java在内存分配方面的效率比较 [英] java efficiency comparison in terms of memory allocation

问题描述

这可能是一个重复的问题，但我无法找到我正在搜索的内容。如果它存在，抱歉重复。

This may be a duplicate question but I couldnt find what I am searching. If it exists, sorry about duplication.

我想知道如果以下部分代码在内存分配方面是相同的。

I want to learn that if the following part of codes are same in terms of memory allocation.

//first
int n = some_number;
for(int i = 0; i < n; i++){
    for(int j = 0; j < n; j++){
        int a = something;
    }
}

//second
int i, j, a;
for(i = 0; i < n; i++){
    for(j = 0; j < n; j++){
        a = something;
    }
}

我想知道，如果java分配变量<$ c第一个代码中$ c> a n ^ 2次和 j n次或者两者仅在第二个代码中分配一次。

I wonder, if java allocates the variable a n^2 times and j n times in the first code or both are allocated only once as in the second code.

我在java中尝试了几次，但结果不一致，就像在一次试验中首先是8秒，第二次是9秒，在另一次试用中反过来。所以，我想确定它们是否相等，

I tried this couple of times in java but the results are inconsistent like in one trial first is 8 sec, second is 9 sec, in another trial reverse. So, I want to make sure if they are equal or not,

谢谢

推荐答案

查看是否存在差异的一种简单方法是检查字节码。

One easy way to see if there's a difference is to examine the bytecodes.

第一个版本编译为：

  public static void f();
    Code:
       0: bipush        100
       2: istore_0      
       3: iconst_0      
       4: istore_1      
       5: goto          26
       8: iconst_0      
       9: istore_2      
      10: goto          18
      13: iconst_3      
      14: istore_3      
      15: iinc          2, 1
      18: iload_2       
      19: iload_0       
      20: if_icmplt     13
      23: iinc          1, 1
      26: iload_1       
      27: iload_0       
      28: if_icmplt     8
      31: return        

而第二个编译为：

  public static void g();
    Code:
       0: bipush        100
       2: istore_3      
       3: iconst_0      
       4: istore_0      
       5: goto          26
       8: iconst_0      
       9: istore_1      
      10: goto          18
      13: iconst_3      
      14: istore_2      
      15: iinc          1, 1
      18: iload_1       
      19: iload_3       
      20: if_icmplt     13
      23: iinc          0, 1
      26: iload_0       
      27: iload_3       
      28: if_icmplt     8
      31: return        

如果你仔细比较它们，你会发现它们基本相同。

If you compare them closely, you'll see that they are essentially identical.

从风格上讲，我认为最好将变量尽可能接近第一次使用。考虑到这一点，我会在第二个版本中选择第一个版本。

Stylistically, I think it's preferable to declare variables as close as possible to their first use. With this in mind, I'd choose the first version over the second.

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