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.

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.

Problem Statement

Let’s 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 every time as it is costly. So, I will create their prototype instances, and every time when i will need a new instance, I will just clone the prototype.

Implementation of Prototype Design Pattern                                                             

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:

1.    Ability to clone prototypes without knowing their actual types

2.    Provides a type reference to be used in registry

Their workflow will look like this:

PrototypeCapable.java

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

Movie.java, Album.java and Show.java

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

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

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

                                   [Design Patterns Interview Questions]

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