On Sunday 06 October 2002 09:57 pm, Michael G Schwern wrote: > On Sun, Oct 06, 2002 at 01:49:26AM -0400, Noah White wrote: > > >OTOH, Java interfaces have a loophole which is considered a design > > >mistake. > > >An interface can declare some parts of the interface optional and then > > >implementors can decide if they want to implement it or not. The > > >upshot > > >being that if you use a subclass in Java you can't rely on the optional > > >parts being there. > > > > Say what?!? I don't think so. If a JAVA class which implements an > > interface does not declare all of the methods of the interface then > > that class will be abstract and you won't be able to instantiate a > > concrete instance of it. You are guaranteed that any concrete instance > > of a class which implements an interface will contain ALL methods > > defined by the interface. >
[snip] > > It is possible in Java to declare methods to be an "optional operation". > This means the subclass may or may not implement that method and if you try > to use an unimplemented method an UnsupportedOperationException is thrown. > > Effectively, this means that for every optional method you want to use, > you've got to wrap it in a try block to catch the > UnsupportedOperationException, recover and try to do something else if it's > not there. Makes using an optional operation complicated and error prone. To be clear, UnsupportedOperationException is a subclass of RuntimeException, and therefore one is not required to wrap a try around methods that throw it. > > In effect, an optional interface is declaring "this method may or may not > exist" which ain't that useful. > > I can't find the syntax or this or when it was introduced, might be 1.2, > but I see it mentioned in the Collection API and tutorial docs: > > http://java.sun.com/docs/books/tutorial/collections/interfaces/collection.h >tml > > The Collection interface is shown below: > public interface Collection { > // Basic Operations > int size(); > boolean isEmpty(); > boolean contains(Object element); > boolean add(Object element); // Optional > boolean remove(Object element); // Optional > Iterator iterator(); > I think there may be some confusion here. In java, there's no special syntax to declare a method an optional part of the interface. All concrete classes that implement the Collection interface still must define full-bodied C<add(Object element)> methods. It just so happens that by convention some classes simply throw an UnsupportedOperationException, perhaps like so: public class MyStaticCollection implements Collection { ... public boolean add (Object element) { throw new UnsupportedOperationException("add not supported"); } ... } This is really no different than the following from Tie::Handle: sub READ { my $pkg = ref $_[0]; croak "$pkg doesn't define a READ method"; } No special syntax, just convention enforced by the method implementations. - Dan Boorstein
