Question

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Is it possible to reference a nested generic parameter in java?

A quick question on Java generics if I may. Is there syntax for declaring a generic class-wide type that is NOT used as a generic parameter for instantiation. For example:

public class <U extends FooUType> BarType<T extends FooType<U>>{

   public U getU(){return U;}
}

To create a BarType I want to write the following, which in itself contains U, but I don’t want to have to specify U separately. So:

BarType<SomeT<FooUType>> instance

We get the type of U implicitly from the parameterized SomeT without having to specify U separately. As opposed to :

public class BarType<U extends FooUType, T extends FooType<U>> 

which would require:

BarType<FooUType,SomeT<FooUType>>

I guess I’m looking for something akin to the same idea in methods:

public <U> boolean StupidMethod(String s){
    ...
}

I would rather not use <? extends FooUType> as this leads to problems with method return types inside the class that return <U extends FooUType> U.

Many thanks for the clarification!

Answer 1

Thanks all,

I went for a different solution in the end. Basically the class I was trying to hide the U in was abstract and so I added U as a generic parameter as there is no way round this. I then created the concrete classes that extend this and had them fill in the type of U silently so that the caller wouldn’t need to bloat their code.

Something like this:

public abstract class MyAbsClass<U extends Foo, T extends Bar<U>>{...}

public class ConcreteClass<T extends SomeBar> extends MyAbsClass<SilentUType, T>

where SomeBar is not parameterized. The user can then just instantiate with T:

 new ConcreteClass<SomeDerivedBar>()

This worked for my scenario neatly, so I hope it helps others.