Builder Design Pattern

Builder pattern builds a complex object using simple objects and using a step by step approach. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.
A Builder class builds the final object step by step. This builder is independent of other objects.

Implementation 
We've considered a business case of fast-food restaurant where a typical meal could be a burger and a cold drink. Burger could be either a Veg Burger or Chicken Burger and will be packed by a wrapper. Cold drink could be either a coke or pepsi and will be packed in a bottle.
 We're going to create an Item interface representing food items such as burgers 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 Mealobject by combining Item. BuilderPatternDemo, our demo class will use MealBuilder to build a Meal.

Class Diagram









Steps
Use the following steps to implement the above mentioned design pattern.

Step 1
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();
}

Step 2
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";
            }
}

Step 3
Create abstract classes implementing the item interface providing default functionalities.
Burger.java
public abstract class Burger 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();
}

Step 4
Create concrete classes extending Burger and ColdDrink classes.
VegBurger.java
public class VegBurger extends Burger {
            @Override
            public float price() {
                        return 25.0f;
            }
            @Override
            public String name() {
                        return "Veg Burger";
            }
}

ChickenBurger.java
public class ChickenBurger extends Burger {
            @Override
            public float price() {
                        return 50.5f;
            }
            @Override
            public String name() {
                        return "Chicken Burger";
            }
}
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 35.0f;
            }
            @Override
            public String name() {
                        return "Pepsi";
            }
}

Step 5
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());
                        }
            }
}

Step 6
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 VegBurger());
                        meal.addItem(new Coke()); return meal;
            }
            public Meal prepareNonVegMeal (){
                        Meal meal = new Meal();
                        meal.addItem(new ChickenBurger());
                        meal.addItem(new Pepsi()); return meal;
            }
}

Step 7
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("\n\nNon-Veg Meal");
                        nonVegMeal.showItems();
                        System.out.println("Total Cost: " +nonVegMeal.getCost());
            }
}
Step 8
Verify the output.
Veg Meal Item : Veg Burger, Packing : Wrapper, Price : 25.0
Item : Coke, Packing : Bottle, Price : 30.0 Total Cost: 55.0

Non-Veg Meal
Item : Chicken Burger, Packing : Wrapper, Price : 50.5
Item : Pepsi, Packing : Bottle, Price : 35.0 Total Cost: 85.5