CODEWITHSUNDEEP
javagenericstypeswildcard
scriptkiddie
scriptkiddie

Reputation: 41

Wildcards for Nested Types

I'm trying to understand why the type erasure of this method doesn't work with wildcards.

public boolean foo(TypeA<TypeB<?>, TypeB<?>> bar) {
    return true;
}

// instantiate an object
TypeA<TypeB<TypeC>, TypeB<TypeC>> somethingOtherThanFooBar = ...;

// TypeA<TypeB<?>, TypeB<?>> cannot be applied to TypeA<TypeB<TypeC>, TypeB<TypeC>>
foo(somethingOtherThanFooBar);

I know it will work if the method signature is pulic <T, U> boolean foo(TypeA<TypeB<T>, TypeB<U>> bar) but this doesn't work with wildcards for a reason I'm failing to understand.

Upvotes: 1

Views: 54

Answers (1)

Judge Mental
Judge Mental

Reputation: 5239

The rule is: A is-a B implies G<A> is-a G<? extends B> and G<B> is-a G<? super A>.

So

  • TypeC is-a Object, therefore
  • TypeB<TypeC> is-a TypeB<?>, therefore
  • TypeA<TypeB<TypeC>> is-a TypeA<? extends TypeB<?>>.

There is no type rule that allows you to get from your argument type to your parameter type.

You should check out Scala, which allows you to declare the variance of generic type parameters; if the compiler could know that your generic class would always use the parameter in covariant position, it could allow the conversion you want.

Upvotes: 1

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