Inherit generic type and force the <type>

Question

Is it possible to inherit generic type and to force in the child class the type received?

Something like:

class A<GenericType>{}
class B extends A<GenericType>{}

Or:

class B <PreciseType> extends A <GenericType>{}

But where do I define the GenericType used in B?

Answer 1

Given

class A<T> {}

It depends on what you try to do, but both options are possible:

class B extends A<SomeType> {};
B bar = new B();
A<SomeType> foo = bar; //this is ok

and

class B<T> extends A<T>{}; //you could use a name different than T here if you want
B<SomeType> bar = new B<SomeType>();
A<SomeType> foo = bar; //this is ok too

But keep in mind that in the first case SomeType is an actual class (like String) and in the second case T is a generic type argument that needs to be instantiated when you declare/create objects of type B.

As a piece of advice: using generics in collections is easy and straightforward, but if you want to create your own generic classes you really need to understand them properly. There are a few important gotchas about their variance properties, so read the tutorial carefully and many times to master them.