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Factory Design Pattern in Java

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Welcome to the Factory Design Pattern in Java tutorial. Factory Pattern is one of the Creational Design pattern and it’s widely used in JDK as well as frameworks like Spring and Struts.

Factory Design Pattern

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Factory design pattern is used when we have a super class with multiple sub-classes and based on input, we need to return one of the sub-class. This pattern take out the responsibility of instantiation of a class from client program to the factory class.

Let’s first learn how to implement factory design pattern in java and then we will look into factory pattern advantages. We will see some of factory design pattern usage in JDK. Note that this pattern is also known as Factory Method Design Pattern.

Factory Design Pattern Super Class

Super class in factory design pattern can be an interface, abstract class or a normal java class. For our factory design pattern example, we have abstract super class with overridden toString() method for testing purpose.

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package com.journaldev.design.model;

public abstract class Computer {
	
	public abstract String getRAM();
	public abstract String getHDD();
	public abstract String getCPU();
	
	@Override
	public String toString(){
		return "RAM= "+this.getRAM()+", HDD="+this.getHDD()+", CPU="+this.getCPU();
	}
}

Factory Design Pattern Sub Classes

Let’s say we have two sub-classes PC and Server with below implementation.

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package com.journaldev.design.model;

public class PC extends Computer {

	private String ram;
	private String hdd;
	private String cpu;
	
	public PC(String ram, String hdd, String cpu){
		this.ram=ram;
		this.hdd=hdd;
		this.cpu=cpu;
	}
	@Override
	public String getRAM() {
		return this.ram;
	}

	@Override
	public String getHDD() {
		return this.hdd;
	}

	@Override
	public String getCPU() {
		return this.cpu;
	}

}

Notice that both the classes are extending Computer super class.

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package com.journaldev.design.model;

public class Server extends Computer {

	private String ram;
	private String hdd;
	private String cpu;
	
	public Server(String ram, String hdd, String cpu){
		this.ram=ram;
		this.hdd=hdd;
		this.cpu=cpu;
	}
	@Override
	public String getRAM() {
		return this.ram;
	}

	@Override
	public String getHDD() {
		return this.hdd;
	}

	@Override
	public String getCPU() {
		return this.cpu;
	}

}

Factory Class

Now that we have super classes and sub-classes ready, we can write our factory class. Here is the basic implementation.

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package com.journaldev.design.factory;

import com.journaldev.design.model.Computer;
import com.journaldev.design.model.PC;
import com.journaldev.design.model.Server;

public class ComputerFactory {

	public static Computer getComputer(String type, String ram, String hdd, String cpu){
		if("PC".equalsIgnoreCase(type)) return new PC(ram, hdd, cpu);
		else if("Server".equalsIgnoreCase(type)) return new Server(ram, hdd, cpu);
		
		return null;
	}
}

Some important points about Factory Design Pattern method are;

  1. We can keep Factory class Singleton or we can keep the method that returns the subclass as static.
  2. Notice that based on the input parameter, different subclass is created and returned. getComputer is the factory method.

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Here is a simple test client program that uses above factory design pattern implementation.

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package com.journaldev.design.test;

import com.journaldev.design.factory.ComputerFactory;
import com.journaldev.design.model.Computer;

public class TestFactory {

	public static void main(String[] args) {
		Computer pc = ComputerFactory.getComputer("pc","2 GB","500 GB","2.4 GHz");
		Computer server = ComputerFactory.getComputer("server","16 GB","1 TB","2.9 GHz");
		System.out.println("Factory PC Config::"+pc);
		System.out.println("Factory Server Config::"+server);
	}

}

Output of above program is:

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Factory PC Config::RAM= 2 GB, HDD=500 GB, CPU=2.4 GHz
Factory Server Config::RAM= 16 GB, HDD=1 TB, CPU=2.9 GHz

Factory Design Pattern Advantages

  1. Factory design pattern provides approach to code for interface rather than implementation.
  2. Factory pattern removes the instantiation of actual implementation classes from client code. Factory pattern makes our code more robust, less coupled and easy to extend. For example, we can easily change PC class implementation because client program is unaware of this.
  3. Factory pattern provides abstraction between implementation and client classes through inheritance.

Factory Design Pattern Examples in JDK

  1. java.util.Calendar, ResourceBundle and NumberFormat getInstance() methods uses Factory pattern.
  2. valueOf() method in wrapper classes like Boolean, Integer etc.

Factory Design Pattern YouTube Video Tutorial

I recently uploaded a video on YouTube for Factory Design pattern, please check it out. Please like and share the video and subscribe to my YouTube channel.

You can download the example code from my GitHub Project.
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Comments

  8. I understand that there’s always a benefit to get the instance of a class using the provided factory implementation.
    How can I force users to compulsorily use the factory implementation to get the instance, instead of getting away creating an instance directly using new operator of the actual class?
    i.e. How can I force people to use ServerFactory.getInstance() instead of new Server()?

    Reply

      • If you can make the constructor private as per singleton, then i assume that you cannot create the instance using new Server(); and only enforces the user to access the object only through getInstance(). Also as an additional rule getInstance() method can be enforced as a rule to be implemented in the sub-classes extending computer. i.e. an abstract method of getInstance(), be present in Computer class which is already an abstract class. Hope m right, others correct me if m wrong

        Reply

  12. Where below are implemented

    import com.journaldev.design.abstractfactory.PCFactory;
    import com.journaldev.design.abstractfactory.ServerFactory;

    Reply

  13. It’s a very good tutorial. But I have a doubt,

    You mentioned Calendar#getInstance() as factory pattern implementation. But in this there is a small difference right?

    There is no separate factory class. The super class Calendar itself is acting as the factory class.

    Does an implementation like this have any advantage or disadvantage?

    Reply

  14. Hi, is there a place in which I can download all the source code for the several design pattern examples.

    These are great examples, but I have to copy-paste each single text box into Intellij, and it is very cumbersome.

    Thank you very much, and congratulations for such good material.

    Reply

  16. I am relatively new to design patterns but I need to ask this question. What if we need to add another subclass of computer say Laptop to the application.? Does this mean we will have to modify the computer factory class? This looks like violating the OO principle which says classes should be closed to modification but open to extension.

    Reply

  17. It seems to me that you’re showing what is called a simple factory with ComputerFactory; It is not the Factory Method Pattern.
    The client TestFactory delegates the creation to another class which it is composed with.

    If you want to implement the Factory Method Pattern,:
    1. ComputerFactory should define an asbtract method getComputer(String ram, String hdd, String cpu)
    2. ComputerFactory should have two subclasses PCFactory and ServerFactory.which implements the superclass abstract method to return either a PC or a server
    3. The client should be given one of the two concrete factories and call the factory method to get PC or servers, depending which one was instanciated

    Reply

  20. Thanks for the clear explanation.
    I have one doubt here in Factory pattern. We have two concrete classes implementing the interface/Abstract class whose instances are created inside Factory class.But, instead of below line
    Computer pc = ComputerFactory.getComputer(“pc”,”2 GB”,”500 GB”,”2.4 GHz”);

    we can also use
    Computer pc=new PC(“pc”,”2 GB”,”500 GB”,”2.4 GHz”); to get new instance of PC. Then what is the advantage of using ComputerFactory.getComputer() method on the client side directly.?

    Reply

    • That’s why it’s called an creation all design pattern cause then we don’t have to dirty our code keeping the instance creation all logic here and there cause we have implanted a factory out there which is just doing it for us you just name it..name the object and it’s ready for you….

      Reply

    • Thats because u will be bound to the object, ie if you create Computer pc=new PC(“pc”,”2 GB”,”500 GB”,”2.4 GHz”) u will always get the instance of PC and it would be hardcoding, So if you use factory you wil not worry of the implementation u will always get the object of reference Computer.

      Reply

  21. Some body asked me that Why do we have to implement singleton pattern when we have static.
    And here in your post you say that either we can use static method or implement it as Singleton. Can you please detail on this?

    Reply

    • May be its a late reply, but worth share thought here.

      Factory classes(In general design patterns) are meant for maintainability and re-usability. Factory pattern states that, the objective of this pattern is to decouple the object instantiation from the client program. And this can be achieved either by static method or singleton factory class.

      Why we use singleton pattern when we have static?
      we use “static” when a piece of code/data same across all instances of a class OR important piece of code that needs to be executed even when class is not instantiated OR instantiation is not required.

      Question here is, how to make outer class itself static? the answer is “Singleton Pattern”.

      The “singleton pattern” is a mechanism, which gives the flexibility to reuse the same instance of a class(avoid multiple instantiation of a class when it is not required OR execute mandatory functionality only once) OR have the single instance to achieve the expected functionality.
      This can be achieved by using static + additional checks on the pre-existence of the instance(of self).
      Ex: Thread pool.

      Reply

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