What is the difference between A 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 and A
         


        

Answer 1

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

A is a part of generic type declaration such as

public class A { ... }

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 is a parameterized type with wildcard, it can be used in variable and method declarations, etc, as a normal type:

A a = ...;

public void foo(A a) { ... }

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

For example, given this declaration

List l;

you can:

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

  l = new ArrayList(); 
  

• 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