Java8ずいぶん変わってるなw
class Pair {
final double first;
final int second;
Pair(double first, int second) { this.first = first; this.second = second; }
}
double[] a = {2.0, 1.2, 8.5, 1.2};
int[] b = {1, 2, 3, 4};
ArrayList<Pair> list = new ArrayList<Pair>() { { for (int i = 0; i < a.length; i++) add(new Pair(a[i], b[i])); } };
list.sort(Comparator.comparing(x -> x.first));
list.forEach(p -> System.out.println(p.second)); //=> 2 4 1 3