Why the first call to constructor takes 10 times more time than other ones?

Question

class testx
{
  public testx()
  {
    long startTime = System.nanoTime();
    System.out.println((System.nanoTime() - startTime));
  }

  public static void main(String args[])
  {
      new testx();
      new testx();
      new testx();
  }
}

I always get results similar to this 7806 660 517. Why the first call takes 10 times more time than other ones?

Answer 1

Because the JVM loads a bunch o' classes for the first time at that point. Once that first System.nanoTime() returns, you have already loaded System.class and testx.class, but once System.out.println comes into the picture, I suspect a lot of I/O classes get loaded up, and that takes some time.

In any event, this is not a good benchmarking technique; you should really be warming up the JIT by running something for ~10000 iterations before you start measuring it. Alternately (and preferably), use a pre-built benchmarking tool like Caliper.

Answer 2

It is definitely as Louis Wasserman, It takes longer on its first round as it has to load all the necessary System classes, you can get around this by calling a blank println() before creating new instances of the class, because look what happens when we do this:

public class testx
{
  public testx()
  {
    long startTime = System.nanoTime();
    System.out.println((System.nanoTime() - startTime));
  }

  public static void main(String args[])
  {
    //loads all System.* classes before calling constructor to decrease time it takes
    System.out.println();
      new testx();
      new testx();
      new testx();
  }
}

output:

405 0 405

where as your initial code outputted:

7293 0 405