Variable Management in Loops

[rp181]

Which is better, if any are better?

for(int i = 0; i < 100; i++){
for(int d = 0; d < 200; d++){
int z = i*d;
...
}
}

int z;
for(int i = 0; i < 100; i++){
for(int d = 0; d < 200; d++){
z = i*d;
...
}
}

This is a totally random example. Does "recreating" the variable everytime actually have an effect? Or should you declare changing variables beforehand?

[iluxa]

Depends on Java version and operating system, but I've seen declaration of variable outside the loop help performance in some configurations.

[paul pasciak]

Your two code snippets are somewhat different from each other.
They might not be use to perform the same task.
Both of them are below.
One will not compile (I did this to emphasize the difference)
because the variable is out of 'scope'.

class TryZ{

  public static void main(String[] args){

    int z=0;
    for(int i = 0; i < 100; i++){
      for(int d = 0; d < 200; d++){
        z = i*d;
      }
    }
    System.out.println(z);

/*
    for(int i = 0; i < 100; i++){
      for(int d = 0; d < 200; d++){
        int zz = i*d;
      }
    }
    System.out.println(zz);
*/
  }
}

Otherwise, you raise a valid question.
I have read in one book that:

  int length = array.length;
  for(int i=0; i<length; i++){
    ;
  }

is faster than

  for(int i=0; i<array.length; i++){
    ;
  }