for (CapabilityClass17 cap : getAdapter (CapabilityClass17.class)) {
// do something with the extended API
}

Ironically, if you think your class had to behave like a list, and you add that adapter later, you could actually get some kind of real looking for loop :)

for (Object s : getAdapter (List.class)) {
// Booya :)
}

As for the type of the iterator, maybe super type tokens can help.

I’ve read the GoF Book 15 years ago, taught them to lots of students and used them in lots of projects. I think the Null object (though not from the GoF book but from Fowler several years later) is used not often enough. Recently I introduced NullId to a project (with a nice isValid() method) with very good results. More people should do that.

“Search for design partners or GoF.”

Search for Refactoring and Fowler for the Null object pattern.

Nonetheless Option/Maybe is something different.

if (x != null && y != null) {
return x y;
}

…and much more readable.

Perhaps you should use try to some patterns that are better fit
for Java like Visitor mentioned by James or Null object. Search
for design partners or GoF.

Don’t get me wrong, monads are cool but can’t be expressed
in Java at this time. Perhaps if Java7 incorporates closure
proposal, we’ll be able to express something like:

withObjectsThatArentNull(x, y) {
return x y;
}

The double loop is a nice construct. Hats off for thinking of that and the anology to Scala and Haskell with for and do. Especially “for” :-)