In object-oriented programming, the **factory design pattern** is one of the most famous design patterns. In this pattern, objects are created without specifying their exact class. 

This is achieved by specifying in an interface and implementing by child classes, or, by implementing in a base class and overriding by child classes.

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# Example

In the class diagram below, the GenerateBill class can not generate a bill directly. Instead, the Factory class generates the correct type of bill using `computeBill()`.

public class Factory {
	
   // computeBill method
   public Billing computeBill(String Type){
      if(Type == null){
         return null;
      }		
      if(Type.equalsIgnoreCase("DOMESTIC")){
         return new Domestic();
         
      } else if(Type.equalsIgnoreCase("COMMERCIAL")){
         return new Commercial();
         
      } else if(Type.equalsIgnoreCase("INSTITUTIONAL")){
         return new Institutional();
      }
      
      return null;
   }
}



 // Inteface for billing.
public interface Billing {
   void getBill();
}

// Commercial class that implements the billing interface.
public class Commercial implements Billing {
   
   @Override
   public void getBill() {
      System.out.println("This is a bill for Commercial.");
   }
}

// Domestic class that implements the billing interface.
public class Domestic implements Billing {

   @Override
   public void getBill() {
      System.out.println("This is a Domestic bill.");
   }
}

// Institutional class that implements the billing interface.
public class Institutional implements Billing {

   @Override
   public void getBill() {
      System.out.println("This is a institutional bill.");
   }
}

public class main {

   public static void main(String[] arguments) {
      Factory bill = new Factory();

      //get an object of Domestic Billing and call its getBill() method.
      Billing bill_1 = bill.computeBill("Domestic");

      bill_1.getBill();
   }
}

What is the factory design pattern in Java?

Objects are created without specifying their exact class, using interfaces or base class methods.

What is the factory design pattern in Java?

Example

Implementation