Java wildcard inheritance

Can you tell which ArrayList<?>

is the superclass ArrayList<Integer>

?

Not.

ArrayList<?>

is the same class as ArrayList<Integer>

. There is only one declaration for ArrayList<?>

and ArrayList<Integer>

, but these are different types:

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{

      

        
        
        
      

    

<?>

means that the generic type of the parameter can be any type.

Is the above example therefore displaying polymorphism?

I think yes.

What you are basically doing is assigning an instance of a type to a variable of a different type. These are "morphs".

Then it's fair to say what A<?>

is super class B<Integer>

?

Yes. This can be shown with the following code:

// BClass extends AClass
AClass<?> a = new AClass<String>();
BClass<Integer> b = new BClass<Integer>();
// prints true
System.out.println(b.getClass().getSuperclass().equals(a.getClass()));

      

        
        
        
      

    

This is because the type erasure occurs at runtime, so generic types are not displayed. Adoption A<T>

is superclass B<T>

means that A<Anything>

is a superclass B<Anything>

.