Java Advantages of Converting Global Variables to Local Variables in Methods

Question

I am working on an android launcher based on the stock launcher. I am just interested why are there lots of global variables converted to local variables in methods e.g.

final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000);

instead of just

mVelocityTracker.computeCurrentVelocity(1000);

Is it some android thing or a general java rule? It makes no sense allocating a new VelocityTracker when it can be accessed directly.

EDIT Yes this code is being repeated many times.

It might be a weird optimization, see my answer here. Note I have no experience with Android. — Tomasz Nurkiewicz
– Tomasz Nurkiewicz, Commented Jul 23, 2011 at 17:16

Peter Lawrey · Accepted Answer · 2011-07-23 17:17:24Z

4

This can be useful if you are using a field many times. Some JVM and I assume Android VMs don't optimise access to fields as efficiently.

However it can be overused and I don't see the point if only accessed once.

It can also be useful of you are accessing a volatile field. This ensures when you use that field many times you are taking about the same object. e.g.

volatile String text;

String text = this.text;
if(text != null)
    doSomething(text);

If you didn't use a local variable text could be non-null for the if statement and null for the doSomething().

answered Jul 23, 2011 at 17:17

Peter Lawrey

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Comments

AlexR · Accepted Answer · 2011-07-23 17:18:04Z

1

You are right, in your short example this assignment does not have any sense. But generally it is a good practice to minimize scope of variables. This allows you to concentrate on specific code that deals with specific variables.

answered Jul 23, 2011 at 17:18

AlexR

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1 Comment

Kirk Woll Over a year ago

How is it ever useful to assign a field to a local variable when the field is just as accessible as the variable? How does this "minimize the scope of variables"?

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