java - Calling getters on an object vs. storing it as a local variable (memory footprint, performance)

Question

In the following piece of code we make a call listType.getDescription() twice:

for (ListType listType: this.listTypeManager.getSelectableListTypes())
{
    if (listType.getDescription() != null)
    {
        children.add(new SelectItem( listType.getId() , listType.getDescription()));
    }
}

I would tend to refactor the code to use a single variable:

for (ListType listType: this.listTypeManager.getSelectableListTypes())
{
    String description = listType.getDescription();

    if (description != null)
    {
        children.add(new SelectItem(listType.getId() ,description));
    }
}

My understanding is the JVM is somehow optimized for the original code and especially nesting calls like children.add(new SelectItem(listType.getId(), listType.getDescription()));.

Comparing the two options, which one is the preferred method and why? That is in terms of memory footprint, performance, readability/ease, and others that don't come to my mind right now.

When does the latter code snippet become more advantageous over the former, that is, is there any (approximate) number of listType.getDescription() calls when using a temp local variable becomes more desirable, as listType.getDescription() always requires some stack operations to store the this object?

Answer 1

I'd nearly always prefer the local variable solution.

Memory footprint

A single local variable costs 4 or 8 bytes. It's a reference and there's no recursion, so let's ignore it.

Performance

If this is a simple getter, the JVM can memoize it itself, so there's no difference. If it's a expensive call which can't be optimized, memoizing manually makes it faster.

Readability

Follow the DRY principle. In your case it hardly matters as the local variable name is character-wise as about as long as the method call, but for anything more complicated, it's readability as you don't have to find the 10 differences between the two expressions. If you know they're the same, so make it clear using the local variable.

Correctness

Imagine your SelectItem does not accept nulls and your program is multithreaded. The value of listType.getDescription() can change in the meantime and you're toasted.

Debugging

Having a local variable containing an interesting value is an advantage.

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The only thing to win by omitting the local variable is saving one line. So I'd do it only in cases when it really doesn't matter:

• very short expression
• no possible concurrent modification
• simple private final getter