Java: Wildcard Types Mismatch Results in Compilation Error

Question

I\'ve created a factory class in my project which would allow me (in theory) to create managers for any (supported) given type. Interacting with the manager allows me to alt

Answer 1

Use the following:

IManager> test3Manager =
        Factory.>createManager(test3);

This is just a case of the compiler's type inference falling on its face, so it's necessary explicitly provide the type argument for T.

More technically:

test3 is declared to have the type IAnotherRandomType, where ? is a wildcard capture - a sort of one-use type parameter representing some specific unknown type. That's what the compiler's referring to when it says capture#1-of ?. When you pass test3 into createManager, T gets inferred as IAnotherRandomType.

Meanwhile, test3Manager is declared to have the type IManager>, which has a nested wildcard - it does not behave like a type parameter but rather represents any type.

Since generics aren't covariant, the compiler can't convert from IManager> to IManager>.

More reading on nested wildcards:

• Multiple wildcards on a generic methods makes Java compiler (and me!) very confused
• Java Generic List>
• Which super-subset relationships exist among wildcards?