Factory Method Pattern (Virtual Constructor)

When we want to return one sub-class object from multiple sub-classes using an input, should use Factory design pattern. Factory class takes responsibility of instantiation the class (We can return Singleton instance from static factory method).

In Factory pattern, we create object without exposing the creation logic to the client and refer to newly created object using a common interface.

       

Example: Coffee/Vending machine, give input from options and as per input coffee, lemon tea, plain milk or hot water will be an output.

interface Drink {

       void prepare();

}

class Coffee implements Drink {

       @Override

       public void prepare() {

              System.out.println("Coffee is prepared !!");

       }

}

class LemonTea implements Drink {

       @Override

       public void prepare() {

              System.out.println("Lemon Tea is prepared !!");

       }

}

class PlainWater implements Drink {

       @Override

       public void prepare() {

              System.out.println("Plain Water is prepared !!");

       }

}

class VedingMachine {

       public static Drink getDrink(String str) {

              if("PlainWater".equals(str)) {

                     return new PlainWater();

              } else if("Coffee".equals(str)) {

                     return new Coffee();

              } else if("LemonTea".equals(str)) {

                     return new LemonTea();

              }

              return null;

       }

}

public class FactoryPatternTest {

       public static void main(String[] args) {

              Drink drink = VedingMachine.getDrink("Coffee");

              drink.prepare();

       }

}

Output: Coffee is prepared !!

Benefits of Factory Method Pattern

Factory Method Pattern provides approach to code for interface rather than implementation and it provides abstraction between implementation and client classes through inheritance.

Factory Method Pattern allows the sub-classes to choose the type of objects to create.

We can easily change the implementation of sub-class because client program is unaware of this. It makes code more robust, less coupled and easy to extend (client interacts solely with the resultant interface or abstract class).

Usage in JDK

java.util.Calendar, ResourceBundle and NumberFormat getInstance() methods uses Factory pattern.

valueOf() method in wrapper classes like Boolean, Integer etc.

Spring and hibernate frameworks.