How to implement Factory Design Pattern in Java with a simple program ?

Program to demonstrate how to implement Factory Design Pattern in Java

In order to implement Factory Design Pattern, we will create following classes/interface and properties file :-

Step 1 : Create a Interface "Animal"

package com.hubberspot.designpatterns.factory.example;

// Step 1 : Create a Simple interface for polymorphic
// behavior ... here it is Animal having two simple 
// method as eat() and sleep()

public interface Animal {

 public void eat();

 public void sleep();

}

Step 2 : Create a concrete implementation of Animal interface as "Dog"

package com.hubberspot.designpatterns.factory.example;


// Step 2 : create object of Animal say Dog 
// implementing Animal interface.
// Provide concrete implementation to the 
// two methods eat() and sleep() respective to Dog()

public class Dog implements Animal{


 @Override
 public void eat() {

  System.out.println("Dog : eat()");
 }

 @Override
 public void sleep() {

  System.out.println("Dog : sleep()");
 }

}

Step 3 : Create a concrete implementation of Animal interface as "Cat"

package com.hubberspot.designpatterns.factory.example;

//Step 3 : create object of Animal say Cat 
//implementing Animal interface.
//Provide concrete implementation to the 
//two methods eat() and sleep() respective to Cat

public class Cat implements Animal
{

 @Override
 public void eat() {
  System.out.println("Cat : eat()");

 }

 @Override
 public void sleep() {

  System.out.println("Cat : sleep()");

 }

}

Step 4 : Create a properties file which reads for the dynamic class name

# Step 4 : Create a configuration file which will provide us with 
# name of Concrete Animal class at the runtime.


#Animals Configurations

animal.classname=com.hubberspot.designpatterns.factory.example.Dog

Step 5 : Create a Factory class which produces Animal object based on properties file

package com.hubberspot.designpatterns.factory.example;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.PropertyResourceBundle;


// Step 5 : Create a class which will act as a factory for 
// creating concrete objects of Animal 
public class AnimalFactory {

 // Providing the name of the resource files from where the 
 // properties will be read
 private static final String ANIMALS_PROP = "animal.properties";
 private Class concreteClass ;

 public AnimalFactory()
 {
  FileInputStream propInp = null;

  try {

   // Creating a FileInputStream by passing it the name of the
   // resource
   propInp = new FileInputStream(ANIMALS_PROP);

   // PropertyResourceBundle class takes in the resource file 
   // as a resource bundle
   PropertyResourceBundle bundle = new PropertyResourceBundle(propInp);

   // Taking the name of class from the resource file
   String className = bundle.getString("animal.classname");

   // Class.forName() method will dynamically load the class
   // passed to it as a parameter
   concreteClass = Class.forName(className);


  }catch(Exception ex){



  }finally {
   try {
    // closing the FileInputStream
    propInp.close();
   } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
  }
 }


 // This method will act as a method that will work as a factory
 // and will return the object asked based on the configuration 
 // files entry.
 public Animal createAnimal(){

  try {

   // Returns concrete implementation of Animal by 
   // calling newInstance() method on the Class instance
   // Here in our case properties file points to Dog object
   // so based on the resource file the object will be created .
   return (Animal)concreteClass.newInstance();
  } catch (InstantiationException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } catch (IllegalAccessException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
  return null;
 }


}

Step 6 : Write a Test class to run the application.

package com.hubberspot.designpatterns.factory.example;

public class Test {


 public static void main(String[] args) {

  // Create a AnimalFactory Object and call its createAnimal method
  // This method will read the name of the class from the
  // properties file and returns back the Object of that class
  // dynamically. 
  // Now if we want to create object of Cat object, we just need to 
  // change properties file thats it . Our code remains untouched
  AnimalFactory animalFactory = new AnimalFactory();
  Animal animal = animalFactory.createAnimal();

  // Calling the respective methods
  animal.eat();
  animal.sleep();

 }

}

Output of the program :