Difference between Enumeration<? extends ZipEntry> and Enumeration?

Question

Is there a difference between Enumeration and Enumeration? If so, what is the difference?

Answer 1

There's no practical difference in terms of what you can do when you've got one of them, because the type parameter is only used in an "output" position. On the other hand, there's a big difference in terms of what you can use as one of them.

Suppose you had an Enumeration - you couldn't pass this to a method which took Enumeration as one of its arguments. You could pass it to a method taking Enumeration though.

It's more interesting when you've got a type which uses the type parameter in both input and output positions - List being the most obvious example. Here are three examples of methods with variations on a parameter. In each case we'll try to get an item from the list, and add another one.

// Very strict - only a genuine List will do
public void Foo(List list)
{
    T element = list.get(0); // Valid
    list.add(element); // Valid
}

// Lax in one way: allows any List that's a List of a type
// derived from T.
public void Foo(List list)
{
    T element = list.get(0); // Valid
     // Invalid - this could be a list of a different type.
     // We don't want to add an Object to a List
    list.add(element);   
}

// Lax in the other way: allows any List that's a List of a type
// upwards in T's inheritance hierarchy
public void Foo(List list)
{
    // Invalid - we could be asking a List