What is the difference between List and List<? extends Number>?

Question

I have a very basic question ragarding Java generics . I thought that both List and List are

Answer 1

Generics are compile time language features, means, they don't exist in run-time. In generic mechanism, for compile-time checks, they are not homogeneous, i.e. if you want to use polymorphism in generic type.

Following gives you a compile time error, although it seems a valid definition:

 List list = new ArrayList ();

whereas

 List list = new ArrayList ();

is valid. Moreover, you can't use wildcard types on the right side:

 List list = new ArrayList ();

won't be compiled.