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?

    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.

Just failed an audit on this... I don't get how the EDIT part is acceptable. — Almo, Jun 04 '19 at 16:03

Michael · Accepted Answer · 2019-05-16T16:46:19.693

16

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.

edited May 16 '19 at 16:46

answered May 16 '19 at 09:53

Michael

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Oh, so I got it all wrong.. how then do I sort `nums` by `order`? – shakedzy May 16 '19 at 09:56
@Michael I believe using `order` as the keys for sorting. – justhalf May 16 '19 at 14:55
@shakedzy I'd like to add that the comparator does not fulfil the `Comparator` [specification](https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#compare-T-T-). That implies that not only is the sort order unspecified, but sort methods might throw exception. `ArrayList` for example [might throw](https://hg.openjdk.java.net/jdk/jdk/file/830ca7b43b95/src/java.base/share/classes/java/util/TimSort.java#l288) `AssertionError`. – Lii Jun 06 '19 at 16:13
Just for fun I played with `ArrayList` and different flawed comparators. I never get an exception with comparators that always return -1, and I never get `AssertionError`s. With comparators that return random integers I often get [`IllegalArgumentException`](https://hg.openjdk.java.net/jdk/jdk/file/830ca7b43b95/src/java.base/share/classes/java/util/TimSort.java#l781) however. (Of course, all this is unspecified and implementation dependent, so it might vary between JRE:s and releases.) – Lii Jun 07 '19 at 08:49

score 4 · Answer 2 · answered May 16 '19 at 10:05

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

Lesiak · Answer 3 · 2019-05-16T10:19:13.927

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);

score 1 · Answer 4 · answered May 16 '19 at 09:53

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.

Java Comparator.comparing not comparing?

4 Answers4

Linked