Should I declare variables outside or inside reused methods?

Question

(I'm using String as an example, but it could be replaced for Object > MB's of memory)

do this quite a lot:

private static String mTempString = "";

private static void SomeMethod()
{
    mTempString = "Whatever Result";
}

Now my question is, if I was to write it like so:

private static void SomeMethod()
{
    String mTempString = "Whatever Result";
}

and use it in a loop (that is executing hundreds of times a second as an example) would Java know how to manage the memory as example one? Would be memory be of the same efficiently. (Sorry I can't test this myself at the moment)

Which is more memory efficient (disregarding the fact that they are small variables)

--edit--- Found an excellent article here that explains it http://www.cs.berkeley.edu/~jrs/4/lec/08

Answer 1

Keep the scope of your variables as narrow as possible.

This is important for several reasons:

1. Readability. If you use that variable in three different places with four different values (go figure), you're going to have a hard time discerning what purpose that variable is supposed to serve.

2. Bugs. You reduce the amount of errors that could crop up in your application if you keep a single variable to a single, well-specified scope. Suppose you had that String, and you expected it to be some value in two methods, but it was something completely different.

3. Intent of purpose. I mentioned this a bit in the readability section, but if you have a static variable that keeps getting redefined, its intended purpose has become unclear. Typically, static methods and variables can be used independent of the state of the object, so if the state of the object affects the value of the static variable, the intent has become confusing.

I wouldn't worry much about memory efficiency at this point (unless you had gobs of strings, but then I'd say you have two things to worry about).*

^{*: First rule of optimization: Don't do it.
Second rule of optimization (experts only!): Don't do it yet.}