Design Patterns

Design Patterns

Factory method design pattern

OpenOffice.org development heavily uses the Factory method design pattern. Design patterns are conventional templates that describes how to solve common software problems. Since most developers are familiar with the patterns, they can recognize a pattern in others source code. That makes working in teams easier. There are many popular design patterns. One of them is Factory method pattern. Factory method pattern is a type of creational pattern. Creational pattern pattern solves problems related to creating. Factory pattern solves two major problem generally faced by developers. 1. To reduce too many new operator usage

When working on a large software, numerous instances of classes are created continuously at the runtime. The programmer cannot predict what the user is going to do. So at any given time, the programmer doesn't know what object is create. For example, To create a new document, the user might click new text document or new spreadsheet document. There would several possibilities about what the user is going to do. So, a factory class is assigned to do all these repetitive work of creating a new instance of what the user wants. By separating these repetitive object creations into a factory class, when new classes are added, only the factory class need to be updated.

2. To create object without knowing its class name.

When using the concrete classes, the developer has to remember the class names. In factory pattern, choosing what type of object to be created is delegated to the factory class. Usually this is done by sending a parameter. Based on the parameter passed to the factory, the factory creates an instance of a certain type/class.

Factory Method Pattern

public final class DocumentFactory {
   XDocument document;
   XDocument getDocument(String type){
   if(type.equals("text"){
      document = new TextDocument();
   }
   else if(type.equals("sheet"){
      document = new SpreadSheet();
   }
   return document;
   }
}

public interface XDocument{
   open();
}

public class TextDocument implements XDocument{
   //concrete class for Text documents
   open(){
      //method to open text document
      System.out.println("opening a text document...");
   }
}

public class SpreadSheet implements XDocument{
  //concrete class for spreadsheet documents
  open(){
     //method to open spreadsheet document
     System.out.println("opening a spreadsheet document...");
  }
}

class DocumentProgram{
   public static void main(String[] args){
      XDocument doc = df.getDocument("text");  //this just created an instance of TextDocument without knowing its class name.
      doc.open();
   }
}

Singleton Pattern

In Singleton Pattern, the class can only create a single instance. We want a class to have only a single instance for various reasons. Sometimes, we want use a global object to keep information about your program. This object should not have any copies. This information might be things like configuration of the program, or a master object that manages pools of resources. So when you need a resource, you ask the master object to get it for you. Now if there were many copies of this master object, you would not know whom to ask for that resource. This single object should not be allowed to have copies. Singleton Pattern forces this rule so that programmer doesn't have to remember about not creating copies. Singleton pattern will create an instance if it doesn't exist and will not create any new instance if an instance already exist. It will just return a reference to that single instance.

class ProgramConfiguration{
    public ProgramConfiguraiton(){
        //default constructor code
    }
}

A new instance of a class is created by the constructor. Most of the time, we have a public constructor, which is called to create a new instance. Since we want to prohibit multiple instance, we have to restrict access to the constructor. This is done by making the constructor private.

class ProgramConfiguration{
    private ProgramConfiguration(){
        //default private constructor code
    }
}

then we create a static public method that will make sure that only one instance lives in the whole program.

class ProgramConfiguration{
    private static ProgramConfiguration _configObject;
    private ProgramConfiguration(){
        //default private constructor code
    }
    public getInstance(){
        /*
        if an instance exist return that instance otherwise 
        call the constructor to create an instance and return it.
        */
        if(_configObject = null){
        _configObject = ProgramConfiguration();
        }
        return _configObject;
    }
}

So anytime you want to get that single object, you call the getInstance() method.

main(){
    /*
    no access to default constructor. so if you did 
    ProgramConfiguration pc = new ProgramConfiguration(); 
    you will get compilation error.
    */
    ProgramConfiguration pc = ProgramConfiguration.getInstance();
    ProgramConfiguration newpc = ProgramConfiguration.getInstance();
    /*
    in the above code pc and newpc both point to the same static object. when 
    getinstance() is called for the second time, it finds that _configObject is not null 
    anymore, so it doesn't call the constructor to create any new instance.
    */
}

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