ntro to Design Patterns: Prototype Pattern

Today we’re going to look at the Prototype design pattern.? Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. – Gof

Type

Object Creational

Solution

The Prototype pattern creates a new object by cloning an existing
object. The client using the prototype object does not need to know
what kind of object it is dealing with as long as the concrete
prototype extends or implements the prototype interface/class. The
concrete prototype object is responsible for cloning itself and
returning the cloned object.

The pattern enables a client to create the kind of object required
at runtime by selecting the appropriate prototype. The prototype
classes are created generically by the client without the client
knowing the exact type of the concrete prototype. New concrete
prototypes can be added at runtime as long as they conform to the
abstract prototype.

Structure

Java Sample Code

Example 1:

Download Example 1

The following is an example of the Prototype Pattern. The prototype object is an Animalobject. The Animal prototype contains two concrete prototype subclasses called Sheepand Chicken. The AnimalCreator class contains references to the two concrete prototypes. During the initialization of the AnimalCreator class the two concrete prototypes, Sheep and Chicken are created and stored as the two concrete prototypes members of the AnimalCreator class. The AnimalCreator class contains a retrieveAnimalmethod that clones a prototype Animal depending on the parameter that is passed to it.

Animal.java
The Animal class is the
abstract prototype of the two concrete prototypes in the example. The
client invokes methods on the two different concrete prototypes through
the Animal type to ensure the client does not know the type of the concrete prototypes.

Most importantly, the Animal prototype defines a clone method to
assist the two subtypes or concrete prototypes to clone themselves.

Code:

public Animal clone() {     Animal clonedAnimal = null;  try {   clonedAnimal = (Animal) super.clone();   clonedAnimal.setDescription(description);   clonedAnimal.setNumberOfLegs(numberOfLegs);   clonedAnimal.setName(name);  } catch (CloneNotSupportedException e) {  e.printStackTrace();  } // catch  return clonedAnimal; } // method clone

Sheep.java
The Sheep object is a concrete prototype that extends the Animal prototype. TheSheep prototype has a clone method to clone itself to create a new object.

Code:

public class Sheep extends Animal {

Chicken.java
The Chicken object is a concrete prototype that extends the Animal prototype. TheChicken prototype has a clone method to clone itself to create a new object.

Code:

public class Chicken extends Animal {

AnimalCreator.java
The AnimalCreator class is used to create and manage prototype objects.
The AnimalCreator class contains two concrete prototypes that are
initialized during the initialization of the class. The AnimalCreator
class forms part of the "Prototype" pattern by returning a cloned
object (Animal) to the client without the client knowing the type of
the prototype.

Code:

public Animal retrieveAnimal(String kindOfAnimal) {   if ("Chicken".equals(kindOfAnimal)) {    return (Animal) chicken.clone();  } else if ("Sheep".equals(kindOfAnimal)) {   return (Animal) sheep.clone();  } // if    return null; } // method retrieveAnimal

AnimalClient.java
The AnimalClient class makes use of the AnimalCreator class to create a concrete prototypes of type Animal. The AnimalClient class does not know the type of the concrete prototypes but references them through the Animal prototype.

Code:

AnimalCreator animalCreator = new AnimalCreator();  Animal[] animalFarm = new Animal[8]; animalFarm[0] = animalCreator.retrieveAnimal("Chicken");animalFarm[1] = animalCreator.retrieveAnimal("Chicken"); animalFarm[2] = animalCreator.retrieveAnimal("Chicken"); animalFarm[3] = animalCreator.retrieveAnimal("Chicken"); animalFarm[4] = animalCreator.retrieveAnimal("Sheep"); animalFarm[5] = animalCreator.retrieveAnimal("Sheep"); animalFarm[6] = animalCreator.retrieveAnimal("Sheep"); animalFarm[7] = animalCreator.retrieveAnimal("Sheep"); for (int i= 0; i<=7; i++) {  System.out.println(animalFarm[i].helloAnimal()); } // for

Console
The following is the output of the AnimalClient calling different concrete prototype objects.

Cluck cluck World. I am Chicken1. I have 2 legs.
Cluck cluck World. I am Chicken2. I have 2 legs.
Cluck cluck World. I am Chicken3. I have 2 legs.
Cluck cluck World. I am Chicken4. I have 2 legs.
Meeeeeee World. I am Sheep1. I have 4 legs.
Meeeeeee World. I am Sheep2. I have 4 legs.
Meeeeeee World. I am Sheep3. I have 4 legs.
Meeeeeee World. I am Sheep4. I have 4 legs.


Class Diagram Example

Sequence Diagram

Sequence Diagram by "Yanic Inghelbrecht" with Trace Modeler