Java generic collection, cannot add list to list

Question

Why does the following

public class ListBox {
    private Random random = new Random();
    private List> box;

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Answer 1

You've declared box to be a List of something that extends Collection of Object. But according to the Java compiler, it could be anything that extends Collection, i.e. List<Vector<Object>>. So it must disallow add operations that take the generic type parameter for this reason. It can't let you add an ArrayList<Object> to a List that could be List<Vector<Object>>.

Try removing the wildcard:

private List<Collection<Object>> box;

This should work because you can certainly add an ArrayList<Object> to a List of Collection<Object>.