CODEWITHSUNDEEP
genericslambdajava-8type-inferencemethod-reference
marsouf
marsouf

Reputation: 1147

Java 8 [cannot infer type variables] problem

I have a ThrowingFunction<T, R, E extends Exception> functional interface with a single abstract method having this signature : R apply(T t) throws E.

I have a static utility method inside the functional interface that permits users converting ThrowingFunction<T, R, E extends Exception> instances into Function<T, R> instances :

static <T, R> Function<T, R> unchecked (final ThrowingFunction<T, R, ?> tf)
{
    // code here
}

The problem here is why the type parameters T and R don't get inferred by the compiler if I pass the method reference URL::new into the unchecked static utility method ?

final URL url = unchecked(URL::new).apply("http://www.examples.com");

I mean, isn't URL::new a valid condidate for ThrowingFunction<String, URL, MalformedURLException> and therefore T and R should be inferred as String and URL respectively ?

Upvotes: 4

Views: 3348

Answers (2)

luk2302
luk2302

Reputation: 57114

I do not know what the issue is but the answer is yes, it is a valid candidate and it works if you split it into two statements:

ThrowingFunction<String, URL, MalformedURLException> ctor = URL::new;
final URL url = unchecked(ctor).apply("http://www.examples.com");

Upvotes: 2

Eugene
Eugene

Reputation: 120848

You are right, it should work, it most probably has to do with the fact that you are using a method reference and the compiler is somehow getting confused, there are two solutions:

// a lambda
ThrowingFunction.unchecked((String x) -> new URL(x));


// an explicit cast
ThrowingFunction.unchecked((ThrowingFunction<String, URL, MalformedURLException>)URL::new);

Upvotes: 3

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