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Android Development
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Cache Field Lookups

Accessing object fields is much slower than accessing local variables. Instead of writing:

for (int i = 0; i < this.mCount; i++)
      dumpItem(this.mItems[i]);

You should write:

  int count = this.mCount;
  Item[] items = this.mItems;
 
  for (int i = 0; i < count; i++)
      dumpItems(items[i]);

(We're using an explicit "this" to make it clear that these are member variables.)

A similar guideline is never call a method in the second clause of a "for" statement. For example, the following code will execute the getCount() method once per iteration, which is a huge waste when you could have simply cached the value as an int:

for (int i = 0; i < this.getCount(); i++)
    dumpItems(this.getItem(i));

It's also usually a good idea to create a local variable if you're going to be accessing an instance field more than once. For example:

    protected void drawHorizontalScrollBar(Canvas canvas, int width, int height) {
        if (isHorizontalScrollBarEnabled()) {
            int size = mScrollBar.getSize(false);
            if (size <= 0) {
                size = mScrollBarSize;
            }
            mScrollBar.setBounds(0, height - size, width, height);
            mScrollBar.setParams(
                    computeHorizontalScrollRange(),
                    computeHorizontalScrollOffset(),
                    computeHorizontalScrollExtent(), false);
            mScrollBar.draw(canvas);
        }
    }

That's four separate lookups of the member field mScrollBar. By caching mScrollBar in a local stack variable, the four member field lookups become four stack variable references, which are much more efficient.

Incidentally, method arguments have the same performance characteristics as local variables.

Android Development
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  Published under the terms fo the Apache 2.0 License Design by Interspire