Variable declaration placement guidelines in Java [closed]

Question

There seems to be two accepted variable declaration placements for Java variables, each with different raison d'être.

From the Sun's code conventions we can see:

Put declarations only at the beginning of blocks. (A block is any code surrounded by curly braces "{" and "}".) Don't wait to declare variables until their first use; it can confuse the unwary programmer and hamper code portability within the scope.

However in the highly praised "Code Complete" and some other authors advocate for reducing the scope of a variable to a minimum. That is basically waiting to declare variables until their first use.

These two approaches are clearly contradictory although I can see the point for both of them.

Which should I follow? Is there any consensus on the matter?

Answer 1

Variables should be declared as close to their use as possible, for two reasons:

• Vertical locality
• Tool hinting

Vertical locality makes it easier to reason about chunks of code. The less scanning a reader has to do the easier it is to understand what code does, and what side-effects it

Reducing variable scope also allows better tool hinting for automated tools, like refactoring. The closer together related things are the more obvious they are.

That said: if a method is long enough that the above points come in to play, it's likely the method is already too long, and should be refactored anyway.

Answer 2

That is basically waiting to declare variables until their first use.

This is actually not true, and these two styes are not conflicting. Limiting the scope of the variable means that that variable, in fact, does not exist outside of that scope. E.g.

for(int i=0; i<10;i++){
   int a = 5;
   doSomething(a);
}

In this case, a is scope limited to the for block and this is what Code complete is referencing.

In any case I agree with sun, that variables within a scope (class, method, if block, etc.) should be declared at the beginning.