Return generic functional interface in Java 8

Question

I want to write kind of function factory. It should be a function, which is called once which different strategies as parameters. It should return a function, which selects one of this strategies dependent on the parameter, which is to be fulfilled by a predicate. Well, better look at condition3 for better understanding. The problem is, that it is not compiling. I think because the compiler can't figure out, that the functional interface H can be realized by the implementation. Without generics it is working fine.

@FunctionalInterface
public interface Finder<T, S> {

    Stream<S> findBest (T t);

    // Valid code
    static <P, Q> Finder<P, Q> condition1 () {
        return p -> null;
    }

    // Valid code, but selects just one of the H's when the method is invoked
    static <P, Q, H extends Finder<P, Q>> H condition2 (Pair<Predicate<P>, H>... hs) {
        return hs[0].getRight ();
    }

    // Should return a method, which selects the appropiate H 
    // whenever it is invoked with an P
    // Compiler complain: 
    // The target type of this expression must be a functional interface
    static <P, Q, H extends Finder<P, Q>> H condition3 (Pair<Predicate<P>, H>... hs) {
        return p -> stream (hs).filter (pair -> pair.getLeft ().test (p))
                               .findFirst ()
                               .map (Pair::getRight)
                               .map (h -> h.findBest (p))
                               .orElseGet (Stream::empty);
    }
}

So what's the problem here? Can I solve it and if it's possible with Java: how?

Answer 1

Look at the signature of your method and try to tell what the exact return type is:

static <P, Q, H extends Finder<P, Q>> H condition3(…

Lambdas can only implement an interface known at compile-time. But the actual type argument for H is not known to the compiler.

Your first method works because it returns the type Finder<P, Q> which the lambda can implement, your second works because it doesn't use a lambda for implementing the return type H extends Finder<P, Q> .

Only your third method attempts to specify a lambda expression for a type argument H extends Finder<P, Q> .

---

A solution is not to give the caller the freedom to mandate a particular sub-type of Finder as the method's return type:

static <P, Q, H extends Finder<P, Q>>
    Finder<P, Q> condition3(Pair<Predicate<P>, H>... hs) {

---

To illustrate what implications your original method signature has, look at the following example:

final class HImpl implements Finder<String,String> {
    public Stream<String> findBest(String t) {
        return null; // just for illustration, we never really use the class
    }
}

…

HImpl x=Finder.<String,String,HImpl>condition3();

Given your original method signature this compiles without any error. But how ought the method condition3 provide an instance of HImpl here using your lambda expression?