1. JCF Collection classes and interfaces
Iterators
CS-2851
Dr. Mark L. Hornick

2. The Java Collections Framework…
Is implemented as a series of hierarchies with:
interfaces at the top
abstract classes in the middle
and concrete (instantiable) classes at the bottom
CS-2851
Dr. Mark L. Hornick

3. A small part of the hierarchy:
CS-2851
Dr. Mark L. Hornick

4. The Collection interface declares methods for inserting, removing and searching for an element in a collection
All Collections support the following:
add(E element)
remove(int index)
contains(E element)
...
CS-2851
Dr. Mark L. Hornick

5. The List interface extends the Collection interface by supplying some index-oriented methods
List-specific behavior that, for example, doesn’t make sense for all Collections
add(int index, E element)
get(int index)
set(int index)
...
Some data structures, like queues, are Collections but not Lists – meaning their elements cannot be accessed by index.
CS-2851
Dr. Mark L. Hornick

6. The Collection interface extends the Iterable interface
The Iterable interface declares a single method: iterator(), which returns an Iterator object
An Iterator is an object that allows a user to loop through a Collection without accessing the elements directly
CS-2851
Dr. Mark L. Hornick

7. Every JCF class that implements Collection can be iterated
Associated with each class that implements the Collection interface, there is a private inner class that implements the Iterator interface
CS-2851
Dr. Mark L. Hornick

8. Accessing and using a collection’s Iterator
To get an Iterator from a Collection, use the iterator() method
ArrayList<Double> al= new ArrayList<Double)();
// Get an Iterator object to iterate over // this collection.
Iterator<Double> itr = al.iterator();
while( itr.hasNext() ) { // while elements exist… double x = itr.next(); // …get the next element
System.out.println( x ); }
CS-2851
Dr. Mark L. Hornick

9. The Iterator<E> methods
public interface Iterator<E> {
// Returns true if this Iterator object is positioned
// at an element in the collection.
public boolean hasNext();
// Returns the element this Iterator object is
// positioned at, and advances this Iterator object.
public E next();
// Removes the element returned by the most
// recent call to next().
public void remove();
} // interface Iterator
CS-2851
Dr. Mark L. Hornick

10. The for-each loop uses a collection’s Iterator automatically
// create a collection
List<Double> al = new ArrayList<Double>();
// use the built-in Iterator…
for( Double x : al ) // “for each x in al”
System.out.println(x);
CS-2851
Dr. Mark L. Hornick