Why do we need "? extends" in Java

There is a good free tutorial on generics: https://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf. It explains the design of generics, and tells a bit about why Sun engineers came up with this implementation.

Wow, I'm so glad that I don't need to worry about such a mess when using duck-typing in Python.

Why do we need Java?

We had to learn it when doing an oo programming course at university. And I immediately forgot all about it.

I hope when writing Java code sometimes at a job, I can refer to this again.

This is all done in the name of type safe programming (I don't know the exact english terminology).

is called covariance. It helps to access attributes of B and derived classes with get() but prevents to use set().

ArrayList list;

list = new ArrayList();

list = new ArrayList();

list = new ArrayList();

list.set(index, myInteger); // throws error

Compiler has to know the exact type to set the value, but Long, Double and Integer are all from type Number, so you can read it into a Number variable.

Is that correct?