Java 8 Comparator comparing doesn't chain

Question

Let\'s say I have a Pair class

public class Pair {
    public P p;
    public Q q;


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


        

Answer 1

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.

Answer 2

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));
//                                        ^^^^^