Java type errors can be nasty

error

> javac -Xlint:unchecked Transpose.java 
Transpose.java:8: error: incompatible types: inference variable R has incompatible bounds
        return map.entrySet().stream().collect(toMap(e -> e.getValue(), e -> new HashSet<>(Set.of(e.getKey())), (x, y) -> {x.addAll(y); return x;}));
                                              ^
    equality constraints: Map<K,U>
    upper bounds: HashMap<B,HashSet<A#2>>,Object
  where R,A#1,T#1,K,T#2,U,B,A#2 are type-variables:
    R extends Object declared in method <R,A#1>collect(Collector<? super T#1,A#1,R>)
    A#1 extends Object declared in method <R,A#1>collect(Collector<? super T#1,A#1,R>)
    T#1 extends Object declared in interface Stream
    K extends Object declared in method <T#2,K,U>toMap(Function<? super T#2,? extends K>,Function<? super T#2,? extends U>,BinaryOperator<U>)
    T#2 extends Object declared in method <T#2,K,U>toMap(Function<? super T#2,? extends K>,Function<? super T#2,? extends U>,BinaryOperator<U>)
    U extends Object declared in method <T#2,K,U>toMap(Function<? super T#2,? extends K>,Function<? super T#2,? extends U>,BinaryOperator<U>)
    B extends Object declared in method <A#2,B>transposeMap(HashMap<A#2,B>)
    A#2 extends Object declared in method <A#2,B>transposeMap(HashMap<A#2,B>)
1 error

# This is probably an instance of this bug: https://bugs.openjdk.java.net/browse/JDK-8196762 the proper solution (adding a type witness) is described there and here: https://stackoverflow.com/questions/38089827/collector-doesnt-match-wildcard-in-result-type
# though the wildcard in `toMap` makes it quite puzzling to understand what to fill in. Casting the result of the computation is unfortunately much easier :/

Transpose.java

import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import static java.util.stream.Collectors.toMap;

class Transpose{
    static <A, B> HashMap<B, HashSet<A>> transposeMap(HashMap<A, B> map){
        return map.entrySet().stream().collect(toMap(e -> e.getValue(), e -> new HashSet<>(Set.of(e.getKey())), (x, y) -> {x.addAll(y); return x;}));
    }
}

Questo compila:

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import static java.util.stream.Collectors.toMap;

public class Transpose {
    static <A, B> Map<B, HashSet<A>> transposeMap(HashMap<A, B> map) {
        return map
                .entrySet()
                .stream()
                .collect(toMap(
                        Map.Entry::getValue,
                        e -> new HashSet<>(Set.of(e.getKey())),
                        (x, y) -> {
                            x.addAll(y);
                            return x;
                        }
                ));
    }

    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("one", 1);
        map.put("two", 2);
        map.put("three", 3);
        map.put("other-three", 3);

        System.out.println(transposeMap(map));
    }
}

Output:

{1=[one], 2=[two], 3=[other-three, three]}

Il problema è che toMap è un Collector<T, ?, Map<K,U>> e non un Collector<T, ?, HashMap<K,U>>. Stavi assumendo uno specifico tipo di mappa, cosa che toMap non garantisce in nessun modo. Detto questo, il messaggio di errore è terribile.

Inoltre io mi limiterei a dipendere dalle interfacce Map e Set anziché da specifiche implementazioni. La signature di trasposeMap dovrebbe essere:

static <A, B> Map<B, Set<A>> transposeMap(Map<A, B> map)

👍

As you noticed, a concrete Set implementation is still needed since as you noticed addAll is an optional operation