Local variables: Programming Practices

Question

Given the following methods:

public int methodOne() {
    int total = local_int_one + local_int_two;
    return total;
}

public int methodTwo() {
    return local_int_one + local_int_two;
}

1) Is the only difference in the above methods readability or is there a micro-optimization "benefit" in methodTwo()?

2) Should the defining of local variables in a narrow scope be shunned and avoided when possible? (I can see methodTwo becoming unreadable if several calculations must be performed in a single statement)

@EJP: what if you turn on optimizations? Currently I have no access to a java bytecode viewer. But evidently debug mode results in some sort of one-on-one mapping. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Oct 3, 2015 at 20:09
@Commusoft The Java compiler does little or nothing in the way of optimization. — user207421
– user207421, Commented Oct 3, 2015 at 20:13

Alex Shesterov · Accepted Answer · 2015-10-03 20:14:21Z

5

The short answer is: methodTwo() is slightly more efficient.

methodOne() results in the following bytecode:

public int methodOne();
 0  aload_0 [this]
 1  getfield com.example.Test.local_int_one : int [13]
 4  aload_0 [this]
 5  getfield com.example.Test.local_int_two : int [15]
 8  iadd
 9  istore_1 [total]
10  iload_1 [total]
11  ireturn

And here's the bytecode for methodTwo():

public int methodTwo();
 0  aload_0 [this]
 1  getfield com.example.Test.local_int_one : int [13]
 4  aload_0 [this]
 5  getfield com.example.Test.local_int_two : int [15]
 8  iadd
 9  ireturn

But note that this optimization is too minor, and that code readability in this case matters a lot more then a couple of java instructions.

If you think a temporary variable will contribute to code readability, then, by all means, use it.

answered Oct 3, 2015 at 20:14

Alex Shesterov

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2 Comments

VILLAIN bryan Over a year ago

Almost two years later, I'd like to add a follow-up here if I may. Since methodTwo() does not perform istore_1, does this mean that (in Java) there would be no additional gc step to perform (since the value is just returned and not stored)? ... so in a gc-critical environment, instantiating variables in small scopes should be shunned?

Alex Shesterov Over a year ago

@mcw, integers aren't subject to garbage collection, nor the local variables. As for objects - they are created on heap in any case stackoverflow.com/a/27587519/2170192

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