performance - java compiler optimization

Question

Is the Java compiler smart enough to optimize loop below, by extracting the

Double average = new Double( totalTime / callCount ); 

out of the for loop?

public double computeSD( Set values, int callCount, long totalTime ) {
  double diffs = 0.0d; 
  for( Iterator i=values.iterator(); i.hasNext(); ) {
    double value = ( ( Double )i.next() ).doubleValue(); 
    Double average = new Double( totalTime / callCount ); 
    diffs += ( value – average.doubleValue() ) * ( value – average.doubleValue() );
  } 
  double variance = diffs / callCount;
  return Math.sqrt( variance );
}

Answer 1

Nothing prevents the bytecode compiler (java->bytecode) from performing optimizations. When I worked at Symantec, and they did a Java IDE, the compiler write did look into putting some optimizations into our compiler but said nobody (in the outside world) seemed to be interested and the focus was on the Just In Time (JIT) compiler that is roughly the same as HotSpot in modern Sun VMs.

There is nothing that prevents a bytecode compiler from performing optimizations, but I am not aware of any that do so. There is huge focus on runtime optimizations, but those are pretty much hidden at runtime.

So, the source->bytecode compiler probably does not optimize it but the VM probably does. If you are on something like an Android then it probably performs no runtime optimization.