枚举 学习笔记

android 中 我们通常推荐使用 @IntDef 等 代替使用 枚举，很多同学有疑问，为什么抛弃 枚举。我们现在就来一探究竟。

为什么枚举不行

• 枚举占据了较大的内存，而相比较来说 IntDef

枚举

• 每一个枚举就是一个对象。我们来看例子这是一个我们定义的枚举

public enum Test {
    Ins, Ins2, IUns3
}

通过 javac 再 javap 查看他的bytecode

// class version 51.0 (51)
// access flags 0x4031
// signature Ljava/lang/Enum<Lcom/wt/emode/utils/Test;>;
// declaration: com/wt/emode/utils/Test extends java.lang.Enum<com.wt.emode.utils.Test>
public final enum com/wt/emode/utils/Test extends java/lang/Enum  {

  // compiled from: Test.java

  // access flags 0x4019
  public final static enum Lcom/wt/emode/utils/Test; Ins

  // access flags 0x4019
  public final static enum Lcom/wt/emode/utils/Test; Ins2

  // access flags 0x4019
  public final static enum Lcom/wt/emode/utils/Test; IUns3

  // access flags 0x101A
  private final static synthetic [Lcom/wt/emode/utils/Test; $VALUES

  // access flags 0x9
  public static values()[Lcom/wt/emode/utils/Test;
   L0
    LINENUMBER 3 L0
    GETSTATIC com/wt/emode/utils/Test.$VALUES : [Lcom/wt/emode/utils/Test;
    INVOKEVIRTUAL [Lcom/wt/emode/utils/Test;.clone ()Ljava/lang/Object;
    CHECKCAST [Lcom/wt/emode/utils/Test;
    ARETURN
    MAXSTACK = 1
    MAXLOCALS = 0

  // access flags 0x9
  public static valueOf(Ljava/lang/String;)Lcom/wt/emode/utils/Test;
   L0
    LINENUMBER 3 L0
    LDC Lcom/wt/emode/utils/Test;.class
    ALOAD 0
    INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
    CHECKCAST com/wt/emode/utils/Test
    ARETURN
   L1
    LOCALVARIABLE name Ljava/lang/String; L0 L1 0
    MAXSTACK = 2
    MAXLOCALS = 1

  // access flags 0x2
  // signature ()V
  // declaration: void <init>()
  private <init>(Ljava/lang/String;I)V
   L0
    LINENUMBER 3 L0
    ALOAD 0
    ALOAD 1
    ILOAD 2
    INVOKESPECIAL java/lang/Enum.<init> (Ljava/lang/String;I)V
    RETURN
   L1
    LOCALVARIABLE this Lcom/wt/emode/utils/Test; L0 L1 0
    MAXSTACK = 3
    MAXLOCALS = 3

  // access flags 0x8
  static <clinit>()V
   L0
    LINENUMBER 4 L0
    NEW com/wt/emode/utils/Test
    DUP
    LDC "Ins"
    ICONST_0
    INVOKESPECIAL com/wt/emode/utils/Test.<init> (Ljava/lang/String;I)V
    PUTSTATIC com/wt/emode/utils/Test.Ins : Lcom/wt/emode/utils/Test;
    NEW com/wt/emode/utils/Test
    DUP
    LDC "Ins2"
    ICONST_1
    INVOKESPECIAL com/wt/emode/utils/Test.<init> (Ljava/lang/String;I)V
    PUTSTATIC com/wt/emode/utils/Test.Ins2 : Lcom/wt/emode/utils/Test;
    NEW com/wt/emode/utils/Test
    DUP
    LDC "IUns3"
    ICONST_2
    INVOKESPECIAL com/wt/emode/utils/Test.<init> (Ljava/lang/String;I)V
    PUTSTATIC com/wt/emode/utils/Test.IUns3 : Lcom/wt/emode/utils/Test;
   L1
    LINENUMBER 3 L1
    ICONST_3
    ANEWARRAY com/wt/emode/utils/Test
    DUP
    ICONST_0
    GETSTATIC com/wt/emode/utils/Test.Ins : Lcom/wt/emode/utils/Test;
    AASTORE
    DUP
    ICONST_1
    GETSTATIC com/wt/emode/utils/Test.Ins2 : Lcom/wt/emode/utils/Test;
    AASTORE
    DUP
    ICONST_2
    GETSTATIC com/wt/emode/utils/Test.IUns3 : Lcom/wt/emode/utils/Test;
    AASTORE
    PUTSTATIC com/wt/emode/utils/Test.$VALUES : [Lcom/wt/emode/utils/Test;
    RETURN
    MAXSTACK = 4
    MAXLOCALS = 0
}

enum 集成自 java.lang.Enum 这个类 然后定义了三个 public final static enum xxx的实例

@IntDef 为什么比enum 好

@Retention(SOURCE) // 这里 源码级别 说明 只是在javac 编译期间做检查
@Target({ANNOTATION_TYPE})
public @interface IntDef {
    /** Defines the allowed constants for this element */
    int[] value() default {};

    /** Defines whether the constants can be used as a flag, or just as an enum (the default) */
    boolean flag() default false;

    /**
     * Whether any other values are allowed. Normally this is
     * not the case, but this allows you to specify a set of
     * expected constants, which helps code completion in the IDE
     * and documentation generation and so on, but without
     * flagging compilation warnings if other values are specified.
     */
    boolean open() default false;
}