Java Design Patterns Interview Questions and Answers

Observer pattern is used when there is one to many relationship between objects such as if one object is modified, its depenedent objects are to be notified automatically. Observer pattern falls under behavioral pattern category.

Observer pattern uses three actor classes. Subject, Observer and Client. Subject, an object having methods to attach and de-attach observers to a client object. We've created classes Subject, Observer abstract class and concrete classes extending the abstract class the Observer.

ObserverPatternDemo, our demo class will use Subject and concrete class objects to show observer pattern in action.

Step 1

Create Subject class.

Subject.java

import java.util.ArrayList;

import java.util.List;

public class Subject {

   private List<Observer> observers 

      = new ArrayList<Observer>();

   private int state;

   public int getState() {

      return state;

   }

   public void setState(int state) {

      this.state = state;

      notifyAllObservers();

   }

   public void attach(Observer observer){

      observers.add(observer);

   }

   public void notifyAllObservers(){

      for (Observer observer : observers) {

         observer.update();

      }

   }

}

Step 2

Create Observer class.

Observer.java

public abstract class Observer {

   protected Subject subject;

   public abstract void update();

}

Step 3

Create concrete observer classes

BinaryObserver.java

public class BinaryObserver extends Observer{

   public BinaryObserver(Subject subject){

      this.subject = subject;

      this.subject.attach(this);

   }

   @Override

   public void update() {

      System.out.println( "Binary String: " 

      + Integer.toBinaryString( subject.getState() ) ); 

   }

}

OctalObserver.java

public class OctalObserver extends Observer{

   public OctalObserver(Subject subject){

      this.subject = subject;

      this.subject.attach(this);

   }

   @Override

   public void update() {

     System.out.println( "Octal String: " 

     + Integer.toOctalString( subject.getState() ) ); 

   }

}

HexaObserver.java

public class HexaObserver extends Observer{

   public HexaObserver(Subject subject){

      this.subject = subject;

      this.subject.attach(this);

   }

   @Override

   public void update() {

      System.out.println( "Hex String: " 

      + Integer.toHexString( subject.getState() ).toUpperCase() ); 

   }

}

Step 4

Use Subject and concrete observer objects.

ObserverPatternDemo.java

public class ObserverPatternDemo {

   public static void main(String[] args) {

      Subject subject = new Subject();

      new HexaObserver(subject);

      new OctalObserver(subject);

      new BinaryObserver(subject);

      System.out.println("First state change: 15");

      subject.setState(15);

      System.out.println("Second state change: 10");

      subject.setState(10);

   }

}

Step 5

Verify the output:

First state change: 15

Hex String: F

Octal String: 17

Binary String: 1111

Second state change: 10

Hex String: A

Octal String: 12

Binary String: 1010

Builder pattern builds a complex object using simple objects and using a step by step approach. It creates one by one object and wraps on parent object. E.g. burger making, pizza making, house building etc.

Below we have created a Pizza restaurant where pizza and cold-drink is a typical meal. Pizza can be Italian, American, Mexican etc which will be packed in a wrapper and Cold-drink can be Pepsi, Coke, Limca etc which will be packed in a bottle.

Creating an Item interface representing food items such as pizzas and cold drinks and concrete classes implementing the Item interface and a Packing interface representing packaging of food items and concrete classes implementing the Packing interface as burger would be packed in wrapper and cold drink would be packed as bottle.

We then create a Meal class having ArrayList of Item and a MealBuilder to build different types of Meal object by combining Item. BuilderPatternDemo, our demo class will use MealBuilder to build a Meal.

Create an interface Item representing food item and packing.

Item.java

public interface Item {

   public String name();

   public Packing packing();

   public float price();

}

Packing.java

public interface Packing {

   public String pack();

}

Create concreate classes implementing the Packing interface.

Wrapper.java

public class Wrapper implements Packing {

   @Override

   public String pack() {

      return "Wrapper";

   }

}

Bottle.java

public class Bottle implements Packing {

   @Override

   public String pack() {

      return "Bottle";

   }

}

Create abstract classes implementing the item interface providing default functionalities.

Pizza.java

public abstract class Pizza implements Item {

   @Override

   public Packing packing() {

      return new Wrapper();

   }

   @Override

   public abstract float price();

}

ColdDrink.java

public abstract class ColdDrink implements Item {

@Override

public Packing packing() {

       return new Bottle();

}

@Override

public abstract float price();

}

Create concrete classes extending Pizza and ColdDrink classes

ItalianPizza.java

public class ItalianPizza extends Pizza {

   @Override

   public float price() {

      return 200.0f;

   }

   @Override

   public String name() {

      return "Italian Pizza";

   }

}

AmericanPizza.java

public class AmericanPizza extends Pizza {

   @Override

   public float price() {

      return 300.0f;

   }

   @Override

   public String name() {

      return "American Pizza";

   }

}

Coke.java

public class Coke extends ColdDrink {

   @Override

   public float price() {

      return 30.0f;

   }

   @Override

   public String name() {

      return "Coke";

   }

}

Pepsi.java

public class Pepsi extends ColdDrink {

   @Override

   public float price() {

      return 40.0f;

   }

   @Override

   public String name() {

      return "Pepsi";

   }

}

Create a Meal class having Item objects defined above.

Meal.java

import java.util.ArrayList;

import java.util.List;

public class Meal {

   private List<Item> items = new ArrayList<Item>();

   public void addItem(Item item){

      items.add(item);

   }

   public float getCost(){

      float cost = 0.0f;

      for (Item item : items) {

         cost += item.price();

      }

      return cost;

   }

   public void showItems(){

      for (Item item : items) {

         System.out.print("Item : "+item.name());

         System.out.print(", Packing : "+item.packing().pack());

         System.out.println(", Price : "+item.price());

      }

   }

}

Create a MealBuilder class, the actual builder class responsible to create Meal objects.

MealBuilder.java

public class MealBuilder {

   public Meal prepareVegMeal (){

      Meal meal = new Meal();

      meal.addItem(new ItalianPizza());

      meal.addItem(new Coke());

      return meal;

   }   

   public Meal prepareNonVegMeal (){

      Meal meal = new Meal();

      meal.addItem(new AmericanPizza());

      meal.addItem(new Pepsi());

      return meal;

   }

}

BuiderPatternDemo uses MealBuider to demonstrate builder pattern.

BuilderPatternDemo.java

public class BuilderPatternDemo {

   public static void main(String[] args) {

      MealBuilder mealBuilder = new MealBuilder();

      Meal vegMeal = mealBuilder.prepareVegMeal();

      System.out.println("Veg Meal");

      vegMeal.showItems();

      System.out.println("Total Cost: " +vegMeal.getCost());

      Meal nonVegMeal = mealBuilder.prepareNonVegMeal();

      System.out.println("nnNon-Veg Meal");

      nonVegMeal.showItems();

      System.out.println("Total Cost: " +nonVegMeal.getCost());

   }

}

Veg Meal

Item : Italian Pizza, Packing : Wrapper, Price : 200.0

Item : Coke, Packing : Bottle, Price : 30.0

Total Cost: 230.0

Non-Veg Meal

Item : American Pizza, Packing : Wrapper, Price : 300.0

Item : Pepsi, Packing : Bottle, Price : 40.0

Total Cost: 340.0