CODEWITHSUNDEEP
javagenericstype-inferencecontravariance
miedzinski
miedzinski

Reputation: 113

Type mismatch in nested contravariant type

Given such class (part of it)

import java.util.*;
import java.util.stream.Collectors;

public class A {
    private Map<String, Set<String>> map = new LinkedHashMap<>();

    public Map<String, Collection<String>> getMap() {
        return Collections.unmodifiableMap(map);
    }

    public static <K, V> Map<K, V> sorted(Map<K, V> map, Comparator<Map.Entry<? super K, ? super V>> comparator) {
        return map
                .entrySet()
                .stream()
                .sorted(comparator)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    }

    public Map<String, Collection<String>> getSortedMap() {
        Comparator<Map.Entry<String, Collection<String>>> cmp =
                Map.Entry.comparingByValue(Comparator.comparingInt(Collection::size));
        return sorted(getMap(), cmp);
    }
}

I am getting an error on compile

error: method sorted in class A cannot be applied to given types;
    return sorted(getMap(), cmp);
            ^
            required: Map<K,V>,Comparator<Entry<? super K,? super V>>
        found: Map<String,Collection<String>>,Comparator<Entry<String,Collection<String>>>
        reason: cannot infer type-variable(s) K,V
        (argument mismatch; Comparator<Entry<String,Collection<String>>> cannot be converted to Comparator<Entry<? super K,? super V>>)
        where K,V are type-variables:
        K extends Object declared in method <K,V>sorted(Map<K,V>,Comparator<Entry<? super K,? super V>>)
        V extends Object declared in method <K,V>sorted(Map<K,V>,Comparator<Entry<? super K,? super V>>)
        1 error

When I change A.sorted signature to be invariant over comparator parameter i.e. <K, V> Map<K, V> sorted(Map<K, V> map, Comparator<Map.Entry<K, V>> comparator) it compiles without any issues. However, I don't think my code violates any typing relations. Is this a problem with Java's type inference?

I am using OpenJDK 8.

Upvotes: 2

Views: 207

Answers (1)

Jorn Vernee
Jorn Vernee

Reputation: 33885

Generics are invariant. If you declare a parameter Comparater<T> then it expects exactly Comparator<T>. In this case case you have K and V trough the map which makes K = String and V = Collection<String>. That would make the second parameter Comparator<Entry<? super String, ? super Collection<String>>>, and you can not assign a Comparator<Entry<String,Collection<String>>> to that, since the type argument to the comparator does not exactly match.

You could declare the Comparator contra-variant instead:

public static <K, V> Map<K, V> sorted(Map<K, V> map,
        Comparator<? super Map.Entry<K, V>> comparator) {
    ...
}

Upvotes: 2

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