Creational Patterns - Builder Pattern

Builder, as the name suggests builds complex objects from simple ones step-by-step. It separates the construction of complex objects from their representation.

Let’s take a non-software example for this. Say, we have to plan for a children meal at a fast food restaurant. What is it comprised of? Well, a burger, a cold drink, a medium fries and a toy.

This is common to all the fast food restaurants and all the children meals. Here, what is important? Every time a children’s meal is ordered, the service boy will take a burger, a fries, a cold drink and a toy. Now suppose, there are 3 types of burgers available. Vegetable, Fish and Chicken, 2 types of cold drinks available. Cola and Orange and 2 types of toys available, a car and a doll.

So, the order might be a combination of one of these, but the process will be the same. One burger, one cold drink, one fries and one toy. All these items are placed in a paper bag and is given to the customer.

Now let’s see how we can apply software to this above mentioned example.

The whole thing is called a children meal. Each of the four burger, cold drink, fries and toy are items in the meal. So, we can say, there is an interface Item having two methods, pack() and price().

Let’s take a closer look at the interface Item:

Item.java

package creational.builder; public interface Item {
	/** * pack is the method, as every item will be packed * in a different way. * E.g.:- The burger will be packed as wrapped in a paper * The cold drink will be given in a glass * The medium fries will be packed in a card box and * The toy will be put in the bag straight. * The class Packing is an interface for different types of * for different Items. */ public Packing pack(); /** * price is the method as all the items * burger, cold drink, fries will have a price. * The toy will not have any direct price, it will * be given free with the meal. * * The total price of the meal will be the combined * price of the three items. * * @return price, int in rupees. */ public int price();
}// End of class

So, we must now have a class defined for each of the items, as burger, toy, cold drink and fries. All these will implement the Item interface.

Lets start with Burger:

Burger.java

package creational.builder; /** * The class remains abstract as price method will be implemented * according to type of burger. * @see price() * */ public abstract class Burger implements Item {
	/** * A burger is packed in a wrapper. Its wrapped * in the paper and is served. The class Wrapper is * sub-class of Packing interface. * @return new Wrapper for every burger served. */ public Packing pack() { return new Wrapper(); } /** * This method remains abstract and cannot be * given an implementation as the real implementation * will lie with the type of burger. * * E.g.:- Veg Burger will have a different price from * a fish burger. * * @return price, int. */ public abstract int price();
}// End of class

The class Burger can be further extended to VegBurger, FishBurger, ChickenBurger etc. These classes will each implement the price() method and return a price for each type of burger. I, in this example have given the implementation for VegBurger class.

VegBurger.java

package creational.builder; /** * The implementation of price method. */ public class VegBurger extends Burger {
	/** * This is the method implementation from * the super class Burger. * * @return price of a veg burger in rupees. */ public int price() { return 39; }
}// End of class

Let’s concentrate on other items now. I, here for explanation purpose will give another item Fries.

Fries.java

package creational.builder; /** * Implements the Item interface. */ public class Fries implements Item {
	/** * Packing in which fries are served. * * @return new Packing for every fries. * Envelop is a packing in which fries are given */ public Packing pack() { return new Envelop(); } /** * Price of the medium fries. * * @return int , price of medium fries in rupees */ public int price() { return 25; }
}// End of class

Now, let’s see the Builder class, MealBuilder. This class is the one which serves the Children’s meal.

MealBuilder.java

package creational.builder; /** * Main builder class which builds the entire meal * for the customers */ public class MealBuilder {
	public Packing additems() { Item[] items = {new VegBurger(), new Fries(), new Cola(), new Doll()} return new MealBox().addItems(items); } public int calculatePrice() { int totalPrice = new VegBurger().price() + new Cola().price() + new Fries().price() + new Doll().price(); return totalPrice; }
}// End of class

This class gives the total meal and also presents the total price. Here, we have abstracted the price calculation and meal package building activity from the presentation, which is a meal box. The Builder pattern hides the internal details of how the product is built.

Each builder is independent of others. This improves modularity and makes the building of other builders easy.

Because, each builder builds the final product step by step, we have more control on the final product.