Java：枚举与if-then-else的性能 [英] Java: Performance of Enums vs. if-then-else

问题描述

我没有真正的运气，通过使用谷歌而不是简单的回答这个比较，而不是自己耗时的评估，我以为我会先问。

我相当肯定，使用枚举的switch语句的执行速度要比if-then-else语句的速度要快，不过它是否是一个显着的区别是另一个问题。

---

感谢您快速回复的人，我会保留这个对于未来的项目，请记住。

解决方案

是的，因为一般来说，switch语句的工作速度比if / 。

尽管生成的字节码并不总是性能比较的确定来源，但您可以检查它有更好的想法。

例如这个代码：

  class A {
枚举N {ONE，TWO，THREE} 
 void testSwitch（N e）{
 switch（e）{
 case ONE：x（）;打破; 
 case TWO：x（）;打破; 
 case THREE：x（）;打破; 
} 
} 
 void testIf（Enum e）{
 if（e == NONE）{x（）; } 
 else if（e == N.TWO）{x（）; } 
 else if（e == N.THREE）{x（）; } 
} 
 void x（）{} 
} 
  

生成以下内容：

 从A.java编译
 class A扩展java.lang.Object {
 A（）; 
代码：
 0：aload_0 
 1：invokespecial＃1; //方法java / lang / Object。< init>:(）V 
 4：return 
 
 void testSwitch（A $ N）; 
代码：
 0：getstatic＃2; // Field A $ 1. $ SwitchMap $ A $ N：[I 
 3：aload_1 
 4：invokevirtual＃3; //方法A $ N.ordinal :(）I 
 7：iaload 
 8：tableswitch {// 1到3 
 1：36; 
 2：43; 
 3：50; 
 default：54} 
 36：aload_0 
 37：invokevirtual＃4; //方法x :(）V 
 40：goto 54 
 43：aload_0 
 44：invokevirtual＃4; //方法x :(）V 
 47：goto 54 
 50：aload_0 
 51：invokevirtual＃4; //方法x :(）V 
 54：return 
 
 void testIf（java.lang.Enum）; 
代码：
 0：aload_1 
 1：getstatic＃5; //字段A $ N.ONE：LA $ N; 
 4：if_acmpne 14 
 7：aload_0 
 8：invokevirtual＃4; //方法x :(）V 
 11：goto 39 
 14：aload_1 
 15：getstatic＃6; // Field A $ N.TWO：LA $ N; 
 18：if_acmpne 28 
 21：aload_0 
 22：invokevirtual＃4; //方法x :(）V 
 25：goto 39 
 28：aload_1 
 29：getstatic＃7; // Field A $ N.THREE：LA $ N; 
 32：if_acmpne 39 
 35：aload_0 
 36：invokevirtual＃4; //方法x :(）V 
 39：return 
 
 void x（）; 
代码：
 0：return 
 
} 
  

在这两种情况下似乎都相当快。

所以，选择一个更容易维护的。

I've had no real luck getting a concise answer for this comparison by using Google and rather than do my own time consuming evaluations, I thought I would ask first.

I'm fairly sure that a switch statement using Enums would perform faster than an if-then-else statement, though whether or not it is a noticable difference is another question.

Could someone shed some light on this for me?

---

Thanks for the quick responses guys, I will keep this in mind for future projects.

解决方案

Yeap, it does, because in general term a switch statement works faster than if/else chain.

Although bytecode generated is not always definitive source for performance comparisons you can examine it to have a better idea.

For instance this code:

class A { 
    enum N { ONE, TWO, THREE }
    void testSwitch( N e ) { 
        switch( e ) { 
            case ONE : x(); break;
            case TWO : x(); break;
            case THREE : x(); break;
        }
    }
    void testIf( Enum e ) { 
        if( e == N.ONE ) { x(); }
        else if( e == N.TWO ) { x(); }
        else if( e == N.THREE ) { x(); }
    }
    void x(){}
}

Generates the following:

Compiled from "A.java"
class A extends java.lang.Object{
A();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

void testSwitch(A$N);
  Code:
   0:   getstatic   #2; //Field A$1.$SwitchMap$A$N:[I
   3:   aload_1
   4:   invokevirtual   #3; //Method A$N.ordinal:()I
   7:   iaload
   8:   tableswitch{ //1 to 3
        1: 36;
        2: 43;
        3: 50;
        default: 54 }
   36:  aload_0
   37:  invokevirtual   #4; //Method x:()V
   40:  goto    54
   43:  aload_0
   44:  invokevirtual   #4; //Method x:()V
   47:  goto    54
   50:  aload_0
   51:  invokevirtual   #4; //Method x:()V
   54:  return

void testIf(java.lang.Enum);
  Code:
   0:   aload_1
   1:   getstatic   #5; //Field A$N.ONE:LA$N;
   4:   if_acmpne   14
   7:   aload_0
   8:   invokevirtual   #4; //Method x:()V
   11:  goto    39
   14:  aload_1
   15:  getstatic   #6; //Field A$N.TWO:LA$N;
   18:  if_acmpne   28
   21:  aload_0
   22:  invokevirtual   #4; //Method x:()V
   25:  goto    39
   28:  aload_1
   29:  getstatic   #7; //Field A$N.THREE:LA$N;
   32:  if_acmpne   39
   35:  aload_0
   36:  invokevirtual   #4; //Method x:()V
   39:  return

void x();
  Code:
   0:   return

}

Which seems to be pretty fast in both cases.

So, pick the one that is easier to maintain.

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