Java Performance Tuning

Chapter 7. Loops and Switches

I have made this letter longer than usual because I lack the time to make it shorter.

—Blaise Pascal

Programs spend most of their time in loops. There are many optimizations that can speed up loops :

• Take out of the loop any code that does not need to be executed on every pass. This includes assignments, accesses, tests, and method calls that need to run only once.

• Method calls are more costly than the equivalent code without the call, and by repeating method calls again and again, you just add overhead to your application. Move any method calls out of the loop, even if this requires rewriting. Inline method calls in loops when possible.

• Array access (and assignment) always has more overhead than temporary variable access because the VM performs bounds-checking for array-element access. Array access is better done once (and assigned to a temporary) outside the loop rather than repeated at each iteration. For example, consider this next loop:

  for(int i = 0; i < Repeat; i++)
    countArr[0]+=10;

  The following loop optimizes the last loop using a temporary variable to execute the addition within the loop. The array element is updated outside the loop. This optimized loop is significantly better (twice as fast) than the original loop:

  count = countArr[0];
  for(int i = 0; i < Repeat; i++)
    count+=10;
  countArr[0]=count;

• Comparison to 0 is faster than comparisons to most other numbers. The VM has optimizations for comparisons to the integers -1, 0, 1, 2, 3, 4, and 5. ...