CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2002 Assignment 1 Comments & Null Object
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NullObject

Structure
Binary Search Tree Example

Refactoring

Applicability
Consequences
Implementation

Doc 3, Assignment 1 Comments & Null Object Slide # 17

NullObject

Structure

Doc 3, Assignment 1 Comments & Null Object Slide # 18

Binary Search Tree Example

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, Assignment 1 Comments & Null Object Slide # 19

Doc 3, Assignment 1 Comments & Null Object Slide # 20

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, Assignment 1 Comments & Null Object Slide # 21

• 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, Assignment 1 Comments & Null Object Slide # 22

Refactoring

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

• Create NullCustomer subclass of Customer with:

   NullCustomer>>plan
      ^BillingPlan basic

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

plan := customer plan

• Often one makes a Null Object a singleton

Doc 3, Assignment 1 Comments & Null Object Slide # 23

Applicability

• 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

• 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, Assignment 1 Comments & Null Object Slide # 24

Consequences

• Uses polymorphic classes

• Simplifies client code

• Encapsulates do nothing behavior

• Makes do nothing behavior reusable

• 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 can not transform themselves into a RealObject

Doc 3, Assignment 1 Comments & Null Object Slide # 25

Implementation

Eliminate one class by making NullObject a subclass of RealObject

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

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

Use the proxy pattern

Doc 3, Assignment 1 Comments & Null Object Slide # 26

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