What is the difference between A<T extends B> and A<? extends B>?

Question

I am a new java learner. Recently I was reading Generic programming and got my self confused with this...

A<T extends B> and A<? extends B>

Answer 1

First of all, those are completely different constructs used in different contexts.

A<T extends B> is a part of generic type declaration such as

public class A<T extends B> { ... }

It declares generic type A with type parameter T, and introduces a bound on T, so that T must be a subtype of B.

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A<? extends B> is a parameterized type with wildcard, it can be used in variable and method declarations, etc, as a normal type:

A<? extends B> a = ...;

public void foo(A<? extends B> a) { ... }

Variable declaration such as A<? extends B> a means that type of a is A parameterized with some subtype of B.

For example, given this declaration

List<? extends Number> l;

you can:

• Assign a List of some subtype of Number to l:

  l = new ArrayList<Integer>(); 
  

• Get an object of type Number from that list:

  Number n = l.get(0);
  

However, you can't put anything into list l since you don't know actual type parameter of the list:

Double d = ...;
l.add(d); // Won't compile

Answer 2

? is what is called a wildcard. It means anything that extends B.

When you write List<T extends String>, your List can contains only elements of type T.

When you write List<? extends String>, your List can contains any element that extends String

I hope I'm clear, since these concepts are complicated.