Java枚举类型

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大家有没有考虑过Java枚举类型在Java中是如何实现的？接下来通过一个小例子来看看Java是如何实现枚举类型的。
这是一个Week的枚举类型。

public enum Week {    Sunday(0),    Monday(1),    Tuesday(2),    Wednesday(3),    Thursday(4),    Friday(5),    Saturday(6);    private int digit;    Week(int digit) {        this.digit = digit;    }    public Week valueOf(int digit) {        for (Week week : values()) {            if (week.getDigit() == digit) {                return week;            }        }        return null;    }    public int getDigit() {        return digit;    }    public void setDigit(int digit) {        this.digit = digit;    }}

枚举类型定义Week好了之后，通过
javac Week.java 及
javap -c Week.class 命令查看Week的字节码

Compiled from "Week.java"public final class Week extends java.lang.Enum<Week> {  public static final Week Sunday;  public static final Week Monday;  public static final Week Tuesday;  public static final Week Wednesday;  public static final Week Thursday;  public static final Week Friday;  public static final Week Saturday;  static {};    Code:       0: new           #4                  // class Week       3: dup       4: ldc           #10                 // String Sunday       6: iconst_0       7: iconst_0       8: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      11: putstatic     #12                 // Field Sunday:LWeek;      14: new           #4                  // class Week      17: dup      18: ldc           #13                 // String Monday      20: iconst_1      21: iconst_1      22: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      25: putstatic     #14                 // Field Monday:LWeek;      28: new           #4                  // class Week      31: dup      32: ldc           #15                 // String Tuesday      34: iconst_2      35: iconst_2      36: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      39: putstatic     #16                 // Field Tuesday:LWeek;      42: new           #4                  // class Week      45: dup      46: ldc           #17                 // String Wednesday      48: iconst_3      49: iconst_3      50: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      53: putstatic     #18                 // Field Wednesday:LWeek;      56: new           #4                  // class Week      59: dup      60: ldc           #19                 // String Thursday      62: iconst_4      63: iconst_4      64: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      67: putstatic     #20                 // Field Thursday:LWeek;      70: new           #4                  // class Week      73: dup      74: ldc           #21                 // String Friday      76: iconst_5      77: iconst_5      78: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      81: putstatic     #22                 // Field Friday:LWeek;      84: new           #4                  // class Week      87: dup      88: ldc           #23                 // String Saturday      90: bipush        6      92: bipush        6      94: invokespecial #11                 // Method "<init>":(Ljava/lang/String;II)V      97: putstatic     #24                 // Field Saturday:LWeek;     100: bipush        7     102: anewarray     #4                  // class Week     105: dup     106: iconst_0     107: getstatic     #12                 // Field Sunday:LWeek;     110: aastore     111: dup     112: iconst_1     113: getstatic     #14                 // Field Monday:LWeek;     116: aastore     117: dup     118: iconst_2     119: getstatic     #16                 // Field Tuesday:LWeek;     122: aastore     123: dup     124: iconst_3     125: getstatic     #18                 // Field Wednesday:LWeek;     128: aastore     129: dup     130: iconst_4     131: getstatic     #20                 // Field Thursday:LWeek;     134: aastore     135: dup     136: iconst_5     137: getstatic     #22                 // Field Friday:LWeek;     140: aastore     141: dup     142: bipush        6     144: getstatic     #24                 // Field Saturday:LWeek;     147: aastore     148: putstatic     #1                  // Field $VALUES:[LWeek;     151: return}

通过字节码，可以看出枚举类型的内部实现其实是一个继承了Java Enum类，然后将所有枚举值声明成static final的Week类类型，并通过静态初始化方式初始化了这些值。

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