7.7 Performance Checklist

Most of these suggestions apply only after a bottleneck has been identified:

• Make the loop do as little as possible.

  • Remove from the loop any execution code that does not need to be executed on each pass.

  • Move any code that is repeatedly executed with the same result, and assign that code to a temporary variable before the loop ("code motion").

  • Avoid method calls in loops when possible, even if this requires rewriting or inlining.

  • Multiple access or update to the same array element should be done on a temporary variable and assigned back to the array element when the loop is finished.

  • Avoid using a method call in the loop termination test.

  • Use int data types preferentially, especially for the loop variable.

  • Use System.arraycopy( ) for copying arrays.

  • Try to use the fastest tests in loops.

  • Convert equality comparisons to identity comparisons when possible.

  • Phrase multiple boolean tests in one expression so that they "short circuit" as soon as possible.

  • Eliminate unneeded temporary variables from loops.

  • Try unrolling the loop to various degrees to see if this improves speed.

• Rewrite any switch statements to use a contiguous range of case values.

• Identify whether a recursive method can be made faster.

  • Convert recursive methods to use iteration instead.

  • Convert recursive methods to use tail recursion.

  • Try caching recursively calculated values to reduce the depth of recursion.

  • Use temporary variables in place of passed parameters to convert a recursive method using a single search path into an iterative method.

  • Use temporary stacks in place of passed parameters to convert a recursive method using multiple search paths into an iterative method.