Bag implementation Add(T item) – Enlarge bag if necessary; allocate larger array Remove(T item) – Reduce bag if necessary; allocate smaller array Iterator – Visit each element of the Bag, in turn

Iterators

Iterator pattern Horstmann book, pages Carrano text, chapter 8 Various websites (here are just some): sourcemaking.com/design_patterns/iterator c2.comc2.com (down at the moment?)

Collection classes Each collection defines its own Iterator. Each collection needs to keep its implementation details private. Each iterator is defined as a separate class (so it can be instantiated independently of the class). Each iterator encapsulates iteration logic for a specific collection. It must know the implementation details in order to serve as the bridge between clients of the collection and the internal workings of that collection. How do we grant access to Iterator classes while denying access to others?

java.util.Iterator javadoc comments public interface Iterator { /** * Returns true if the iteration has more elements. (In other * words, returns true if next would return an element * rather than throwing an exception.) * true if the iterator has more elements. */ boolean hasNext(); /** * Returns the next element in the iteration. * the next element in the iteration. NoSuchElementException iteration has no more elements. */ E next(); /** * * Removes from the underlying collection the last element returned by the * iterator (optional operation). This method can be called only once per * call to next. The behavior of an iterator is unspecified if * the underlying collection is modified while the iteration is in * progress in any way other than by calling this method. * UnsupportedOperationException if the remove * operation is not supported by this Iterator. IllegalStateException if the next method has not * yet been called, or the remove method has already * been called after the last call to the next * method. */ void remove(); }

Implementation for our Bag class

Inner classes Java’s Collection classes – concrete collection class must hide its implementation – must provide Iterator – Iterator must know implementation details – Inner class structure embeds Iterator definition within scope of collection class – Iterator can access internals of collection without collection having to break encapsulation