Prototype design pattern in java

A prototype is a template of any object before the actual object is constructed. In java also, it holds the same meaning. Prototype design pattern is used in scenarios where application needs to create a number of instances of a class, which has almost same state or differs very little.

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In this design pattern, an instance of actual object (i.e. prototype) is created on starting, and thereafter whenever a new instance is required, this prototype is cloned to have another instance. The main advantage of this pattern is to have minimal instance creation process which is much costly than cloning process.

Sections in this post:
Design participants
Problem statement
Implementation

Important: Please ensure that you want to deep clone or shallow clone your prototype because both will have different behavior on runtime. If deep copy is needed, you can use a good technique given here using in memory serialization.

Design participants

  • Prototype : This is the prototype of actual object as discussed above.
  • Prototype registry : This is used as registry service to have all prototypes accessible using simple string parameters.
  • Client : Client will be responsible for using registry service to access prototype instances.

Problem statement

Lets understand this pattern using an example. I am creating an entertainment application that will require instances of Movie, Album and Show classes very frequently. I do not want to create their instances everytime as it is costly. So, I will create their prototype instances, and everytime when i will need a new instance, I will just clone the prototype.

Implementation of prototype pattern

Lets start by creating class diagram.

prototype-pattern-class-diagram

Above class diagram explains the necessary classes and their relationship.

Only one interface, “PrototypeCapable” is new addition in solution. The reason to use this interface is broken behavior of Cloneable interface. This interface helps in achieving following goals:

  • Ability to clone prototypes without knowing their actual types
  • Provides a type reference to be used in registry

Their workflow will look like this:

Prototype pattren sequence diagram

Lets hit the keyboard and compose these classes.

PrototypeCapable.java

package com.howtodoinjava.prototypeDemo.contract;

public interface PrototypeCapable extends Cloneable
{
	public PrototypeCapable clone() throws CloneNotSupportedException;
}

Movie.java, Album.java and Show.java

package com.howtodoinjava.prototypeDemo.model;

import com.howtodoinjava.prototypeDemo.contract.PrototypeCapable;

public class Movie implements PrototypeCapable
{
	private String name = null;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@Override
	public Movie clone() throws CloneNotSupportedException {
		System.out.println("Cloning Movie object..");
		return (Movie) super.clone();
	}
	@Override
	public String toString() {
		return "Movie";
	}
}

public class Album implements PrototypeCapable
{
	private String name = null;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@Override
	public Album clone() throws CloneNotSupportedException {
		System.out.println("Cloning Album object..");
		return (Album) super.clone();
	}
	@Override
	public String toString() {
		return "Album";
	}
}

public class Show implements PrototypeCapable
{
	private String name = null;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@Override
	public Show clone() throws CloneNotSupportedException {
		System.out.println("Cloning Show object..");
		return (Show) super.clone();
	}
	@Override
	public String toString() {
		return "Show";
	}
}

PrototypeFactory.java

package com.howtodoinjava.prototypeDemo.factory;

import com.howtodoinjava.prototypeDemo.contract.PrototypeCapable;
import com.howtodoinjava.prototypeDemo.model.Album;
import com.howtodoinjava.prototypeDemo.model.Movie;
import com.howtodoinjava.prototypeDemo.model.Show;

public class PrototypeFactory
{
	public static class ModelType
	{
		public static final String MOVIE = "movie";
		public static final String ALBUM = "album";
		public static final String SHOW = "show";
	}
	private static java.util.Map<String , PrototypeCapable> prototypes = new java.util.HashMap<String , PrototypeCapable>();

	static
	{
		prototypes.put(ModelType.MOVIE, new Movie());
		prototypes.put(ModelType.ALBUM, new Album());
		prototypes.put(ModelType.SHOW, new Show());
	}

	public static PrototypeCapable getInstance(final String s) throws CloneNotSupportedException {
		return ((PrototypeCapable) prototypes.get(s)).clone();
	}
}

TestPrototypePattern

package com.howtodoinjava.prototypeDemo.client;

import com.howtodoinjava.prototypeDemo.factory.PrototypeFactory;
import com.howtodoinjava.prototypeDemo.factory.PrototypeFactory.ModelType;

public class TestPrototypePattern
{
	public static void main(String[] args)
	{
		try
		{
			String moviePrototype  = PrototypeFactory.getInstance(ModelType.MOVIE).toString();
			System.out.println(moviePrototype);

			String albumPrototype  = PrototypeFactory.getInstance(ModelType.ALBUM).toString();
			System.out.println(albumPrototype);

			String showPrototype  = PrototypeFactory.getInstance(ModelType.SHOW).toString();
			System.out.println(showPrototype);

		}
		catch (CloneNotSupportedException e)
		{
			e.printStackTrace();
		}
	}
}

When you run the client code, following is the output.

Cloning Movie object..
Movie
Cloning Album object..
Album
Cloning Show object..
Show

I hope, you liked this post. If any question, drop a comment.

Download source code

Happy Learning !!

16 thoughts on “Prototype design pattern in java”

      1. Hi Lokesh,

        Can you please let me know why creating instance is more costly than cloning? In your cloning session you have not provided the explanation. Please reply.

        Thanks,
        Kunal

        1. Well creating instances is not expensive all the times. It is only when object creation process involve other expensive processes such as opening DB connection, Opening network connection or doing some IO operations in file system etc. It can be expensive too when it involves creating many other instances as well inform of associated properties.

          In above cases, creating instances is mostly expensive process in comparison to cloning an existing object.

  1. Very good article,easy to understand the design patterns with real examples and also with perfect class diagram.
    Really useful

  2. I didn’t see much of value of introducing PrototypeCapable interface to enable clone() method, which is build-in in Object. It is not the contract obligation broken on Cloneable in java, it is the implementation was build-in for a purpose to make sure the Clone mechanism is working even across inheritance hierarchy, otherwise, the type inheritance will be mismatch. That means, there is no free way to correctly implement “clone” behavior defined in PrototypeCapable interface other than call “super.clone()” which binding to Object.clone build-in mechanism. What’s the point?

    1. In java, to make an object cloning supported, it takes two steps: 1) Implement Cloneable 2) Override clone()

      Lets say, a new developer joined a team which knows only basics in java. He is assigned a task in which he has to add one more object in prototype registry(factory) i.e. “Ticket”.

      By which design, you can force that developer to perform above both steps??

      It is only possible when you tie both steps together otherwise behavior will be surprising.

      1. Lokesh,
        So, in other words what you mean to say is the only purpose of this prototype pattern is to introduce an interface which actually has the method clone( ), that somehow Cloneable interface missed in it, so that the rule of overriding the clone( ) method is followed in future.

        That’s the only only only thing in this whole jargon I guess. That’s it. Nothing else. I wonder why we create so many classes and confuse things when all we are doing here is just creating one interface! This whole thing is confusing and not worth the effort.

        By the way, that little java programmer has to learn stuff, we cannot design software components keeping worries of junior programmers in focus. Someone who can spend 1 or 2 hours to understand how java is handling the concept of cloning its objects can easily understand how to override clone and why it’s important to call the clone of super class.

        Frankly, I think this is a design pattern is confusing and not worth studying and remembering, rather programmers should spend time on understanding the basics of cloning in java, that is enough.

        Your thoughts?

        Thanks,
        Saurabh

        1. Hi Saurabh, Thanks for sharing your thoughts. Much appreciated.

          I believe that design patterns will feel like a burden if someone will try to apply them forcefully. They are solution of some specific problem. Until you encounter the problem in your design/implementation, you shouldn’t inject patterns just to make code fancy. I agree with you that simplicity is the best thing you can do with your code.

          Regarding prototype design pattern, I do not feel that it is not worth studying. Yes, proper use of clone() will also achieve the target, but you need some code to wrap the actual logic somewhere centralized. So when in future, a junior programmer find a tiny fault in cloning process, he does not have to change it multiple places.

          Remember that if you have to change you code in more than place, to solve one specific problem, then your code need redesign/refactoring.

  3. Dear Lokesh
    ModelType is static subclass of PropertyFactory class ,so Following correction is required in running class TestPropertytypePattern

    String moviePrototype = PrototypeFactory.getInstance(PrototypeFactory.ModelType.MOVIE).toString();

    rest is fine ,I have tested it

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