Java 8 Comparator comparing doesn't chain

Question

Let's say I have a Pair class

public class Pair<P, Q> {
    public P p;
    public Q q;


    public Pair(P p, Q q) {
        this.p = p;
        this.q = q;
    }

    public int firstValue() {
        return ((Number)p).intValue();
    }

    public int secondValue() {
        return ((Number)q).intValue();
    }
}

And I wish to sort it, first by first value, then by second value. Now' if I do this

List<Pair<Integer, Integer>> pairList = new ArrayList<>();
pairList.add(new Pair<>(1, 5));
pairList.add(new Pair<>(2, 2));
pairList.add(new Pair<>(2, 22));
pairList.add(new Pair<>(1, 22));
pairList.sort(Comparator.comparing(Pair::firstValue));

Everything works well and good, the list is sorted by first values of pair, but if I do this

pairList.sort(Comparator.comparing(Pair::firstValue).thenComparing(Pair::secondValue));

It fails with error

Error:(24, 38) java: incompatible types: cannot infer type-variable(s) T,U
(argument mismatch; invalid method reference
  method firstValue in class DataStructures.Pair<P,Q> cannot be applied to given types
    required: no arguments
    found: java.lang.Object
    reason: actual and formal argument lists differ in length)

Ok,so it might not be able to infer the arguments, so if I do this

pairList.sort(Comparator.<Integer, Integer>comparing(Pair::firstValue)
                                          .thenComparing(Pair::secondValue));

It fails with error

Error:(24, 39) java: invalid method reference
non-static method firstValue() cannot be referenced from a static context

Why does it work for comparing() and not for comparing().thenComparing() ?

@shmosel wow that worked, do you mind adding it as an answer with a reason why it worked! Thanks! — Mayur Kulkarni, Feb 09 '17 at 03:36
I suspect you would have fewer problems if you used [Comparator.comparingInt](https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparingInt-java.util.function.ToIntFunction-). — VGR, Feb 09 '17 at 03:51
@VGR Suprisingly not. Type inference with chained comparators is an [old problem](http://stackoverflow.com/q/24436871/1553851), but can usually be avoided with method references. But there's something about the generic parameters in `Pair` that's causing problems here. — shmosel, Feb 09 '17 at 03:55
@MayurKulkarni It's still the better option since it avoids boxing. — shmosel, Feb 09 '17 at 04:10

score 7 · Accepted Answer · answered Feb 09 '17 at 04:12

The error seems to be related to Pair's generic parameters. One workaround it to use an explicit type, as you've attempted:

pairList.sort(Comparator.<Pair>comparingInt(Pair::firstValue).thenComparingInt(Pair::secondValue));
//                       ^^^^^^

Note the comparingInt() which reduces the number of parameters you need to specify, and improves performance by avoiding boxing.

Another solution is to parameterize the type reference:

pairList.sort(Comparator.comparingInt(Pair<?,?>::firstValue).thenComparingInt(Pair::secondValue));
//                                        ^^^^^

score 4 · Answer 2 · answered Feb 09 '17 at 03:37

It should be:

pairList.sort(Comparator.<Pair, Integer>comparing(Pair::firstValue)
                                       .thenComparing(Pair::secondValue));

First type parameter refers to the type being passed to Comparator. Second type parameter refers to the type that comparator should effectively compare with.

Java 8 Comparator comparing doesn't chain

2 Answers2

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