Java：if-return-if-return vs if-return-elseif-return

问题描述

询问我的代码如下：

public boolean equals(Object obj)
{
    if (this == obj)
        return true;

    if (obj == null)
        return false;

    if (getClass() != obj.getClass())
        return false;

    // Check property values
}

我有一个声称这不是最优的评论，而是（如果我理解正确）它应该这样做：

I got a comment which claimed that this was not optimal, and that it instead (if I understood correctly) should do this:

public boolean equals(Object obj)
{
    if (this == obj)
        return true;

    else if (obj == null)
        return false;

    else if (getClass() != obj.getClass())
        return false;

    // Check property values
}

因为返回语句，我真的不明白为什么它们中的任何一个应该比另一个更高效或更快。鉴于某个对象，就我所见，这两种方法都必须进行相同数量的检查。并且由于返回语句，其中任何一个都不会运行额外的代码。

Because of the return statements, I can't really see why any of them should be more efficient or faster than the other. Given a certain object, both methods would have to do an equal number of checks as far as I can see. And because of the return statements, no extra code would run in any of them.

我在这里遗漏了什么吗？有东西吗？是否有一些编译器优化或正在进行的事情或其他什么？

Am I missing something here? Is there something to it? Are there some compiler optimizations or something going on or whatever?

我知道这是微优化，我很可能坚持第一种方式，因为我认为它看起来更干净，所有的ifs都在同一个位置。但我无能为力;我很好奇！

推荐答案

生成的字节代码对于这两种情况是相同的，所以这纯粹是一个问题风格。

The generated byte code is identical for those two cases, so it's purely a matter of style.

我制作了两种方法 e1 和 e2 并且都生成了这个字节代码（使用 javap -v 读取）：

I produced two methods e1 and e2 and both produced this byte code (read using javap -v):

public boolean e1(java.lang.Object);
  Code:
   Stack=2, Locals=2, Args_size=2
   0:   aload_0
   1:   aload_1
   2:   if_acmpne   7
   5:   iconst_1
   6:   ireturn
   7:   aload_1
   8:   ifnonnull   13
   11:  iconst_0
   12:  ireturn
   13:  aload_0
   14:  invokevirtual   #25; //Method java/lang/Object.getClass:()Ljava/lang/Class;
   17:  aload_1
   18:  invokevirtual   #25; //Method java/lang/Object.getClass:()Ljava/lang/Class;
   21:  if_acmpeq   26
   24:  iconst_0
   25:  ireturn

我遗漏了代码I然后把它编成。

I left out the code I put after that to make it compile.

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