	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2002 Builder
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Contents of Doc 7, Builder

References
Design Patterns: Elements of Reusable Object-Oriented Software , Gamma, Helm, Johnson, Vlissides, Addison-Wesley, 1995, pp. 97-106

The Design Patterns Smalltalk Companion, Alpert, Brown, Woolf, 1998, pp. 47-62


Doc 7, Builder Slide # 2

Builder

Intent
Separate the construction of a complex object from its representation so that the same construction process can create different representations

Applicability

Use the Builder pattern when

The algorithm for creating a complex object should be independent of the parts that make up the object and how they're assembled

The construction process must allow different representations for the object that's constructed


Doc 7, Builder Slide # 3

Collaborations
The client creates the Director object and configures it with the desired Builder object

Director notifies the builder whenever a part of the product should be built

Builder handles requests from the director and adds parts to the product

The client retrieves the product from the builder


Doc 7, Builder Slide # 4

Example – XML Parser

Director

XML Parser

Abstract Builder Class

XML.SAXDriver (Smalltalk)
org.xml.sax.helpers.DefaultHandler (Java)
DefaultHandler (C++)

Concrete Builder Class

Your subclass of the abstract builder

Client
Your code that uses the tree built


Doc 7, Builder Slide # 5

Java Example

   public static void main(String argv[])
      {
      SAXDriverExample handler = new SAXDriverExample();
   
        // Use the default (non-validating) parser
      SAXParserFactory factory = SAXParserFactory.newInstance();
      try 
         {
         SAXParser saxParser = factory.newSAXParser();
         saxParser.parse( new File("sample"), handler );
         } 
      catch (Throwable t) 
         {
         t.printStackTrace();
         }
      System.out.println( handler.root());
      }


Doc 7, Builder Slide # 6

Smalltalk Example

| builder exampleDispatcher |

builder := SAXDriverExample new.
exampleDispatcher := SAXDispatcher new contentHandler: builder.
XMLParser 
   processDocumentInFilename: 'page' 
   beforeScanDo: 
      [:parser | 
      parser
         saxDriver:(exampleDispatcher); 
         validate: true].
builder root.


Doc 7, Builder Slide # 7

Consequences

It lets you vary a product's internal representation

It isolates code for construction and representation

It gives you finer control over the construction process

Implementation

Assembly and construction interface

Builder may have to pass parts back to director, who will then pass them back to builder

Why no abstract classes for products

Empty methods as default in Builder

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