Java: select from collection only elements of provided type

Question

Hi!

I have a collection of elements of type B and C, which all extends A. I need to filter the collection to get elements of only B type.

Is there any way to do it except:

for (A a : initCollection) {
  if (a instanceof B) {
    newCollection.add(a)?
  }
}

Thanks

Answer 1

AFAIK, there is no plain way to do it without adding code to classes A, B, and C. The real question is: Why do you need to filter out elements of a specific type? It goes against the very idea of sub-typing in OOP.

Answer 2

I don't see anything wrong with doing it as per your example. However, if you want to get fancy you could use Google's Guava library, specifically the Collections2 class, to do a functional-style filtering on your collection. You get to provide your own predicate, which can of course do the instanceof thing.

Collection<Object> newCollection = Collections2.filter(initCollection, new Predicate<Object>() { 
    public boolean apply(Object o) {
        return !(o instanceof String);
    } 
});

Answer 3

If you don't want to create new collection you can remove odd elements from existing one:

Iterator<A> it = initCollection.iterator();
while (it.hasNext()){
   if (it.next() instanceof C) {
      it.remove();
   }
}

Answer 4

Guava was alluded to in other answers, but not the specific solution, which is even simpler than people realize:

Iterable<B> onlyBs = Iterables.filter(initCollection, B.class);

It's simple and clean, does the right thing, only creates a single instance and copies nothing, and doesn't cause any warnings.

(The Collections2.filter() method does not have this particular overload, though, so if you really want a Collection, you'll have to provide Predicates.instanceOf(B.class) and the resulting collection will still sadly be of type Collection<A>.)