Factory method design pattern in Java
It is a creational design pattern that talks about the creation of an object. The factory design pattern says that define an interface ( A java interface or an abstract class) for creating object and let the subclasses decide which class to instantiate. The factory method in the interface lets a class defers the instantiation to one or more concrete subclasses. Since these design patterns talk about the instantiation of an object and so it comes under the category of creational design pattern. If we notice the name Factory method, that means there is a method which is a factory, and in general, factories are involved with creational stuff and here with this, an object is being created. It is one of the best ways to create an object where object creation logic is hidden from the client. Now Let’s look at the implementation.
Implementation:
- Define a factory method inside an interface.
- Let the subclass implements the above factory method and decides which object to create.
- In Java, constructors are not polymorphic, but by allowing subclass to create an object, we are adding polymorphic behavior to the instantiation. In short, we are trying to achieve Pseudo polymorphism by letting the subclass to decide what to create, and so this Factory method is also called a virtual constructor.
Let us try to implement it with a real-time problem and some coding exercises.
Problem Statement: Consider we want to implement a notification service through email, SMS, and push notifications. Let’s try to implement this with the help of the factory method design pattern. First, we will design a UML class diagram for this.
In the above class diagram, we have an interface called Notification, and three concrete classes are implementing the Notification interface. A factory class NotificationFactory is created to get a Notification object. Let’s jump into the coding now.
Create Notification interface
Creating factory classes in java
Я понимаю что в 1м примере с кофе нужно разделить создание экземпляров от приготовления кофе. Мне не понятно, а не проще ли просто было создать метод createCoffee в этом же классе CoffeeShop, а не городить дополнительный класс? Я не настаиваю что я прав, но хочу понять что бы разобраться. public class CoffeeShop < public Coffee createCoffee (CoffeeType type) < Coffee coffee = null; switch (type) < case AMERICANO: coffee = new Americano(); break; case ESPRESSO: coffee = new Espresso(); break; case CAPPUCCINO: coffee = new Cappuccino(); break; case CAFFE_LATTE: coffee = new CaffeLatte(); break; >return coffee; > public Coffee orderCoffee(CoffeeType type) < /*Coffee coffee = null; switch (type) < case AMERICANO: coffee = new Americano(); break; case ESPRESSO: coffee = new Espresso(); break; case CAPPUCCINO: coffee = new Cappuccino(); break; case CAFFE_LATTE: coffee = new CaffeLatte(); break; >*/ Coffee coffee = createCoffee(type); coffee.grindCoffee(); coffee.makeCoffee(); coffee.pourIntoCup(); System.out.println(«Вот ваш кофе! Спасибо, приходите еще!»); return coffee; > >
У данного примера с кофе, мы все также не решаем проблему маштабирования. Если у нас появляется новый вид кофе, то нам нужно вносить изменения в 2 класса SimpleCoffeeFactory и CoffeeType. «Если ассортимент изменится, нам не придется править код везде, где будет использоваться создание кофе. Достаточно будет изменить код только в одном месте.» — остается проблемой.
А как это дело запустить то? В мэйне мы же не можем создать экземпляр кофешоп. Статик сделать тоже не получается. Как вывести то в консоль что кофе готово?
Хотя все могло быть еще проще, если сделать метод createCoffee статичным. Но тогда мы потеряли бы две возможности: 1. Наследоваться от SimpleCoffeeFactory и переопределять метод createCoffee . Почему не будет возможности наследоваться и тем более переопределять метод, если createCoffe будет с модификатором static?
Использовать switch тоже не совсем хорошая практика. Это претензия не к автору, почему то во всех примерах используют case. Но это как-то не красиво, да и в реальных проектах я такого не встречал
Java Factory Pattern Explained
What is the most usual method of creating an instance of a class in java? Most people will answer this question: “using new keyword“. Well, it is considered old-fashioned now. Let’s see how??
If object creation code is spread in the whole application, and if you need to change the process of object creation then you need to go in each and every place to make necessary changes. After finishing this article, while writing your application, consider using the Java factory pattern.
In my previous post, “Singleton design pattern in java“, we discussed various ways to create an instance of a class such that there can not exist another instance of same class in same JVM.
In this post, I will demonstrate another creational pattern, i.e. Factory pattern, for creating instances for your classes. Factory, as the name suggests, is a place to create some different products which are somehow similar in features yet divided into categories.
In Java, factory pattern is used to create instances of different classes of the same type.
Table of Contents 1. When to implement factory pattern? 2. Factory Pattern Implementation 3. Advantages of factory pattern 4. Final notes
1. When to use factory pattern?
Factory pattern introduces loose coupling between classes which is the most important principle one should consider and apply while designing the application architecture. Loose coupling can be introduced in application architecture by programming against abstract entities rather than concrete implementations. This not only makes our architecture more flexible but also less fragile.
A picture is worth a thousand words. Let’s see how a factory implementation will look like.
Above class-diagram depicts a common scenario using an example of a car factory which is able to build 3 types of cars i.e. small, sedan and luxury. Building a car requires many steps from allocating accessories to final makeup. These steps can be written in programming as methods and should be called while creating an instance of a specific car type.
If we are unfortunate then we will create instances of car types (e.g. SmallCar ) in our application classes and thus we will expose the car building logic to the outside world and this is certainly not good. It also prevents us in making changes to car making process because the code is not centralized, and making changes in all composing classes seems not feasible.
2. Java Factory Pattern Example
So far we have design the classes need to be designed for making a CarFactory. Let’s create them now.
2.1. Object types
CarType will hold the types of car and will provide car types to all other classes.
package designPatterns.creational.factory; public enum CarType
2.2. Object implementations
Car is parent class of all car instances and it will also contain the common logic applicable in car making of all types.
package designPatterns.creational.factory; public abstract class Car < public Car(CarType model) < this.model = model; arrangeParts(); >private void arrangeParts() < // Do one time processing here >// Do subclass level processing in this method protected abstract void construct(); private CarType model = null; public CarType getModel() < return model; >public void setModel(CarType model) < this.model = model; >>
LuxuryCar is concrete implementation of car type LUXURY .
package designPatterns.creational.factory; public class LuxuryCar extends Car < LuxuryCar() < super(CarType.LUXURY); construct(); >@Override protected void construct() < System.out.println("Building luxury car"); // add accessories >> SmallCar is concrete implementation of car type SMALL .
package designPatterns.creational.factory; public class SmallCar extends Car < SmallCar() < super(CarType.SMALL); construct(); >@Override protected void construct() < System.out.println("Building small car"); // add accessories >> SedanCar is concrete implementation of car type SEDAN .
package designPatterns.creational.factory; public class SedanCar extends Car < SedanCar() < super(CarType.SEDAN); construct(); >@Override protected void construct() < System.out.println("Building sedan car"); // add accessories >> 2.3. Factory to create objects
CarFactory.java is our main class implemented using factory pattern. It instantiates a car instance only after determining its type.
package designPatterns.creational.factory; public class CarFactory < public static Car buildCar(CarType model) < Car car = null; switch (model) < case SMALL: car = new SmallCar(); break; case SEDAN: car = new SedanCar(); break; case LUXURY: car = new LuxuryCar(); break; default: // throw some exception break; >return car; > >
2.4. Test factory pattern
In TestFactoryPattern , we will test our factory code. Lets run this class.
package designPatterns.creational.factory; public class TestFactoryPattern < public static void main(String[] args) < System.out.println(CarFactory.buildCar(CarType.SMALL)); System.out.println(CarFactory.buildCar(CarType.SEDAN)); System.out.println(CarFactory.buildCar(CarType.LUXURY)); >>
Building small car designPatterns.creational.factory.SmallCar@7c230be4 Building sedan car designPatterns.creational.factory.SedanCar@60e1e567 Building luxury car designPatterns.creational.factory.LuxuryCar@e9bfee2
As you can see, the factory is able to return any type of car instance it is requested for. It will help us in making any kind of changes in car making process without even touching the composing classes i.e. classes using CarFactory .
3. Benefits of factory pattern
By now, you should be able to count the main advantages of using the factory pattern. Let’s note down:
- The creation of an object precludes its reuse without significant duplication of code.
- The creation of an object requires access to information or resources that should not be contained within the composing class.
- The lifetime management of the generated objects must be centralized to ensure a consistent behavior within the application.
Factory pattern is most suitable where there is some complex object creation steps are involved. To ensure that these steps are centralized and not exposed to composing classes, factory pattern should be used. We can see many realtime examples of factory pattern in JDK itself e.g.
I hope, I have included enough information in this Java factory pattern example to make this post informative.
If you still have some doubt on abstract factory design pattern in Java, please leave a comment. I will be happy to discuss with you.
Design Pattern — Factory Pattern
Factory pattern is one of the most used design patterns in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.
In Factory pattern, we create object without exposing the creation logic to the client and refer to newly created object using a common interface.
Implementation
We’re going to create a Shape interface and concrete classes implementing the Shape interface. A factory class ShapeFactory is defined as a next step.
FactoryPatternDemo, our demo class will use ShapeFactory to get a Shape object. It will pass information (CIRCLE / RECTANGLE / SQUARE) to ShapeFactory to get the type of object it needs.
Step 1
Step 2
Create concrete classes implementing the same interface.
public class Rectangle implements Shape < @Override public void draw() < System.out.println("Inside Rectangle::draw() method."); >> public class Square implements Shape < @Override public void draw() < System.out.println("Inside Square::draw() method."); >> public class Circle implements Shape < @Override public void draw() < System.out.println("Inside Circle::draw() method."); >> Step 3
Create a Factory to generate object of concrete class based on given information.
public class ShapeFactory < //use getShape method to get object of type shape public Shape getShape(String shapeType)< if(shapeType == null)< return null; >if(shapeType.equalsIgnoreCase("CIRCLE")) < return new Circle(); >else if(shapeType.equalsIgnoreCase("RECTANGLE")) < return new Rectangle(); >else if(shapeType.equalsIgnoreCase("SQUARE")) < return new Square(); >return null; > > Step 4
Use the Factory to get object of concrete class by passing an information such as type.
FactoryPatternDemo.java
public class FactoryPatternDemo < public static void main(String[] args) < ShapeFactory shapeFactory = new ShapeFactory(); //get an object of Circle and call its draw method. Shape shape1 = shapeFactory.getShape("CIRCLE"); //call draw method of Circle shape1.draw(); //get an object of Rectangle and call its draw method. Shape shape2 = shapeFactory.getShape("RECTANGLE"); //call draw method of Rectangle shape2.draw(); //get an object of Square and call its draw method. Shape shape3 = shapeFactory.getShape("SQUARE"); //call draw method of square shape3.draw(); >> Step 5
Inside Circle::draw() method. Inside Rectangle::draw() method. Inside Square::draw() method.