migrating java classes to use generics [duplicate]

Question

I have problem to use generics in one of my class. These are my classes:

public interface InterE<PK> {}

public interface Inter2<O extends ClassO, P extends ClassP> {}

public class ClassU<O extends ClassO, P extends ClassP> implements InterE<Long> {}

public class ClassP {}

public class ClassO {}

public class Class1<PK, T extends InterE<PK>> {
    public Class1 (Class<T> clazz) {}
}

public class Class2<O extends ClassO, P extends ClassP> extends Class1<Long, ClassU<O, P>> implements Inter2<O, P> {
    public Class2 () {
        //old constructor
        //super(ClassU.class);
        //new - here is problem
        super(ClassU<O, P>.class);
    }
}

If ClassU does not use generics, then old constructor in Class2 works fine, but I need to use generics in ClassU, so I have problem to call super constructor in Class2.

Answer 1

Short story: change Class1 constructor parameter type - from Class<T> to Class<?>.

Why?

Class1 only needs T to extend InterE. Class2 declares ClassU to be this T argument. Since ClassU implements InterE, everything is OK for Class1.

Do you really need this <T> information in Class1 constructor? I think no:
1) T is already present in class declaration: Class1<PK, T extends InterE<PK>>, you can use it inside the class to generify code and make it compile-time safe:

 public class Class1<PK, T extends InterE<PK>> {
     Map<PK, T> map = new HashMap<>(); // can still use T, compile-time safe
 }

2) What can Class<T> do, that Class<?> can not do? - it can create newInstance() safe and do some more stuff, but you most likely won't use this possibility. Use still can operate with T, for example call T instance = (T) class.newInstance();, so you will not have any insuperable limitations.