Question

I come from a C background, so I admit that I’m still struggling with letting go of memory management when writing in Java. Here’s one issue that’s come up a few times that I would love to get some elaboration on. Here are two ways to write the same routine, the only difference being when double[] array is declared:

Code Sample 1:

double[] array;
for (int i=0; i<n; ++i) {
    array = calculateSomethingAndReturnAnArray(i);
    if (someFunctionOnArrays(array)) {
        // DO ONE THING
    } else {
        // DO SOME OTHER THING
    }
}

Code Sample 2:

for (int i=0; i<n; ++i) {
    double[] array = calculateSomethingAndReturnAnArray(i);
    if (someFunctionOnArrays(array)) {
        // DO ONE THING
    } else {
        // DO SOME OTHER THING
    }
}

Here, private double[] calculateSomethingAndReturnAnArray(int i) always returns an array of the same length. I have a strong aversion to Code Sample 2 because it creates a new array for each iteration when it could just overwrite the existing array. However, I think this might be one of those times when I should just sit back and let Java handle the situation for me.

What are the reasons to prefer one of the ways over the other or are they truly identical in Java?

Answer 1

There’s nothing special about arrays here because you’re not allocating for the array, you’re just creating a new variable, it’s equivalent to:

Object foo;
for(...){
    foo = func(...);
}

In the case where you create the variable outside the loop it, the variable (which will hold the location of the thing it refers to) will only ever be allocated once, in the case where you create the variable inside the loop, the variable may be reallocated for in each iteration, but my guess is the compiler or the JIT will fix that in an optimization step.

I’d consider this a micro-optimization, if you’re running into problems with this segment of your code, you should be making decisions based on measurements rather than on the specs alone, if you’re not running into issues with this segment of code, you should do the semantically correct thing and declare the variable in the scope that makes sense.

See also this similar question about best practices.