Builder pattern - Java

Guidelines for Builder design pattern in Java

1) Make a static nested class called Builder inside the class whose object will be build by Builder. In this example its Cake.

2) Builder class will have exactly same set of fields as original class.

3) Builder class will expose method for adding ingredients e.g. sugar() in this example. each method will return same Builder object. Builder will be enriched with each method call.

4) Builder.build() method will copy all builder field values into actual class and return object of Item class.

5) Item class (class for which we are creating Builder) should have private constructor to create its object from build() method and prevent outsider to access its constructor.

public class BuilderPatternExample {
 
    public static void main(String args[]) {
     
        //Creating object using Builder pattern in java
        Cake whiteCake = new Cake.Builder().sugar(1).butter(0.5).  eggs(2).vanila(2).flour(1.5). bakingpowder(0.75).milk(0.5).build();
     
        //Cake is ready to eat :)
        System.out.println(whiteCake);
    }
}

class Cake {

    private final double sugar;   //cup
    private final double butter;  //cup
    private final int eggs;
    private final int vanila;     //spoon
    private final double flour;   //cup
    private final double bakingpowder; //spoon
    private final double milk;  //cup
    private final int cherry;

    public static class Builder {

        private double sugar;   //cup
        private double butter;  //cup
        private int eggs;
        private int vanila;     //spoon
        private double flour;   //cup
        private double bakingpowder; //spoon
        private double milk;  //cup
        private int cherry;

        //builder methods for setting property
        public Builder sugar(double cup){this.sugar = cup; return this; }
        public Builder butter(double cup){this.butter = cup; return this; }
        public Builder eggs(int number){this.eggs = number; return this; }
        public Builder vanila(int spoon){this.vanila = spoon; return this; }
        public Builder flour(double cup){this.flour = cup; return this; }
        public Builder bakingpowder(double spoon){this.sugar = spoon; return this; }
        public Builder milk(double cup){this.milk = cup; return this; }
        public Builder cherry(int number){this.cherry = number; return this; }
     
     
        //return fully build object
        public Cake build() {
            return new Cake(this);
        }
    }

    //private constructor to enforce object creation through builder
    private Cake(Builder builder) {
        this.sugar = builder.sugar;
        this.butter = builder.butter;
        this.eggs = builder.eggs;
        this.vanila = builder.vanila;
        this.flour = builder.flour;
        this.bakingpowder = builder.bakingpowder;
        this.milk = builder.milk;
        this.cherry = builder.cherry;      
    }

    @Override
    public String toString() {
        return "Cake{" + "sugar=" + sugar + ", butter=" + butter + ", eggs=" + eggs + ", vanila=" + vanila + ", flour=" + flour + ", bakingpowder=" + bakingpowder + ", milk=" + milk + ", cherry=" + cherry + '}';

    }
 
}

Output:
Cake{sugar=0.75, butter=0.5, eggs=2, vanila=2, flour=1.5, bakingpowder=0.0, milk=0.5, cherry=0}

Builder design pattern in Java – Pros and Cons

Live everything Builder pattern also has some disadvantages, but if you look at below, advantages clearly outnumber disadvantages of Builder design pattern. Any way here are few advantages and disadvantage of Builder design pattern for creating objects in Java.

Advantages:

1) more maintainable if number of fields required to create object is more than 4 or 5.

2) less error-prone as user will know what they are passing because of explicit method call.

3) more robust as only fully constructed object will be available to client.

Disadvantages:

1) verbose and code duplication as Builder needs to copy all fields from Original or Item class.

When to use Builder Design pattern in Java

Builder Design pattern is a creational pattern and should be used when number of parameter required in constructor is more than manageable usually 4 or at most 5. Don't confuse with Builder and Factory pattern there is an obvious difference between Builder and Factory pattern, as Factory can be used to create different implementation of same interface but Builder is tied up with its Container class and only returns object of Outer class.

That's all on Builder design pattern in Java. we have seen why we need Builder pattern , what problem it solves, Example of builder design pattern in Java and finally when to use Builder patter with pros and cons.

Read more: http://javarevisited.blogspot.com/2012/06/builder-design-pattern-in-java-example.html#ixzz3v1lzfv00
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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 have 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 are 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 Meal objects by combining Item. BuilderPatternDemo, our demo class will use MealBuilder to build a Meal.

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 concrete 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

Source: http://www.tutorialspoint.com/design_pattern/builder_pattern.htm