Why generic type is not applicable for argument extends super class for both?

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You can create a List<T> list = new ArrayList<T>(); directly, this can allow all subtypes of T into the list. This is actually little difficult to understand. when you declare it as 

List<? extends T> list = ...

It means that it can allow any unknown subtypes of T into the list. But, from that declaration we cannot ensure which is the exact sub-type of T. so, we can only add null into it

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