数据,只需要Season.toInt(Season.SPRING)即可。
但是这种写法有一个隐患:如果想要扩展功能,需要写大量的if-else判断。
这时,枚举来啦。
枚举定义四季
我们还是以四季作为栗子:
public?enum?Season?{
SPRING,?SUMMER,?AUTUMN,?WINTER;
}
好啦,枚举定义完了。我们来看看怎么使用它:
class?Test{
public?static?void?main(String[]?args){
System.out.println(Season.SUMMER);??//输出:SUMMER
}
}
在枚举中,默认的toString()方法返回的就是枚举类中对应的名称。但是我们上面要求打印出来的是如”春季“等,而不是名称本身,且四季对应的int值也是必要的。所以我们还得自己完善枚举:
public?enum?Season?{
SPRING(0),?SUMMER(1),?AUTUMN(2),?WINTER(3);
private?int?value;
private?Season(int?value){
this.value?=?value;
}
public?static?Season?getNextSeason(Season?nowSeason){
int?nextDayValue?=?nowSeason.value;
if(++nextDayValue?==?3){
nextDayValue?=?0;
}
return?getSeasonByValue(nextDayValue);
}
public?static?Season?getSeasonByValue(int?value){
for(Season?s?:?Season.values()){
if(s.value?==?value){
return?s;
}
}
return?null;
}
}
class?Test{
public?static?void?main(String[]?args){
System.out.println("nowSeason->"+Season.SPRING+",?value->"+Season.SPRING.ordinal());
System.out.println("nextSeason->"+Season.getNextSeason(Season.SPRING));
}
}
这样,我们就实现了既定的目标,和之前的代码相比,没有那么多if-else,是不是感觉少了很多烦恼呢?
所以,我们在定义有限的序列时,如星期、性别等,一般会通过静态变量的形式进行定义,但是这种形式在添加功能的时候,就会需要很多不利于扩展和维护的代码,所以枚举的实现,可以简化这些操作。
2. 枚举的用法
枚举类中有些方法还是比较常用的,在此演示几个比较重要的方法。以四季为例:
public?enum?Season?{
SPRING,?SUMMER,?AUTUMN,?WINTER
}
Season.valueOf()方法
此方法的作用是传来一个字符串,然后将它转换成对应的枚举变量。前提是传入的字符串和定义枚举变量的字符串一模一样,须区分大小写。如果传入了一个不存在的字符串,那么会抛出异常。
System.out.println(Season.valueOf("spring".toUpperCase()));
System.out.println(Season.valueOf("nyfor2020"));
运行结果为:
Exception?in?thread?"main"?SPRING
java.lang.IllegalArgumentException:?No?enum?constant?Season.nyfor2020
at?java.lang.Enum.valueOf(Enum.java:238)
at?Season.valueOf(Season.java:5)
at?Test.main(Season.java:11)
Season.values()方法和Season.ordinal()方法
Season.values()方法会返回包括所有枚举变量的数据。
默认情况下,枚举会给所有的枚举变量提供一个默认的次序,该次序类似数组的下标,从0开始,而Season.ordinal()方法正是可以获取其次序的方法。
for (Season s: Season.values()){
System.out.println(s + ".ordinal() --> "+s.ordinal());
}
运行结果为:
SPRING.ordinal()?-->?0
SUMMER.ordinal()?-->?1
AUTUMN.ordinal()?-->?2
WINTER.ordinal()?-->?3
Season.toString()方法和Season.name()方法
Season.toString()方法会返回枚举定义枚举变量时的字符串。此方法同Season.name()方法是一样的。
System.out.println("SEASON.SPRING.name?-->?"+Season.SPRING.name());
System.out.println("SEASON.SPRING.toString?-->?"+Season.SPRING.toString());
运行结果为:
SEASON.SPRING.name?-->?SPRING
SEASON.SPRING.toString?-->?SPRING
从实现过程来看,name()方法和toString()方法可以说是一样的。
public?abstract?class?Enum
implements?Comparable,?Serializable?{
...
public?final?String?name()?{
return?name;
}
public?String?toString()?{
return?name;
}
...
}
但它们之间唯一的区别是,toString()方法可以重写,但name()方法被final修饰了,不能重写。
Season.compareTo()方法
这个方法用于比较两个枚举变量的“大小”,实际上比较的是两个枚举变量之间的次序,并返回次序相减之后的结果。
System.out.println("SEASON.SPRING.compareTo(SEASON.WINTER)?-->?"+?Season.SPRING.compareTo(Season.WINTER));
运行结果为:
SEASON.SPRING.compareTo(SEASON.WINTER)?-->?-3
我们来看看它的源码:
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;
}
在这里其实我们就已经可以看到了,compareTo()方法中会先判断是否属于同一个枚举的变量,然后再返回差值。
那么枚举有什么要注意的东西呢?
枚举使用的是enum关键字,而不是class;
枚举变量之间用逗号隔开,且枚举变量最好用大写,多个单词之间使用“_"隔开(INT_SUM)。
定义完变量之后,以分号结束,如果只是有枚举变量,而不是自定义变量,分号可以省略。
只需要类名.变量名就可以召唤枚举变量了,跟使用静态变量一样。
枚举与switch
枚举是JDK1.5才有的特性,同时switch也更新了。使用switch进行条件判断的时候,条件整数一般只能是整型,字符型,而枚举型确实也被switch所支持。还是用“四季“举个栗子:
public?enum?Season?{
SPRING,?SUMMER,?AUTUMN,?WINTER
}
class?SeasonSwitch{
public?void?judge(Season?s){
switch?(s){
case?SPRING:
System.out.println("spring");
break;
case?SUMMER:
System.out.println("summer");
break;
case?AUTUMN:
System.out.println("autumn");
break;
case?WINTER:
System.out.println("winter");
break;
}
}
public?static?void?main(String[]?args){
Season?s?=?Season.SPRING;
SeasonSwitch?seasonSwitch?=?new?SeasonSwitch();
seasonSwitch.judge(s);
}
}
运行结果为:
spring
枚举的高级使用方法
我们还是拿四季来做个例子:
public?enum?Season?{
SPRING,?SUMMER,?AUTUMN,?WINTER
}
在这里,SPRING对应的ordinal值对应的就是0,SUMMER对应的就是1。如果我们想将SPRING的值为1,那么就需要自己定义变量:
public?enum?Season?{
SPRING(1),?SUMMER(2),?AUTUMN(3),?WINTER(4);
private?int?value;
private?Season(int?value){
this.value?=?value;
}
}
如果我们想对一个枚举变量做两个维度的描述呢?
public?enum?Season?{
SPRING(1,?"spring"),?SUMMER(2,?"summer"),?AUTUMN(3,?"autumn"),?WINTER(4,?"winter");
private?int?value;
private?String?lab;
private?Season(int?value,?String?lab){
this.value?=?value;
this.lab?=?lab;
}
}
总结一下,如果需要自定义枚举变量,需要注意一下几点:
一定要把枚举变量的定义放在第一行,并且以分号结尾;
构造函数必须私有化,但也不是一定要写private,事实上枚举的构造函数默认并强制为private,写public是无法通过编译的。
ordinal还是按照它的规则给每个枚举变量按次序赋值,自定义变量与默认的ordinal属性并不冲突。
3. 枚举的原理
我们还是拿“四季”作为栗子:
public?enum?Season?{
SPRING()?{
@Override
public?Season?getNextSeason()?{
return?SUMMER;
}
},?SUMMER()?{
@Override
public?Season?getNextSeason()?{
return?AUTUMN;
}
},?AUTUMN()?{
@Override
public?Season?getNextSeason()?{
return?WINTER;
}
},?WINTER()?{
@Override
public?Season?getNextSeason()?{
return?SPRING;
}
};
public?abstract?Season?getNextSeason();
}
反编译之后,我们可以看到:
>javap?Season.class
Compiled?from?"Season.java"
public?abstract?class?Season?extends?java.lang.Enum?{
public?static?final?Season?SPRING;
public?static?final?Season?SUMMER;
public?static?final?Season?AUTUMN;
public?static?final?Season?WINTER;
public?static?Season[]?values();
public?static?Season?valueOf(java.lang.String);
public?abstract?Season?getNextSeason();
Season(java.lang.String,?int,?Season$1);
static?{};
}
经过编译器编译之后,Season是一个继承了Enum类的抽象类,而且枚举中定义的枚举变量变成了相应的public static final属性,其类型为抽象类Season类型,名字就是枚举变量的名字。
同时我们可以看到,Season.class的相同路径下看到四个内部类的.class文件:
也就是说,这四个枚举常量分别使用了内部类来实现。
同时还添加了两个方法values()和valueOf(String s)。我们使用的是默认的无参构造函数,但现在的构造函数有两个参数。还生成了一个静态代码块。下面我们来详细看下是怎么回事儿:
>javap?-c?-v?Season.class
Classfile?/E:/Intellij?IDEA/project/JVMTest/src/Season.class
Last?modified?2020-5-6;?size?1114?bytes
MD5?checksum?5fb619a1f14495913ba7820312371ded
Compiled?from?"Season.java"
public?abstract?class?Season?extends?java.lang.Enum
minor?version:?0
major?version:?52
flags:?ACC_PUBLIC,?ACC_SUPER,?ACC_ABSTRACT,?ACC_ENUM
Constant?pool:
#1?=?Methodref??????????#5.#50?????????//?Season."":(Ljava/lang/String;
I)V
#2?=?Fieldref???????????#5.#51?????????//?Season.$VALUES:[LSeason;
#3?=?Methodref??????????#52.#53????????//?"[LSeason;".clone:()Ljava/lang/Obje
ct;
#4?=?Class??????????????#32????????????//?"[LSeason;"
#5?=?Class??????????????#54????????????//?Season
#6?=?Methodref??????????#24.#55????????//?java/lang/Enum.valueOf:(Ljava/lang/
Class;Ljava/lang/String;)Ljava/lang/Enum;
#7?=?Methodref??????????#24.#50????????//?java/lang/Enum."":(Ljava/lang
/String;I)V
#8?=?Class??????????????#56????????????//?Season$1
#9?=?String?????????????#26????????????//?SPRING
#10?=?Methodref??????????#8.#50?????????//?Season$1."":(Ljava/lang/Strin
g;I)V
#11?=?Fieldref???????????#5.#57?????????//?Season.SPRING:LSeason;
#12?=?Class??????????????#58????????????//?Season$2
#13?=?String?????????????#28????????????//?SUMMER
#14?=?Methodref??????????#12.#50????????//?Season$2."":(Ljava/lang/Strin
g;I)V
#15?=?Fieldref???????????#5.#59?????????//?Season.SUMMER:LSeason;
#16?=?Class??????????????#60????????????//?Season$3
#17?=?String?????????????#29????????????//?AUTUMN
#18?=?Methodref??????????#16.#50????????//?Season$3."":(Ljava/lang/Strin
g;I)V
#19?=?Fieldref???????????#5.#61?????????//?Season.AUTUMN:LSeason;
#20?=?Class??????????????#62????????????//?Season$4
#21?=?String?????????????#30????????????//?WINTER
#22?=?Methodref??????????#20.#50????????//?Season$4."":(Ljava/lang/Strin
g;I)V
#23?=?Fieldref???????????#5.#63?????????//?Season.WINTER:LSeason;
#24?=?Class??????????????#64????????????//?java/lang/Enum
#25?=?Utf8???????????????InnerClasses
#26?=?Utf8???????????????SPRING
#27?=?Utf8???????????????LSeason;
#28?=?Utf8???????????????SUMMER
#29?=?Utf8???????????????AUTUMN
#30?=?Utf8???????????????WINTER
#31?=?Utf8???????????????$VALUES
#32?=?Utf8???????????????[LSeason;
#33?=?Utf8???????????????values
#34?=?Utf8???????????????()[LSeason;
#35?=?Utf8???????????????Code
#36?=?Utf8???????????????LineNumberTable
#37?=?Utf8???????????????valueOf
#38?=?Utf8???????????????(Ljava/lang/String;)LSeason;
#39?=?Utf8???????????????
#40?=?Utf8???????????????(Ljava/lang/String;I)V
#41?=?Utf8???????????????Signature
#42?=?Utf8???????????????()V
#43?=?Utf8???????????????getNextSeason
#44?=?Utf8???????????????()LSeason;
#45?=?Utf8???????????????(Ljava/lang/String;ILSeason$1;)V
#46?=?Utf8???????????????
#47?=?Utf8???????????????Ljava/lang/Enum;
#48?=?Utf8???????????????SourceFile
#49?=?Utf8???????????????Season.java
#50?=?NameAndType????????#39:#40????????//?"":(Ljava/lang/String;I)V
#51?=?NameAndType????????#31:#32????????//?$VALUES:[LSeason;
#52?=?Class??????????????#32????????????//?"[LSeason;"
#53?=?NameAndType????????#65:#66????????//?clone:()Ljava/lang/Object;
#54?=?Utf8???????????????Season
#55?=?NameAndType????????#37:#67????????//?valueOf:(Ljava/lang/Class;Ljava/lan
g/String;)Ljava/lang/Enum;
#56?=?Utf8???????????????Season$1
#57?=?NameAndType????????#26:#27????????//?SPRING:LSeason;
#58?=?Utf8???????????????Season$2
#59?=?NameAndType????????#28:#27????????//?SUMMER:LSeason;
#60?=?Utf8???????????????Season$3
#61?=?NameAndType????????#29:#27????????//?AUTUMN:LSeason;
#62?=?Utf8???????????????Season$4
#63?=?NameAndType????????#30:#27????????//?WINTER:LSeason;
#64?=?Utf8???????????????java/lang/Enum
#65?=?Utf8???????????????clone
#66?=?Utf8???????????????()Ljava/lang/Object;
#67?=?Utf8???????????????(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
{
public?static?final?Season?SPRING;
descriptor:?LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC,?ACC_FINAL,?ACC_ENUM
public?static?final?Season?SUMMER;
descriptor:?LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC,?ACC_FINAL,?ACC_ENUM
public?static?final?Season?AUTUMN;
descriptor:?LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC,?ACC_FINAL,?ACC_ENUM
public?static?final?Season?WINTER;
descriptor:?LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC,?ACC_FINAL,?ACC_ENUM
public?static?Season[]?values();
descriptor:?()[LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC
Code:
stack=1,?locals=0,?args_size=0
0:?getstatic?????#2??????????????????//?Field?$VALUES:[LSeason;
3:?invokevirtual?#3??????????????????//?Method?"[LSeason;".clone:()Ljav
a/lang/Object;
6:?checkcast?????#4??????????????????//?class?"[LSeason;"
9:?areturn
LineNumberTable:
line?7:?0
public?static?Season?valueOf(java.lang.String);
descriptor:?(Ljava/lang/String;)LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC
Code:
stack=2,?locals=1,?args_size=1
0:?ldc???????????#5??????????????????//?class?Season
2:?aload_0
3:?invokestatic??#6??????????????????//?Method?java/lang/Enum.valueOf:(
Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
6:?checkcast?????#5??????????????????//?class?Season
9:?areturn
LineNumberTable:
line?7:?0
public?abstract?Season?getNextSeason();
descriptor:?()LSeason;
flags:?ACC_PUBLIC,?ACC_ABSTRACT
Season(java.lang.String,?int,?Season$1);
descriptor:?(Ljava/lang/String;ILSeason$1;)V
flags:?ACC_SYNTHETIC
Code:
stack=3,?locals=4,?args_size=4
0:?aload_0
1:?aload_1
2:?iload_2
3:?invokespecial?#1??????????????????//?Method?"":(Ljava/lang/Str
ing;I)V
6:?return
LineNumberTable:
line?7:?0
static?{};
descriptor:?()V
flags:?ACC_STATIC
Code:
stack=4,?locals=0,?args_size=0
0:?new???????????#8??????????????????//?class?Season$1
3:?dup
4:?ldc???????????#9??????????????????//?String?SPRING
6:?iconst_0
7:?invokespecial?#10?????????????????//?Method?Season$1."":(Ljava/lang/String;I)V
10:?putstatic?????#11?????????????????//?Field?SPRING:LSeason;
13:?new???????????#12?????????????????//?class?Season$2
16:?dup
17:?ldc???????????#13?????????????????//?String?SUMMER
19:?iconst_1
20:?invokespecial?#14?????????????????//?Method?Season$2."":(Ljava/lang/String;I)V
23:?putstatic?????#15?????????????????//?Field?SUMMER:LSeason;
26:?new???????????#16?????????????????//?class?Season$3
29:?dup
30:?ldc???????????#17?????????????????//?String?AUTUMN
32:?iconst_2
33:?invokespecial?#18?????????????????//?Method?Season$3."":(Ljava/lang/String;I)V
36:?putstatic?????#19?????????????????//?Field?AUTUMN:LSeason;
39:?new???????????#20?????????????????//?class?Season$4
42:?dup
43:?ldc???????????#21?????????????????//?String?WINTER
45:?iconst_3
46:?invokespecial?#22?????????????????//?Method?Season$4."":(Ljava/lang/String;I)V
49:?putstatic?????#23?????????????????//?Field?WINTER:LSeason;
52:?iconst_4
53:?anewarray?????#5??????????????????//?class?Season
56:?dup
57:?iconst_0
58:?getstatic?????#11?????????????????//?Field?SPRING:LSeason;
61:?aastore
62:?dup
63:?iconst_1
64:?getstatic?????#15?????????????????//?Field?SUMMER:LSeason;
67:?aastore
68:?dup
69:?iconst_2
70:?getstatic?????#19?????????????????//?Field?AUTUMN:LSeason;
73:?aastore
74:?dup
75:?iconst_3
76:?getstatic?????#23?????????????????//?Field?WINTER:LSeason;
79:?aastore
80:?putstatic?????#2??????????????????//?Field?$VALUES:[LSeason;
83:?return
LineNumberTable:
line?8:?0
line?13:?13
line?18:?26
line?23:?39
line?7:?52
}
Signature:?#47??????????????????????????//?Ljava/lang/Enum;
SourceFile:?"Season.java"
InnerClasses:
static?#20;?//class?Season$4
static?#16;?//class?Season$3
static?#12;?//class?Season$2
static?#8;?//class?Season$1
下面分析一下字节码中各部分内容,先拿静态代码块下手:
静态代码块
static?{};
descriptor:?()V
flags:?ACC_STATIC
Code:
stack=4,?locals=0,?args_size=0
//创建一个Season$1的内部类对象
0:?new???????????#8??????????????????//?class?Season$1
3:?dup
//接下来的两条指令,是将两个参数推送到栈顶,调用Season$1的编译器生成的方法
4:?ldc???????????#9??????????????????//?String?SPRING
6:?iconst_0
//调用Season$1的方法
7:?invokespecial?#10?????????????????//?Method?Season$1."":(Ljava/lang/String;I)V
//设置SPRING属性的值为新创建的对象
10:?putstatic?????#11?????????????????//?Field?SPRING:LSeason;
//接下来说是分别初始化另外三个属性SUMMER、AUTUMU、WINTER,此处就不赘述了
13:?new???????????#12?????????????????//?class?Season$2
16:?dup
17:?ldc???????????#13?????????????????//?String?SUMMER
19:?iconst_1
20:?invokespecial?#14?????????????????//?Method?Season$2."":(Ljava/lang/String;I)V
23:?putstatic?????#15?????????????????//?Field?SUMMER:LSeason;
26:?new???????????#16?????????????????//?class?Season$3
29:?dup
30:?ldc???????????#17?????????????????//?String?AUTUMN
32:?iconst_2
33:?invokespecial?#18?????????????????//?Method?Season$3."":(Ljava/lang/String;I)V
36:?putstatic?????#19?????????????????//?Field?AUTUMN:LSeason;
39:?new???????????#20?????????????????//?class?Season$4
42:?dup
43:?ldc???????????#21?????????????????//?String?WINTER
45:?iconst_3
46:?invokespecial?#22?????????????????//?Method?Season$4."":(Ljava/lang/String;I)V
49:?putstatic?????#23?????????????????//?Field?WINTER:LSeason;
52:?iconst_4
53:?anewarray?????#5??????????????????//?class?Season
56:?dup
57:?iconst_0
58:?getstatic?????#11?????????????????//?Field?SPRING:LSeason;
61:?aastore
62:?dup
63:?iconst_1
64:?getstatic?????#15?????????????????//?Field?SUMMER:LSeason;
67:?aastore
68:?dup
69:?iconst_2
70:?getstatic?????#19?????????????????//?Field?AUTUMN:LSeason;
73:?aastore
74:?dup
75:?iconst_3
76:?getstatic?????#23?????????????????//?Field?WINTER:LSeason;
79:?aastore
//将刚创建的数组设置为属性$VALUES的值
80:?putstatic?????#2??????????????????//?Field?$VALUES:[LSeason;
83:?return
静态代码块部分做的工作,就是分别设置生成的四个公共静态常量字段的值,同时编译器还生成一个静态字段$VALUES,保存的是枚举类型定义的所有枚举常量。相当于以下代码:
Season?SPRING?=?new?Season1();
Season?SUMMER?=?new?Season2();
Season?AUTUMN?=?new?Season3();
Season?WINTER?=?new?Season4();
Season[]?$VALUES?=?new?Season[4];
$VALUES[0]?=?SPRING;
$VALUES[1]?=?SUMMER;
$VALUES[2]?=?AUTUMN;
$VALUES[3]?=?WINTER;
values()方法
接下来我们来看看编译器为我们生成的values()方法:
public?static?Season[]?values();
descriptor:?()[LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC
Code:
stack=1,?locals=0,?args_size=0
0:?getstatic?????#2??????????????????//?Field?$VALUES:[LSeason;
3:?invokevirtual?#3??????????????????//?Method?"[LSeason;".clone:()Ljav
a/lang/Object;
6:?checkcast?????#4??????????????????//?class?"[LSeason;"
9:?areturn
values()方法是一个公共的静态方法,所以我们可以直接调用该方法,返回枚举的数组。而这个方法实现的是,将静态代码块中初始化的$VALUES字段的值克隆出来,并且强制转换成Season[]类型返回,就相当于以下代码:
public?static?Season[]?values(){
return?(Season[])$VALUES.clone();
}
valueOf()方法
接下来我们来看另一个由编译器生成的valueOf()方法:
public?static?Season?valueOf(java.lang.String);
descriptor:?(Ljava/lang/String;)LSeason;
flags:?ACC_PUBLIC,?ACC_STATIC
Code:
stack=2,?locals=1,?args_size=1
0:?ldc???????????#5??????????????????//?class?Season
2:?aload_0
3:?invokestatic??#6??????????????????//?Method?java/lang/Enum.valueOf:(
Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
6:?checkcast?????#5??????????????????//?class?Season
9:?areturn
valueOf()也是一个公共的静态方法,所以可以直接调用这个方法并返回参数字符串表示的枚举变量,另外,这个方法的实现是调用Enum.valueOf()方法,并把类型强制转换为Season,它相当于如下的代码:
public?static?Season?valueOf(String?s){
return?(Season)Enum.valueOf(Season.class,?s);
}
最后,我们来看下编译器生成的内部类是什么样的。
内部类
我们以Season$1为例:
>javap?Season$1.class
Compiled?from?"Season.java"
final?class?Season$1?extends?Season?{
Season$1(java.lang.String,?int);
public?Season?getNextSeason();
}
可以看到,Season1的构造函数有两个入参呢?
关于这个问题,我们还是得从Season的父类Enum说起。
public?abstract?class?Enum
implements?Comparable,?Serializable?{
private?final?String?name;
public?final?String?name()?{
return?name;
}
private?final?int?ordinal;
public?final?int?ordinal()?{
return?ordinal;
}
protected?Enum(String?name,?int?ordinal)?{
this.name?=?name;
this.ordinal?=?ordinal;
}
......
}
从Enum中我们可以看到,每个枚举都定义了两个属性,name和ordinal,name表示枚举变量的名称,而ordinal则是根据变量定义的顺序授予的整型值,从0开始。
在枚举变量初始化的时候,会自动初始化这两个字段,设置相应的值,所以会在Season()的构造方法中添加两个参数。
而且我们可以从Enum的源码中看到,大部分的方法都是final修饰的,特别是clone、readObject、writeObject这三个方法,保证了枚举类型的不可变性,不能通过克隆、序列化和反序列化复制枚举,这就保证了枚举变量只是一个实例,即是单例的。
总结一下,其实枚举本质上也是通过普通的类来实现的,只是编译器为我们进行了处理。每个枚举类型都继承自Enum类,并由编译器自动添加了values()和valueOf()方法,每个枚举变量是一个静态常量字段,由内部类实现,而这个内部类继承了此枚举类。
所有的枚举变量都是通过静态代码块进行初始化,也就是说在类加载期间就实现了。
另外,通过把clone、readObject、writeObject这三个方法定义为final,保证了每个枚举类型及枚举常量都是不可变的,也就是说,可以用枚举实现线程安全的单例。
4. 枚举与单例
枚举类实现单例模式相当硬核,因为枚举类型是线程安全的,且只会装载一次。使用枚举类来实现单例模式,是所有的单例实现中唯一一种不会被破坏的单例模式实现。
public?class?SingletonObject?{
private?SingletonObject()?{
}
private?enum?Singleton?{
INSTANCE;
private?final?SingletonObject?instance;
Singleton()?{
instance?=?new?SingletonObject();
}
private?SingletonObject?getInstance()?{
return?instance;
}
}
public?static?SingletonObject?getInstance()?{
return?Singleton.INSTANCE.getInstance();
}
}
结语
在学习Java枚举类的时候,原本列出来了很多问题如Java枚举的线程安全和序列化问题,但是在了解完Java枚举的原理之后,这些问题,都迎刃而解了,也许在未来可能会碰上枚举的特例吧。
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