	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2001 Iterators
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Contents of Doc 6, Iterators

References Design Patterns: Elements of Reusable Object-Oriented Software , Gamma, Helm, Johnson, Vlissides, 1995, pp. 257-271

Reading
Design Patterns: pp. 257-271


Doc 6, Iterators Slide # 2

Iterator

Provides a way to access elements of an aggregate object sequentially without exposing its underlying representation

Java Examples
Enumeration, Iterator, and Streams in Java are iterators

   Vector listOfStudents = new Vector();
   // code to add students not shown
   Iterator list = listOfStudents.iterator();
   while ( list.hasNext() )
      Console.println( list.next() );
   
   Hashtable anIndex = new HashMap();
   // code to add elements to the hashMap not shown
   Iterator list = anIndex.values().iterator();
   while ( list.hasNext() )
      Console.println( list.next() );


Doc 6, Iterators Slide # 3

Smalltalk Examples
Streams, do:, select:, reject:, collect:, detect:, inject:into: are iterators in Smalltalk

| sum  |
sum := 0.
#( 1 7 2 3 9 3 50) do: [:each | sum := sum + each squared].
^sum

#( 1 7 2 3 9 3 50) do: 
   [:partialSum :number | partialSum + number squared]

'this is an example' select: [:each |  each isVowel ]


Doc 6, Iterators Slide # 4

Sample Implementation of Java Enumerator

class VectorIterator implements Enumeration {
   Vector iteratee;
   int count;
   VectorIterator(Vector v) {
      iteratee = v;
      count = 0;
   }
   public boolean hasMoreElements() {
      return count < iteratee.elementCount;
   }
   public Object nextElement() {
      synchronized (iteratee) {
         if (count < iteratee.elementCount) 
            return iteratee.elementData[count++];
      }
   throw new NoSuchElementException("VectorIterator");
   }
}

The iterator is using privileged access to Vectors fields


Doc 6, Iterators Slide # 5

Issues

Concrete vs. Polymorphic Iterators

Concrete
Use Explicit Iterator Type

Reader iterator = new StringReader( "cat");
int c;
while (-1 != (c = iterator.read() ))
System.out.println( (char) c);

Polymorphic
Actual type of iterator is not known

   Vector listOfStudents = new Vector();
   // code to add students not shown
   Iterator list = listOfStudents.iterator();
   while ( list.hasNext() )
      Console.println( list.next() );

Polymorphic iterators can cause problems with memory leaks in C++ because they are on the heap!


Doc 6, Iterators Slide # 6

Who Controls the iteration?

External (Active)

Vector listOfStudents = new Vector();
// code to add students not shown
Iterator list = listOfStudents.iterator();
while ( list.hasNext() )
   Console.println( list.next() );


Doc 6, Iterators Slide # 7

Who Controls the iteration?
Internal (Passive)

'this is an example' select: [:each |  each isVowel ]

Vector listOfStudents = new Vector();
// code to add students not shown
while ( listOfStudents.hasMoreElements() )
   Console.println( listOfStudents.nextElement() );

Fictitious Code

class Vector implements Cloneable, java.io.Serializable {
   protected Object elementData[];
   protected int elementCount;
   protected int currentPosition;
   
   public boolean hasMoreElements() {
      return currentPosition < elementCount;
   }
   public Object nextElement() {
      if (currentPosition < elementCount) 
         return elementData[currentPosition++];
   throw new NoSuchElementException("VectorIterator");
   }
   // all vector methods not shown
}


Doc 6, Iterators Slide # 8

Who Defines the Traversal Algorithm?

Object being Iterated
Iterator can store where we are

In a Vector this could mean the index of the current item

In a tree structure it could mean a pointer to current node and stack of past nodes

BinaryTree searchTree = new BinaryTree();
// code to add items not shown
Iterator aSearch = searchTree.getIterator();
Iterator bSearch = searchTree.getIterator();
Object first = searchTree.nextElement( aSearch );
Object stillFirst = searchTree.nextElement( bSearch );

Iterator
Makes it easier to have multiple iterator algorithms on same type

On Vector class, why not have a reverseIterator which goes backwards?

In a complex structure the iterator may need access to the iteratee's implementation


Doc 6, Iterators Slide # 9

How Robust is the iterator?

What happens when items are added/removed from the iteratee while an iterator exists?

Vector listOfStudents = new Vector();
// code to add students not shown
Enumeration list = listOfStudents.elements();
Iterator failFastList = listOfStudents.iterator();
listOfStudents.add( new Student( "Roger") );
list.hasMoreElements();
failFastList.hasNext();      //Exception thrown here


Doc 6, Iterators Slide # 10

Additional Iterator Operations
Augmenting basic iteration operations may improve their usefulness

previous()

back up one location

add( Object item)

add item to the iteratee at current location

remove()

remove the current item from the iteratee

skipTo( some location, item or condition )

go to the location indicated

mark()

mark current location for future return

Iterators and Privileged Access

An iterator may need privileged access to the aggregate structure for traversal


Doc 6, Iterators Slide # 11

Iterators for Composites

Traversing a complex structure like a graph, tree, or composite can be difficult

An internal iterator can use recursion to keep track of where to go next

For example using a depth-first search algorithm on graph

If each element in the aggregate “knows” how to traverse to the next element and previous element, than an external iterator can be used

Null Iterator

A Null iterator for the empty aggregates can be useful


Doc 6, Iterators Slide # 12

Exercises

1. Explain why polymorphic iterators in C++ must be on the heap?

2. What are the major consequences of the iterator pattern?

3. Contrast Java and Smalltalk (C++) iterators. What are the strengths and weaknesses of each.

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2001 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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