Creating Generics Instances: New ArrayList <?> () Vs new ArrayList <List <? >> ()

Question

Why is this allowed:

List<List<?>> list = new ArrayList<List<?>>()

but not that?

List<?> list = new ArrayList<?>(); //Compile error: "Cannot instantiate the type ArrayList<?>"

Answer 1

When you create an ArrayList, you need to determine the type of objects it will contain:

• in the first example, you create an ArrayList that will contain lists of unknown generic types, but they will all be lists.
• in the second example, you are trying to create an ArrayList of an unknown type that is not allowed.

Answer 2

You cannot create an instance of an object with an unknown common, so the second line does not work.

List<?> list = new ArrayList<String>();

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Answer 3

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:

CharSequence val = new String("foo");

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new ArrayList<List<?>>()

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(1),

List<?> list

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