Is it more efficient to declare a variable inside a loop, or just to reassign it?

Question

While writing loops I often get confused with which one I should choose. For example,

int sum;
for(int i=0; i<10; i++)
{
   sum=0;
   ...
   ....
}

Or

for(int i=0; i<10; i++)
{
   int sum=0;
   ...
   ....
}

Say, the variable is only required in this loop. There is no need of it in the later part of the program. I need the value of the variable sum to be 0 at the beginning of the loop. Which one is the better practice? Re-initializing a variable at the start of the loop or re-declaring it? Which one is more efficient?

Answer 1

If you declare a variable outside of the loop and not use it past the loop, the compiler will move the declaration inside the loop.

That means there is no reason to compare efficiency here, since you end up with the same exact code that the JVM will run for the two approaches.

So the following code:

int sum;
for(int i=0; i<10; i++)
{
   sum=0;
}

... becomes this after compilation:

for(int i = 0; i < 10; i++)
{
    int sum = 0;
}