Declare an object inside or outside a loop?

Question

Is there any performance penalty for the following code snippet?

for (int i=0; i

        

Answer 1

I've always thought that most compilers these days are smart enough to do the latter option. Assuming that's the case, I would say the first one does look nicer as well. If the loop gets very large, there's no need to look all around for where o is declared.

Answer 2

As someone who maintains more code than writes code.

Version 1 is much preferred - keeping scope as local as possible is essential for understanding. Its also easier to refactor this sort of code.

As discussed above - I doubt this would make any difference in efficiency. In fact I would argue that if the scope is more local a compiler may be able to do more with it!

Answer 3

To quote Knuth, who may be quoting Hoare:

Premature optimization is the root of all evil.

Whether the compiler will produce marginally faster code by defining the variable outside the loop is debatable, and I imagine it won't. I would guess it'll produce identical bytecode.

Compare this with the number of errors you'll likely prevent by correctly-scoping your variable using in-loop declaration...

Answer 4

These have different semantics. Which is more meaningful?

Reusing an object for "performance reasons" is often wrong.

The question is what does the object "mean"? WHy are you creating it? What does it represent? Objects must parallel real-world things. Things are created, undergo state changes, and report their states for reasons.

What are those reasons? How does your object model and reflect those reasons?

Answer 5

I've got to admit I don't know java. But are these two equivalent? Are the object lifetimes the same? In the first example, I assume (not knowing java) that o will be eligible for garbage collection immediately the loop terminates.

But in the second example surely o won't be eligible for garbage collection until the outer scope (not shown) is exited?

Answer 6

There's no performance penalty for declaring the Object o within the loop. The compiler generates very similar bytecode and makes the correct optimizations.

See the article Myth - Defining loop variables inside the loop is bad for performance for a similar example.