Be gone ye foul smelling custom type-safe collections!

I used to use STL type-safe collections in C++ all the time and when I moved to using generic collections in Java something just felt wrong. How was I to ensure my collections contained the correct types?

But you know what? Much to my surprise over the years, I’ve never really had a problem. I can’t even remember the last time (if ever) I had a class-cast exception from accidentally adding the wrong type of object to a collection.

Which one of these would you rather?

The generic collections approach:

Collection customers = ...;
for (Iterator i = customers.iterator(); i.hasNext(); ) {
  doSomething((Customer) i.next());
}

Versus the custom type-safe collection:

public class CustomerCollection extends AbstractCollection {
  private final Collection _customers = new HashSet();
  public boolean add(Object object) {
    return addCustomer((Customer) object);
  }

  public boolean addCustomer(Customer customer) {
    return _customers.add(customer);
  }

  public int size() {
    return _customers.size();
  }

  public Iterator iterator() {
    return customerIterator();
  }

  public CustomerIterator customerIterator() {
    return new CustomerIterator(_customers.iterator());
  }

  public final class CustomerIterator implements Iterator {
    private final Iterator _iterator;
    private CustomerIterator(Iterator iterator) {
      _iterator = iterator;
    }

    public void remove() {
      _iterator.remove();
    }

    public boolean hasNext() {
      return _iterator.hasNext();
    }

    public Object next() {
      return nextCustomer();
    }

    public Customer nextCustomer() {
      return (Customer) _iterator.next();
    }
  }
}

CustomerCollection customers = ...;
for (CustomerIterator i = customers.customerIterator(); i.hasNext(); ) {
  doSomething(i.nextCustomer());
}

Not so bad you say? Well try doing this for every type for which you need to hold collections. Then try implement List or Set instead of the basic Collection. Don’t even think about what it takes to implement Map. I admit, you might end up re-factoring some of this into a generic base class. You could even write one without implementing any of the collection interfaces. But then, why bother?

Sure, they might have save me a few keystrokes here and there but because most people (as I have done in my example) create their type-safe collections with names like getCustomer(), etc. it saves me 2 key-strokes over using a simple cast like (Customer). In fact, in the example given, I actually ended up with more key-strokes!

“But it’s not about keystrokes, it’s about type-safety” I hear you exclaim. Well yes, you may be right. But as I mentioned earlier, I don’t recall this ever being a problem on any projects I’ve worked on. Oh, how that I think about it, there may have been one project but I’m pretty sure we simply shot the offending developer for being such an imbecile ;-)

I’m currently enduring the pain of working on a project where the consensus was that we should have type-safe collections. I seem to spend half my day implementing these instead of letting IntelliJ and Eclipse do a fine job of adding the casts in for me.

And so it is that I eagerly await the release of JDK 1.5 and language supported type-safe collections in the hope that it’ll once and for all stop people writing these useless custom implementations. Egads!