Cache Field Lookups

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.