Returning a list of objects

Please review the following code. Methods getFirsts and getSeconds, both of which are private, return a list of objects which implement CommonInterface. Is this a good or bad design?

@Override
public final List getObjects(final CommonEnum type) {
    if (type == null) {
        return new ArrayList();
    }
    switch (type) {
        case FIRST:
            return getFirsts();
        case SECOND:
            return getSeconds();
        default:
            return null;
    }
}

It's hard to say anything since it seems rather a pseudo-code. Anyway, two notes which you might find useful:

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I guess you could replace the switch-case structure with polymorphism. Two useful reading:

• Refactoring: Improving the Design of Existing Code by Martin Fowler: Replacing the Conditional Logic on Price Code with Polymorphism

• Replace Conditional with Polymorphism

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Are you sure that returning null in the default is fine? I'd consider returning an empty list (as it returns when type == null) or throwing an exception (IllegalStateException, for example) if it's a programming error. (See: The Pragmatic Programmer: From Journeyman to Master by Andrew Hunt and David Thomas: Dead Programs Tell No Lies)

StackExchange Code Review Q#17734, answer score: 7