	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2002 Iterator
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Contents of Doc 2, Iterator

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

Reading
Design Patterns: pp. 257-271


Doc 2, Iterator 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 2, Iterator 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 ]

| input words word |
input := 'juliusceasar' asFilename readStream.
words := Set new.
word := OrderedCollection new.
input do:
   [:each |
   (each isSeparator | ('.,;!-[]()' includes: each))
      ifTrue:
         [words add:( String withAll: word) asLowercase.
         word := OrderedCollection new.]
      ifFalse:[ word add: each]].
input close.
words size


Doc 2, Iterator Slide # 4

What's The Big Deal?

void print(ArrayList list)
   {
   for( int k = 0; k < list.size(); k++ )
      System.out.println( list.get(k) );
   } 

void print(LinkedList list )
   {
   Node current = list.first();
   System.out.println( current );
   while (current.hasNext() )
      {
      current = current.next();
      System.out.println( current );
      }
   }

void print(Collection list )
   {
   Iterator items = list.iterator();
   while (items.hasNext() )
      {
      System.out.println( items.next() );
      }
   }

print: aCollection
   aCollection do: 
   [:each | 
   Transcript 
      show: each;
      cr]
Doc 2, Iterator Slide # 5

What's The Big Deal?
Iterators abstract out underlying representation of collection

Programmer does not have to know implementation details of each type of collection

Can write code that works for wide range of collects

Do not have to change code if change the type of collection used


Doc 2, Iterator Slide # 6

Design Principle 1
Program to an interface, not an implementation

Use abstract classes (and/or interfaces in Java) to define common interfaces for a set of classes

Declare variables to be instances of the abstract class not instances of particular classes

Benefits of programming to an interface
Client classes/objects remain unaware of the classes of objects they use, as long as the objects adhere to the interface the client expects

Client classes/objects remain unaware of the classes that implement these objects. Clients only know about the abstract classes (or interfaces) that define the interface.


Doc 2, Iterator Slide # 7

Programming to an InterfaceJava Collections


Doc 2, Iterator Slide # 8

Java Iterators & Arrays?
Arrays are common collections

How can one get an iterator on a Java array?

How would you pass an array to the following function?

void print(Collection list )
   {
   Iterator items = list.iterator();
   while (items.hasNext() )
      {
      System.out.println( items.next() );
      }
   }


Doc 2, Iterator Slide # 9

Java Iterators in Practice

void printA(Collection list )
   {
   Iterator items = list.iterator();
   while (items.hasNext() )
      System.out.println( items.next() );
   }

void printB(String[] list)
   {
   for (int k = 0; k < list.length; k++)
      System.out.println( list[k]);
   }

Programmers are not used to programming to an interface

printA requires as much typing as printB


Doc 2, Iterator Slide # 10

Smalltalk Iterators in Practice

printA: aCollection
   1 to: aCollection size do: [:index | 
   Transcript
      show: (aCollection at: index);
      cr.

printB: aCollection
   aCollection do: [:each | 
      Transcript
         show: each;
         cr.

printB requires less typing

Programmers use iterators just to avoid extra work


Doc 2, Iterator Slide # 11

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 2, Iterator Slide # 12

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 2, Iterator Slide # 13

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() );

Iteration control code is repeated for each use of the iterator


Doc 2, Iterator Slide # 14

Who Controls the iteration?
Internal (Passive)

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

Control code is inside the iterator

Programmer

Does not repeat control code
Can focus on what to do not how to do it


Doc 2, Iterator Slide # 15

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 2, Iterator Slide # 16

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 2, Iterator Slide # 17

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 2, Iterator Slide # 18

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

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2002 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.

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