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为啥用枚举,枚举有哪些用法?_腾讯新闻

天乐
2020-06-20 21:53:33 第一视角

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

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。

本文主要对final关键字进行介绍,如果本文对你有帮助,请给一个赞吧,这会是我最大的动力~

我是敖丙,一个在互联网苟且偷生的工具人。

你知道的越多,你不知道的越多人才们的【三连】就是丙丙创作的最大动力,我们下期见!

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