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

Refer to
http://stackoverflow.com/questions/5211548/what-is-the-difference-between-at-extends-b-and-a-extends-b
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

.

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

.