Java Comparator.comparing not comparing?

Question

Following this question about sorting a list by another list, I tried to do the same thing - but from some reason it doesn\'t work for me. What am I missing?

Answer 1

You are sorting the numbers by their position in the order list, but none of the numbers occur in the order list. In this case, indexOf will return -1 for everything, meaning everything is equal to everything else. In such a case, the resulting sort order is unspecified - though you may realistically assume that it would not change.

Answer 2

You can make a list of pairs :

[3.0, 5.0]
[1.0, 0.9]
[2.0, 10.4]

Then sort this list of pairs by the first value of each array :

[1.0, 0.9]
[2.0, 10.4]
[3.0, 5.0]

Here is the code :

List<Double> nums = Arrays.asList(5.0, 0.9, 10.4);
List<Double> order = Arrays.asList(3.0, 1.0, 2.0);

List<Double[]> pairs = new ArrayList<>();
for (int i = 0; i < nums.size(); i++) {
    pairs.add(new Double[] {order.get(i), nums.get(i)});
}

pairs.sort(Comparator.comparing(pair -> pair[0]));

for (Double[] pair : pairs) {
    System.out.print(pair[1] + " ");
}

Output :

0.9 10.4 5.0 

Answer 3

The Comparator you are supplying calls indexOf for every num passed. The returned values are -1 on all calls, so the order is preserverd as-is.

You need to sort natural.

Sorting by another list of Double should be possible, but unnecessarily complicated, it would be simpler to provide a custom Object which sorts as desired.

Answer 4

Update

List<Double> nums = Arrays.asList(5.0, 0.9, 10.4);
List<Double> order = Arrays.asList(3.0, 1.0, 2.0);
Map<Double,Double> numToOrder = new HashMap<>();
for (int i = 0; i < nums.size(); ++i) {
    numToOrder.put(nums.get(i), order.get(i));
}
nums.sort(Comparator.comparing(num -> numToOrder.get(num)));
System.out.println(nums);

Original (wrong) answer

(nums is modified in place, and the lambda returning key returns wrong results)

List<Double> nums = Arrays.asList(5.0, 0.9, 10.4);
List<Double> order = Arrays.asList(3.0, 1.0, 2.0);
nums.sort(Comparator.comparing(num -> order.get(nums.indexOf(num))));
System.out.println(nums);