	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2004 Iterator & Null Object
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References Design Patterns: Elements of Reusable Object-Oriented Software , Gamma, Helm, Johnson, Vlissides, 1995, pp. 257-271

Refactoring: Improving the Design of Existing Code, Fowler, 1999, pp. 260-266

“Null Object”, Woolf, in Pattern Languages of Program Design 3, Edited by Martin, Riehle, Buschmmann, Addison-Wesley, 1998, pp. 5-18

Reading
Design Patterns: pp. 257-271

Future Readings
Composite & Visitor patterns


Doc 3, Iterator & Null Object Slide # 2

Iterator

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

Java Example
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() )
      {Student x = list.next();
      System.out.println( x );
      }

C# Example

   Student[ ] listOfStudent;
   // code to add students not shown
   foreach (Student x in listOfStudent )
      {
      Console.WriteLine(x.ToString());
      }


Doc 3, Iterator & Null Object 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) inject: 0 into: 
   [:partialSum :number | partialSum + number squared]

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


Doc 3, Iterator & Null Object 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 3, Iterator & Null Object Slide # 5

What's The Big Deal?
Iterator abstracts 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 3, Iterator & Null Object 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 3, Iterator & Null Object Slide # 7

Programming to an InterfaceJava Collections


Doc 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object Slide # 10

Smalltalk/C# 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.

   Print(IEnumerable list )
      {
      foreach (Object x in list)
         {
         Console.WriteLine( x.ToString());
         }
      }

printB requires less typing than printB

Iterators are part of the C# language

Programmers use iterators just to avoid extra work


Doc 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object 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 3, Iterator & Null Object 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
of each.


Doc 3, Iterator & Null Object Slide # 19

NullObject

Structure

NullObject implements all the operations of the real object,

These operations do nothing or the correct thing for nothing


Doc 3, Iterator & Null Object Slide # 20

Binary Search Tree Example

Without Null Object

public class BinaryNode {
   Node left = new NullNode();
   Node right = new NullNode();
   int key;
   
   public boolean includes( int value ) {
      if (key == value)
         return true;
      else if ((value < key) & left == null) )
         return false;
      else if (value < key)
         return left.includes( value );
      else if (right == null)
         return false;
      else
         return right.includes(value);
   }
etc.
}


Doc 3, Iterator & Null Object Slide # 21

Binary Search Tree ExampleClass Structure

Object Structure


Doc 3, Iterator & Null Object Slide # 22

Searching for a Key

public class BinaryNode extends Node {
   Node left = new NullNode();
   Node right = new NullNode();
   int key;
   
   public boolean includes( int value ) {
      if (key == value)
         return true;
      else if (value < key )
         return left.includes( value );
      else
         return right.includes(value);
   }
etc.
}
   
public class NullNode extends Node {
   public boolean includes( int value ) {
      return false;
   }
etc.
}


Doc 3, Iterator & Null Object Slide # 23

Comments on Example

BinaryNode always has two subtrees

No need check if left, right are null

Since NullNode has no state just need one instance

Use singleton pattern for the one instance

Access to NullNode is usually restricted to BinaryNode


Forces indicate that one may not want to use the Null Object pattern
However, familiarity with trees makes it easy to explain the pattern

Implementing an add method in NullNode


Requires reference to parent or
Use proxy


Doc 3, Iterator & Null Object Slide # 24

Refactoring

Introduce Null Object [1]

You have repeated checks for a null value

Replace the null value with a null object

Example

customer isNil
   ifTrue: [plan := BillingPlan basic]
   ifFalse: [plan := customer plan]

becomes:

Create NullCustomer subclass of Customer with:

   NullCustomer>>plan
      ^BillingPlan basic

Make sure that each customer variable has either a real customer or a NullCustomer

Now the code is:

plan := customer plan

Often one makes a Null Object a singleton


Doc 3, Iterator & Null Object Slide # 25

Applicability

Use the Null Object pattern when:

Some collaborator instances should do nothing

You want clients to ignore the difference between a collaborator that does something and one that does nothing

Client does not have to explicitly check for null or some other special value

You want to be able to reuse the do-nothing behavior so that various clients that need this behavior will consistently work in the same way

Use a variable containing null or some other special value instead of the Null Object pattern when:

Very little code actually uses the variable directly

The code that does use the variable is well encapsulated - at least in one class

The code that uses the variable can easily decide how to handle the null case and will always handle it the same way


Doc 3, Iterator & Null Object Slide # 26

Consequences

Advantages

Uses polymorphic classes

Simplifies client code

Encapsulates do nothing behavior

Makes do nothing behavior reusable


Doc 3, Iterator & Null Object Slide # 27

Disadvantages

Forces encapsulation


Makes it difficult to distribute or mix into the behavior of several collaborating objects

May cause class explosion

Forces uniformity


Different clients may have different idea of what “do nothing” means

Is non-mutable


NullObject objects cannot transform themselves into a RealObject
become: message in Smalltalk allows null objects to “transform” themselves into real objects


Doc 3, Iterator & Null Object Slide # 28

Implementation

Too Many classes

Eliminate one class by making NullObject a subclass of RealObject

Multiple Do-nothing meanings


If different clients expect do nothing to mean different things use Adapter pattern to provide different do-nothing behavior to NullObject

Transformation to RealObject

In some cases a message to NullObject should transform it to a real object

Use the proxy pattern


Doc 3, Iterator & Null Object Slide # 29

Generalized Null Object Pattern
A generalized Null Object pattern based on Objective-C with an implementation in Smalltalk can be found at:

http://www.smalltalkchronicles.net/edition2-1/null_object_pattern.htm


Doc 3, Iterator & Null Object Slide # 30

[1] Refactoring Text, pp. 260-266

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2004 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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