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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?

 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(order::indexOf));
 System.out.println(nums);

 OUTPUT: [5.0, 0.9, 10.4]

It should be [0.9, 10.4, 5.0] (according to order). What am I not doing right?

EDIT: As most of you noticed, I got answer to the question I linked to all wrong. Here's what I actually want to do.

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.

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 

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);

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.