Creating generic lambdas in Java

Question

In java, you can add Type parameters to static methods, to create methods that handle generics. Can you do the same with lambdas?

In my code I have

final private static <K,V> Supplier<Map<K, List<V>> supplier=HashMap::new;

I'm trying to do type parameters like it's a function, but it won't let me.

And if I do:

    final private static Supplier<Map<?, List<?>>> supplier=HashMap::new;

It doesn't accept the argument where I try to use it. What can I do?

Answer 1

One workaround for this may be to wrap the method reference into a method, so that target type deduction resolves the type at the call site:

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Supplier;

public class GenericLambda
{
    // Syntactically invalid
    //final private static <K,V> Supplier<Map<K, List<V>> supplier=HashMap::new;

    final private static Supplier<Map<?, List<?>>> supplier=HashMap::new;

    // A workaround
    private static <K,V> Supplier<Map<K, List<V>>> supplier()
    {
        return HashMap::new;
    }


    public static void main(String[] args)
    {
        // Does not work
        //useSupplier(supplier);

        // Works
        useSupplier(supplier());
    }

    private static <K, V> void useSupplier(Supplier<Map<K, List<V>>> s)
    {
        System.out.println(s.get());
    }
}