Question

I have one problem that I can’t explain. Here is the code in main function:

String numberStr = "3151312423412354315";

System.out.println(numberStr + "\n");
System.out.println("Lehman method: ");
long beginTime = System.currentTimeMillis();
System.out.println(Lehman.getFullFactorization(numberStr));
long finishTime = System.currentTimeMillis();
System.out.println((finishTime-beginTime)/1000. + " sec.");

System.out.println();

System.out.println("Lehman method: ");
beginTime = System.currentTimeMillis();
System.out.println(Lehman.getFullFactorization(numberStr));
finishTime = System.currentTimeMillis();
System.out.println((finishTime-beginTime)/1000. + " sec.");

If it is necessary: method Lehman.getFullFactorization(...) returns the ArrayList of prime divisors in String format.

Here is the output:

3151312423412354315

Lehman method: 
[5, 67, 24473, 384378815693]
0.149 sec.

Lehman method: 
[5, 67, 24473, 384378815693]
0.016 sec.

I was surprised, when I saw it. Why a second execution of the same method much faster than first? Firstly, I thought that at the first running of the method it calculates time with time of running JVM and its resources, but it’s impossible, because obviously JVM starts before execution of the “main” method.

Answer 1

Try doing it more than 10,000 times and it will be much faster. This is because the code first has to be loaded (expensive) then runs in interpreted mode (ok speed) and is finally compiled to native code (much faster)

Can you try this?

int runs = 100*1000;
for(int i = -20000 /* warmup */; i < runs; i++) {
   if(i == 0)
       beginTime = System.nanoTime();
   Lehman.getFullFactorization(numberStr);
}
finishTime = System.nanoTime();
System.out.println("Average time was " + (finishTime-beginTime)/1e9/runs. + " sec.");