Question

Assume class B extends class A and I want to declare a variable for B. What is more efficient and why?

1. B b
   OR
2. A<B> b.

Answer 1

You are confusing two different concepts.

class B extends A {

}

means that B is an A.

If you have something like A<B> it means that you class A is defined as

class A<T> {

}

meaning that you class A is a generic class.

For example (over simplified) you have

class List<T> {

}

So if T takes the value String you would have List<String> meaning a list of Strings

So A<B> does not mean that B extends A.

You should use B b.