CS 696: Advanced OO
Spring Semester, 1997
Doc 11 Builder and Factory Method

Doc 11 Builder and Factory Method Slide # 1

Builder

Intent

Applicability

• 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 11 Builder and Factory Method Slide # 2

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 11 Builder and Factory Method Slide # 3

Example - RTF Converter

RTF (Rich Text Format) is one of several standard formats for converting documents between applications and hardware platforms

The problem is to write a program to convert RTF to other document formats:

TeX, html, ascii, etc.

Doc 11 Builder and Factory Method Slide # 4

Doc 11 Builder and Factory Method Slide # 5

class RTF_Reader
     {
     TextConverter builder;
     String RTF_Text;
     
     public RTF_Reader( TextConverter aBuilder, String RTFtoConvert )
          {
          builder = aBuilder;
          RTF_Text = RTFtoConvert;
          }
     
     public void parseRTF()
          {
          RTFTokenizer rtf = new RTFTokenizer( RTF_Text );
          
          while ( rtf.hasMoreTokens() )
               {
               RTFToken next = rtf.nextToken();
               
               switch ( next.type() )
                    {
                    case CHAR:      builder.character( next.char() ); break;
                    case FONT:      builder.font( next.font() ); break;
                    case PARA:      builder.newParagraph( ); break;
                    etc.
                    }
               }
          }
     }
     
     
Doc 11 Builder and Factory Method Slide # 6

abstract class TextConverter
     {          
     public void character( char nextChar ) {  }
     public void font( Font newFont )     { }
     public void newParagraph() {}
     }
     
class ASCII_Converter extends TextConverter
     {
     StringBuffer convertedText = new StringBuffer();

     public void character( char nextChar ) 
          {  
          convertedText.append( nextChar );
          }
          
     public String getASCII_Text()
          {
          return convertedText.toString();
          }
     }
          
class TeX_Converter extends TextConverter
     {          
     public void character( char nextChar )      { blah  }
     public void font( Font newFont )          { blah }
     public void newParagraph()                { blah }
     public TeX getTeX_Text()          { return the correct thing }
     }

Doc 11 Builder and Factory Method Slide # 7

main()
     {
     ASCII_Converter simplerText = new ASCII_Converter();
     String rtfText;

     // read afile of rtf into rtfText

     RTF_Reader myReader = new RTF_Reader( simplerText, rtfText );

     myReader.parseRTF();

     now can deal with simplerText;
     }

Doc 11 Builder and Factory Method Slide # 8

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

• 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

Doc 11 Builder and Factory Method Slide # 9

Factory Method

Example - Maze Game

Classes for Mazes

Now a maze game has to make a maze, so we might have something like:

Doc 11 Builder and Factory Method Slide # 10

Maze Class Version 1

class MazeGame
     {
     
     public Maze createMaze()
          {
          Maze aMaze = new Maze();
          
          Room r1 = new Room( 1 );
          Room r2 = new Room( 2 );
          Door theDoor = new Door( r1, r2);
          
          aMaze.addRoom( r1 );
          aMaze.addRoom( r2 );
          
          r1.setSide( North, new Wall() );
          r1.setSide( East, theDoor );
          r1.setSide( South, new Wall() );
          r1.setSide( West, new Wall() );

          r2.setSide( North, new Wall() );
          r2.setSide( East, new Wall() );
          r2.setSide( South, new Wall() );
          r2.setSide( West, theDoor );
          
          return aMaze;
          }
     }

Doc 11 Builder and Factory Method Slide # 11

class BombedMazeGame extends MazeGame
     {
     
     public Maze createMaze()
          {
          Maze aMaze = new Maze();
          
          Room r1 = new RoomWithABomb( 1 );
          Room r2 = new RoomWithABomb( 2 );
          Door theDoor = new Door( r1, r2);
          
          aMaze.addRoom( r1 );
          aMaze.addRoom( r2 );
          
          r1.setSide( North, new BombedWall() );
          r1.setSide( East, theDoor );
          r1.setSide( South, new BombedWall() );
          r1.setSide( West, new BombedWall() );

Note the amount of cut and paste!

Doc 11 Builder and Factory Method Slide # 12

class MazeGame
     {
     
     public Maze makeMaze() { return new Maze(); }
     public Room makeRoom(int n ) { return new Room( n ); }
     public Wall makeWall() { return new Wall(); }
     public Door makeDoor() { return new Door(); }
     
     public Maze CreateMaze()
          {
          Maze aMaze = makeMaze();
          
          Room r1 = makeRoom( 1 );
          Room r2 = makeRoom( 2 );
          Door theDoor = makeDoor( r1, r2);
          
          aMaze.addRoom( r1 );
          aMaze.addRoom( r2 );
          
          r1.setSide( North, makeWall() );
          r1.setSide( East, theDoor );
          r1.setSide( South, makeWall() );
          r1.setSide( West, makeWall() );

          r2.setSide( North, makeWall() );
          r2.setSide( East, makeWall() );
          r2.setSide( South, makeWall() );
          r2.setSide( West, theDoor );
          
          return aMaze;
          }
     }

Doc 11 Builder and Factory Method Slide # 13

CreateMaze method stays the same

class BombedMazeGame extends MazeGame
     {
     
     public Room makeRoom(int n ) 
          { 
          return new RoomWithABomb( n ); 
          }

     public Wall makeWall() 
          { 
          return new BombedWall(); 
          }

Doc 11 Builder and Factory Method Slide # 14

Implementation

abstract class MazeGame
     {
     
     public Maze makeMaze();
     public Room makeRoom(int n );
     public Wall makeWall();
     public Door makeDoor();

class MazeGame
     {
     
     public:
          virtual Maze* makeMaze() = 0;
          virtual Room* makeRoom(int n ) = 0;
          virtual Wall* makeWall() = 0;
          virtual Door* makeDoor() = 0;

"Create objects in a separate operation so that subclasses can override the way they're created"

Doc 11 Builder and Factory Method Slide # 15

Parameterized Factory Methods

class Hershey 
     {
     
     public Candy makeChocolateStuff( CandyType id )
          {
          if ( id == MarsBars ) return new MarsBars();
          if ( id == M&Ms ) return new M&Ms();
          if ( id == SpecialRich ) return new SpecialRich();
          
          return new PureChocolate();
          }

C++ Templates to Avoid Subclassing

template <class ChocolateType>
class Hershey 
     {
     public:
          virtual Candy* makeChocolateStuff( );
     }

template <class ChocolateType>
Candy* Hershey<ChocolateType>::makeChocolateStuff( )
     {
     return new ChocolateType;
     }

Hershey<SpecialRich> theBest;

Doc 11 Builder and Factory Method Slide # 16

Java forName and Factory methods

class Hershey 
     {
     
     private String chocolateType;

     public Hershey( String chocolate )
          {
          chocolateType = chocolate;
          }

     public Candy makeChocolateStuff( )
          {
          Class candyClass =  Class.forName( chocolateType );
          return (Candy) candyClass.newInstance();          
          }

Hershey theBest = new Heshsey( "SpecialRich" );

Doc 11 Builder and Factory Method Slide # 17

Clients Can Use Factory Methods

public restock()
{
blah

if ( specialRich.amount() < 10 )
{
Hershey supplier = new Hershey( "SpecialRich" );
specialRich.add( supplier.makeChocolateStuff() );
}

blah

Doc 11 Builder and Factory Method Slide # 18

class Room extends MapSite
     {
     public Room makeRoom()  { return new Room(); }
     etc,
     }

class RoomWithBomb extends Room
     {
     public Room makeRoom()  { return new RoomWithBomb(); }
     etc,
     }

etc. 

class MazeGame
     {
     public Maze mazeMaker; 
     public Room roomMaker; 
     etc.
     
     public MazeGame( Maze mfactory, Room rfactory, etc. )
          {
          mazeMaker = mfactory;   
          etc.
          }

     public Maze CreateMaze()
          {
          Maze aMaze = mazeMaker.makeMaze();
          
          Room r1 = roomMaker.makeRoom( 1 );
          etc.