Question

If

    new ArrayList<?>();  // is not Legal

And if

    new ArrayList<? extends Number>();   // is not Legal,

Then why is

    new ArrayList<Set<?>>();    // IS Legal

Can someone explain with example.

Answer 1

This happens, because in your concrete type declaration you’re using interface as generic parameter. Think about how it would be used in the future:

List someList = new ArrayList<Set<?>>();    
Set<?> someSet = new HashSet<Integer>();
someList.add(someSet);

In first line – you’re defining a list of ArrayList type, that will be holding ‘some set’ (of any type). Then, in second line you’re declaring and defining set of concrete type HashSet. It is casted into Set<?> so there is no problem with adding it to previously created list.