网上很多技术，都没有以一种让人理解的方式讲解。今天让一下 Java Enum 枚举类
最简单的枚举类定义如下：
Test.java
1 enum Weekday {
2 MON, TUE, WED, THU, FRI, SAT, SUN;
3 }
这是个用于表示一周的枚举类，说到它是个 class 类，却让人感觉怪怪的。当 java 源代码在编译的时候，会先被改写，然后再生成 .class 文件。所以，反编译 .class 文件就能得到它被改写后的代码。
演示过程 Test.java ---> Weekday.class ---> Weekday.java
PS D:\demo> ls
Mode LastWriteTime Length Name ---- ------------- ------ ---- -a--- 2020/7/24 17:02 466 Test.java PS D:\demo> javac -encoding utf8 Test.java PS D:\demo> ls
Mode LastWriteTime Length Name ---- ------------- ------ ---- -a--- 2020/7/24 17:02 466 Test.java -a--- 2020/7/24 17:07 1370 Weekday.class PS D:\demo> jad -s java -o Weekday.class Parsing Weekday.class... Generating Weekday.java PS D:\demo>
Weekday.java 整理如下：
1 final class Weekday extends Enum {
2
3 private Weekday(String s, int i){
4 super(s, i);
5 }
6
7 public static final Weekday MON;
8 public static final Weekday TUE;
9 public static final Weekday WED;
10 public static final Weekday THU;
11 public static final Weekday FRI;
12 public static final Weekday SAT;
13 public static final Weekday SUN;
14 private static final Weekday $VALUES[];
15
16 static {
17 MON = new Weekday("MON", 0);
18 TUE = new Weekday("TUE", 1);
19 WED = new Weekday("WED", 2);
20 THU = new Weekday("THU", 3);
21 FRI = new Weekday("FRI", 4);
22 SAT = new Weekday("SAT", 5);
23 SUN = new Weekday("SUN", 6);
24
25 $VALUES = (new Weekday[] {
26 MON, TUE, WED, THU, FRI, SAT, SUN
27 });
28 }
29
30 public static Weekday[] values(){
31
32 return (Weekday[])$VALUES.clone();
33 }
34
35 public static Weekday valueOf(String s){
36
37 return (Weekday)Enum.valueOf(Weekday, s);
38 }
39
40 }
查得 Enum 类构造方法就两个参数，名称和索引：
1 protected Enum(String name, int ordinal) {
2 this.name = name;
3 this.ordinal = ordinal;
4 }
再看一个重写了构造方法的枚举类：
1 enum Weekday {
2
3 MON(1, "星期一"), TUE(2, "星期二"), WED(3, "星期三"), THU(4, "星期四"), FRI(5, "星期五"), SAT(6, "星期六"), SUN(0, "星期日");
4
5 public final int number; //数字表星期
6 private final String chinese; //中文名
7
8 private Weekday(int number, String chinese) {
9
10 this.number = number;
11 this.chinese = chinese;
12 }
13
14 @Override
15 public String toString() {
16 return this.chinese;
17 }
18
19 }
反编译后的代码如下：
1 final class Weekday extends Enum {
2
3 public static final Weekday MON;
4 public static final Weekday TUE;
5 public static final Weekday WED;
6 public static final Weekday THU;
7 public static final Weekday FRI;
8 public static final Weekday SAT;
9 public static final Weekday SUN;
10 private static final Weekday $VALUES[];
11
12 public final int number;
13 private final String chinese;
14
15 static {
16 MON = new Weekday("MON", 0, 1, "\u661F\u671F\u4E00");
17 TUE = new Weekday("TUE", 1, 2, "\u661F\u671F\u4E8C");
18 WED = new Weekday("WED", 2, 3, "\u661F\u671F\u4E09");
19 THU = new Weekday("THU", 3, 4, "\u661F\u671F\u56DB");
20 FRI = new Weekday("FRI", 4, 5, "\u661F\u671F\u4E94");
21 SAT = new Weekday("SAT", 5, 6, "\u661F\u671F\u516D");
22 SUN = new Weekday("SUN", 6, 0, "\u661F\u671F\u65E5");
23 $VALUES = (new Weekday[] { MON, TUE, WED, THU, FRI, SAT, SUN });
24 }
25
26 private Weekday(String s, int i, int j, String s1){
27
28 super(s, i);
29 number = j;
30 chinese = s1;
31 }
32
33 public static Weekday[] values(){
34
35 return (Weekday[])$VALUES.clone();
36 }
37
38 public static Weekday valueOf(String s){
39
40 return (Weekday)Enum.valueOf(Weekday, s);
41 }
42
43 public String toString(){
44 return chinese;
45 }
46
47 }
在 Weekday 类的构造方法的参数列表中，追加两个参数。然后，内部依然调用父类的 构造方法 Enum(String name, int ordinal) ，然后就是使用了。通常是和 switch 语句一起使用。
1 class Main {
2
3 public static void main(String[] args) {
4
5 Weekday day = Weekday.SUN;
6
7 switch(day){
8
9 case MON:
10 case TUE:
11 case WED:
12 case THU:
13 case FRI:
14 System.out.println("Today is " + day + ". Work at office!");
15 break;
16 case SAT:
17 case SUN:
18 System.out.println("Today is " + day + ". Work at home!");
19 break;
20 default:
21 throw new RuntimeException("cannot process " + day);
22 }
23
24 }
25 }
用法很简单，但是不免迷惑，为什么 MON、TUE 、WED 可以直接用，当你用 System.out.print(MON) 就报错，显然，就是类似于语法的东西，会被编译器改写。