Design Pattern (GOF): Creational Pattern : Factory Method

Definition

Provides an abstraction or an interface and lets subclass or implementing classes decide which class or method should be instantiated or called, based on the conditions or parameters given.

Example

To illustrate how to use factory design pattern with class level implementation, here is a real world example. A company has a website to display testing result from a plain text file. Recently, the company purchased a new machine which produces a binary data file, another new machine on the way, it is possible that one will produce different data file. How to write a system to deal with such change. The website just needs data to display. Your job is to provide the specified data format for the website.

Here comes a solution. Use an interface type to converge the different data file format. The following is a skeleton of implementation.

//Let's say the interface is Display
interface Display {

 //load a file
 public void load(String fileName);

 //parse the file and make a consistent data type
 public void formatConsistency();

}

//deal with plain text file
class CSVFile implements Display{

  public void load(String textfile) {
      System.out.println("load from a txt file");
  }
  public void formatConsistency() {
      System.out.println("txt file format changed");
  }
}

//deal with XML format file
class XMLFile implements Display {

  public void load(String xmlfile) {
      System.out.println("load from an xml file");
  }
  public void formatConsistency() {
      System.out.println("xml file format changed");
  }
}

//deal with binary format file
class DBFile implements Display {

  public  void load(String dbfile) {
      System.out.println("load from a db file");
  }
  public void formatConsistency() {
      System.out.println("db file format changed");
  }
}

//Test the functionality
class TestFactory {

  public static void main(String[] args) {
      Display display = null;
    
      //use a command line data as a trigger
      if (args[0].equals("1"))
         display = new CSVFile();
      else if (args[0].equals("2"))
         display = new XMLFile();
      else if (args[0].equals("3"))
         display = new DBFile();
      else
         System.exit(1);

      //converging code follows
      display.load("");
      display.formatConsistency();
 }  
}
//after compilation and run it

C:\>java TestFactory 1
load from a txt file
txt file format changed

C:\>java TestFactory 2
load from an xml file
xml file format changed

C:\>java TestFactory 3
load from a db file
db file format changed

In the future, the company may add more data file with different format, a programmer just adds a new class in accordingly. Such design saves a lot of code and is easy to maintain.

Author:
-----------------------
Kazi Masudul Alam
Software Engineer

Design Pattern (GOF): Creational Pattern : Builder

Definition

Construct a complex object from simple objects step by step.

Where to use & benefits

• Make a complex object by specifying only its type and content. The built object is shielded from the details of its construction.
• Want to decouple the process of building a complex object from the parts that make up the object.
• Related patterns include
• Abstract Factory, which focuses on the layer over the factory pattern (may be simple or complex), whereas a builder pattern focuses on building a complex object from other simple objects.
• Composite, which is often used to build a complex object.

Example

To build a house, we will take several steps:

1. build foundation,
2. build frame,
3. build exterior,
4. build interior.

Let's use an abstract class HouseBuilder to define these 4 steps. Any subclass of HouseBuilder will follow these 4 steps to build house (that is to say to implement these 4 methods in the subclass). Then we use a WorkShop class to force the order of these 4 steps (that is to say that we have to build interior after having finished first three steps). The TestBuilder class is used to test the coordination of these classes and to check the building process.

import java.util.*;

class WorkShop {

    //force the order of building process

    public void construct(HouseBuilder hb) {

        hb.buildFoundation();

        hb.buildFrame();

        hb.buildExterior();

        hb.buildInterior();

    }

}

//set steps for building a house

abstract class HouseBuilder {

    protected House house = new House();

    protected String showProgress() {

        return house.toString();

    }

    abstract public void buildFoundation();

    abstract public void buildFrame();

    abstract public void buildExterior();

    abstract public void buildInterior();

}

class OneStoryHouse extends HouseBuilder {

    public OneStoryHouse(String features) {

        house.setType(this.getClass() + " " + features);

    }

    public void buildFoundation() {

        //doEngineering()

        //doExcavating()

        //doPlumbingHeatingElectricity()

        //doSewerWaterHookUp()

        //doFoundationInspection()

        house.setProgress("foundation is done");

    }

    public void buildFrame() {

        //doHeatingPlumbingRoof()

        //doElectricityRoute()

        //doDoorsWindows()

        //doFrameInspection()

        house.setProgress("frame is done");

    }

    public void buildExterior() {

        //doOverheadDoors()

        //doBrickWorks()

        //doSidingsoffitsGutters()

        //doDrivewayGarageFloor()

        //doDeckRail()

        //doLandScaping()

        house.setProgress("Exterior is done");

    }

    public void buildInterior() {

        //doAlarmPrewiring()

        //doBuiltinVacuum()

        //doInsulation()

        //doDryWall()

        //doPainting()

        //doLinoleum()

        //doCabinet()

        //doTileWork()

        //doLightFixtureBlinds()

        //doCleaning()

        //doInteriorInspection()

        house.setProgress("Interior is under going");

    } 

}

class TwoStoryHouse extends HouseBuilder {

    public TwoStoryHouse(String features) {

        house.setType(this.getClass() + " " + features);

    }

    public void buildFoundation() {

        //doEngineering()

        //doExcavating()

        //doPlumbingHeatingElectricity()

        //doSewerWaterHookUp()

        //doFoundationInspection()

        house.setProgress("foundation is done");

    }

    public void buildFrame() {

        //doHeatingPlumbingRoof()

        //doElectricityRoute()

        //doDoorsWindows()

        //doFrameInspection()

        house.setProgress("frame is under construction");

    }

    public void buildExterior() {

        //doOverheadDoors()

        //doBrickWorks()

        //doSidingsoffitsGutters()

        //doDrivewayGarageFloor()

        //doDeckRail()

        //doLandScaping()

        house.setProgress("Exterior is waiting to start");

    }

    public void buildInterior() {

        //doAlarmPrewiring()

        //doBuiltinVacuum()

        //doInsulation()

        //doDryWall()

        //doPainting()

        //doLinoleum()

        //doCabinet()

        //doTileWork()

        //doLightFixtureBlinds()

        //doCleaning()

        //doInteriorInspection()

        house.setProgress("Interior is not started yet");

    }

}

class House {

    private String type = null;

    private List features = new ArrayList();

    public House() {

    }

    public House(String type) {

        this.type = type;

    }

    public void setType(String type) {

        this.type = type;

    }

    public String getType() {

        return type;

    }

    public void setProgress(String s) {

        features.add(s);

    }

    public String toString() {

        StringBuffer ff = new StringBuffer();

        String t = type.substring(6);

        ff.append(t + "\n ");

        for (int i = 0; i <>

             ff.append(features.get(i) + "\n ");

        }

        return ff.toString();

    }

}

class TestBuilder  {

    public static void main(String[] args) {

       HouseBuilder one = new OneStoryHouse("2 bedrooms, 2.5 baths, 2-car garage, 1500 sqft");

       HouseBuilder two = new TwoStoryHouse("4 bedrooms, 4 baths, 3-car garage, 5000 sqft");

       WorkShop shop = new WorkShop();

       shop.construct(one);

       shop.construct(two);

       System.out.println("Check house building progress: \n");

       System.out.println(one.showProgress());

       System.out.println(two.showProgress());

   }

}

//need jdk1.5 above to compile


Author:

-----------------------

Kazi Masudul Alam

Software Engineer