Java – Difference between declaring variables before or in loop

initializationjavaloopsperformancevariables

I have always wondered if, in general, declaring a throw-away variable before a loop, as opposed to repeatedly inside the loop, makes any (performance) difference?
A (quite pointless) example in Java:

a) declaration before loop:

double intermediateResult;
for(int i=0; i < 1000; i++){
    intermediateResult = i;
    System.out.println(intermediateResult);
}

b) declaration (repeatedly) inside loop:

for(int i=0; i < 1000; i++){
    double intermediateResult = i;
    System.out.println(intermediateResult);
}

Which one is better, a or b?

I suspect that repeated variable declaration (example b) creates more overhead in theory, but that compilers are smart enough so that it doesn't matter. Example b has the advantage of being more compact and limiting the scope of the variable to where it is used. Still, I tend to code according example a.

Edit: I am especially interested in the Java case.

Best Answer

Which is better, a or b?

From a performance perspective, you'd have to measure it. (And in my opinion, if you can measure a difference, the compiler isn't very good).

From a maintenance perspective, b is better. Declare and initialize variables in the same place, in the narrowest scope possible. Don't leave a gaping hole between the declaration and the initialization, and don't pollute namespaces you don't need to.

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