Which of these pieces of code is faster in Java?

Question

a) for(int i = 100000; i > 0; i--) {}

b) for(int i = 1; i < 100001; i++) {}

The answer is t

Answer 1

On a modern Java implementation this is not true. Summing up the numbers up to one billion as a benchmark:

Java(TM) SE Runtime Environment 1.6.0_05-b13
Java HotSpot(TM) Server VM 10.0-b19
up 1000000000: 1817ms 1.817ns/iteration (sum 499999999500000000)
up 1000000000: 1786ms 1.786ns/iteration (sum 499999999500000000)
up 1000000000: 1778ms 1.778ns/iteration (sum 499999999500000000)
up 1000000000: 1769ms 1.769ns/iteration (sum 499999999500000000)
up 1000000000: 1769ms 1.769ns/iteration (sum 499999999500000000)
up 1000000000: 1766ms 1.766ns/iteration (sum 499999999500000000)
up 1000000000: 1776ms 1.776ns/iteration (sum 499999999500000000)
up 1000000000: 1768ms 1.768ns/iteration (sum 499999999500000000)
up 1000000000: 1771ms 1.771ns/iteration (sum 499999999500000000)
up 1000000000: 1768ms 1.768ns/iteration (sum 499999999500000000)
down 1000000000: 1847ms 1.847ns/iteration (sum 499999999500000000)
down 1000000000: 1842ms 1.842ns/iteration (sum 499999999500000000)
down 1000000000: 1838ms 1.838ns/iteration (sum 499999999500000000)
down 1000000000: 1832ms 1.832ns/iteration (sum 499999999500000000)
down 1000000000: 1842ms 1.842ns/iteration (sum 499999999500000000)
down 1000000000: 1838ms 1.838ns/iteration (sum 499999999500000000)
down 1000000000: 1838ms 1.838ns/iteration (sum 499999999500000000)
down 1000000000: 1847ms 1.847ns/iteration (sum 499999999500000000)
down 1000000000: 1839ms 1.839ns/iteration (sum 499999999500000000)
down 1000000000: 1838ms 1.838ns/iteration (sum 499999999500000000)

Note that the time differences are brittle, small changes somewhere near the loops can turn them around.

Edit: The benchmark loops are

        long sum = 0;
        for (int i = 0; i < limit; i++)
        {
            sum += i;
        }

and

        long sum = 0;
        for (int i = limit - 1; i >= 0; i--)
        {
            sum += i;
        }

Using a sum of type int is about three times faster, but then sum overflows. With BigInteger it is more than 50 times slower:

BigInteger up 1000000000: 105943ms 105.943ns/iteration (sum 499999999500000000)