CODEWITHSUNDEEP
javagenerics
user118967
user118967

Reputation: 5722

Extension of generic Java class does not recognize super constructor

Can someone please help me understand why this Java code doesn't compile?

The idea is that A is a tree-like class for a type T that takes a collection of children at construction.

Then I define an extension B of A that specializes T for Pair<R,R>.

class A<T> {
    A(T t, Collection<? extends A<? extends T>> cOfAs) {

    }
}

class B<R> extends A<Pair<R,R>> { // Pair is just a POJO class
    B(Pair<R,R> pair, Collection<? extends B<? extends R>> cOfBs) {
        super(pair, cOfBs);
        // ERROR: The constructor A<Pair<R,R>>(Pair<R,R>, Collection<capture#1-of ? extends B<? extends R>>) is undefined
    }
}

Upvotes: 1

Views: 46

Answers (2)

rgettman
rgettman

Reputation: 178263

The type of the second parameter doesn't match. The B constructor takes Collection<? extends B<? extends R>>, but the superclass constructor takes a Collection<? extends A<? extends T>>.

You've defined T to be Pair<R, R> in your B class, so instead of R, use Pair<R, R>. Also the ? extends part must match, so change ? extends B<...> to ? extends A<...>. The signature of B's constructor now looks like this:

B(Pair<R,R> pair, Collection<? extends A<? extends Pair<R, R>>> cOfBs)

It must be ? extends A exactly, because ? extends B may not match ? extends A. The only way to get this to match is to introduce another type parameter in A representing the "self" type, and to use it in the second parameter of the constructor.

class A<T, S extends A<T, S>> {
    A(T t, Collection<? extends S> cOfAs) {

    }
}

Then in B, supply B<R> as "self".

class B<R> extends A<Pair<R,R>, B<R>> { // Pair is just a POJO class
    B(Pair<R,R> pair, Collection<? extends B<R>> cOfBs) {
        super(pair, cOfBs);
    }
}

Upvotes: 2

iwek
iwek

Reputation: 367

The type of second argument of constructor is wrong:

Error:(20, 25) java: incompatible types: java.util.Collection<capture#1 of ? extends Main.B<? extends R>> cannot be converted to java.util.Collection<? extends Main.A<? extends javafx.util.Pair<R,R>>>

To fix it change second argument to Collection<? extends B<R>> cOfBs

Upvotes: 1

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