枚举Enum

1. 什么是枚举？
2. 为什么要用枚举？
3. 枚举可以怎么用？
4. Pros VS Cons

 

1.什么是枚举

首先我们从一个枚举类的反编译文件来认识一下枚举类

Compiled from "ColorEnum.java"
public final class com.james.learning.j2se.enums.ColorEnum extends java.lang.Enum<com.james.learning.j2se.enums.ColorEnum> {
  public static final com.james.learning.j2se.enums.ColorEnum RED;

  public static final com.james.learning.j2se.enums.ColorEnum YELLOW;

  public static com.james.learning.j2se.enums.ColorEnum[] values();
    Code:
       0: getstatic     #1                  // Field $VALUES:[Lcom/james/learning/j2se/enums/ColorEnum;
       3: invokevirtual #2                  // Method "[Lcom/james/learning/j2se/enums/ColorEnum;".clone:()Ljava/lang/Object;
       6: checkcast     #3                  // class "[Lcom/james/learning/j2se/enums/ColorEnum;"
       9: areturn

  public static com.james.learning.j2se.enums.ColorEnum valueOf(java.lang.String);
    Code:
       0: ldc           #4                  // class com/james/learning/j2se/enums/ColorEnum
       2: aload_0
       3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
       6: checkcast     #4                  // class com/james/learning/j2se/enums/ColorEnum
       9: areturn

  static {};
    Code:
       0: new           #4                  // class com/james/learning/j2se/enums/ColorEnum
       3: dup
       4: ldc           #7                  // String RED
       6: iconst_0
       7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      10: putstatic     #9                  // Field RED:Lcom/james/learning/j2se/enums/ColorEnum;
      13: new           #4                  // class com/james/learning/j2se/enums/ColorEnum
      16: dup
      17: ldc           #10                 // String YELLOW
      19: iconst_1
      20: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      23: putstatic     #11                 // Field YELLOW:Lcom/james/learning/j2se/enums/ColorEnum;
      26: iconst_2
      27: anewarray     #4                  // class com/james/learning/j2se/enums/ColorEnum
      30: dup
      31: iconst_0
      32: getstatic     #9                  // Field RED:Lcom/james/learning/j2se/enums/ColorEnum;
      35: aastore
      36: dup
      37: iconst_1
      38: getstatic     #11                 // Field YELLOW:Lcom/james/learning/j2se/enums/ColorEnum;
      41: aastore
      42: putstatic     #1                  // Field $VALUES:[Lcom/james/learning/j2se/enums/ColorEnum;
      45: return
}

 

 

从前面一句可以看出来 

Compiled from "ColorEnum.java"
public final class com.james.learning.j2se.enums.ColorEnum extends java.lang.Enum<com.james.learning.j2se.enums.ColorEnum> {
 

a. ColorEnum是final的，不能被再继承，也就是之后状态不能被覆盖。

b. ColorEnum继承自Enum.

 

public static final com.james.learning.j2se.enums.ColorEnum RED;
public static final com.james.learning.j2se.enums.ColorEnum YELLOW;
public static com.james.learning.j2se.enums.ColorEnum[] values();

这里我们看到成员变量RED、YELLOW 其实是一个 ColorEnum。自动会增加一个 values的方法返回ColorEnum[] 。

 

2. 为什么要用枚举

从effective java里摘出来的，像

public static final int TOTAL = 10;

public static final String MSG = "message";

 这样的有以下几方面的问题：

      1. 编译期无法感知是否用错，像入参的话可以直接用数字代替，数字还能做加减，这样不是常量的本意。

      2. 如果是RPC，服务端的api里包含的常量，当服务端常量改变的时候，客户端需要跟着编译一次，当然也   可以不编译，但是做参数传递的话就会两端不一致。