How does array access affect the performance?

int steps = 256 * 1024 * 1024;
int[] a = new int[2];

// Loop 1
for (int i=0; i<steps; i++) { a[0]++; a[0]++; }

// Loop 2
for (int i=0; i<steps; i++) { a[0]++; a[1]++; }

Can someone explain why the second loop is 20x times slower than the first (19 ms vs 232 ms)?

That is how I'm timing it:

long start_time = System.currentTimeMillis();

// Loop

long end_time = System.currentTimeMillis();
System.out.println(end_time - start_time);