Java results differ for (int)Math.pow(2,x) and 1<<x

Question

Why do the following two operations yield different results in Java for x = 31 or 32 but the same results for x=3?

int         


        

Answer 1

Here's a micro-benchmark for the case of a long. On my laptop (2.8GHz), using shift instead of Math.pow is over 7x faster.

int limit = 50_000_000;
@Test
public void testPower() {
    Random r = new Random(7);
    long t = System.currentTimeMillis();
    for (int i = 0; i < limit; i++) {
        int p = r.nextInt(63);
        long l = (long)Math.pow(2,p);
    }
    long t1 = System.currentTimeMillis();
    System.out.println((t1-t)/1000.0); // 3.758 s
}
@Test
public void testShift() {
    Random r = new Random(7);
    long t = System.currentTimeMillis();
    for (int i = 0; i < limit; i++) {
        int p = r.nextInt(63);
        long l = 1L << p;
    }
    long t1 = System.currentTimeMillis();
    System.out.println((t1-t)/1000.0); // 0.523 s
}