Question

I’m starting to learn Java and I have a question about generics.

In this methods from Collection<E> interface:

boolean containsAll( Collection <?> c);
boolean removeAll(Collection<?> c);
boolean retainAll ( Collection <?> c);

Why is the parameter Collection <?> c instead of Collection <E> c?

Thanks a lot

Answer 1

The JDK designers wanted code like the following to be possible:

Collection<String> strings = Arrays.asList("foo", "bar", "baz");
Collection<Object> objects = Arrays.asList("foo", 123);
strings.removeAll(objects);
// strigns now contains only "bar" and "baz"

(The above code might not exactly compile because I can’t remember how Arrays.asList() captures type parameters, but it should get the point across.)

That is, because you can call .equals() on any pair of objects and get a meaningful result, you don’t really need to restrict those methods to a specific item type.