java efficiency comparison in terms of memory allocation

问题

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

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;
    }
}

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.

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,

Thanks

回答1:

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

The first version compiles to:

  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        

whereas the second compiles to:

  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        

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

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.

来源：https://stackoverflow.com/questions/15942926/java-efficiency-comparison-in-terms-of-memory-allocation