java-design-patterns/prototype/README.md

---
layout: pattern
title: Prototype
folder: prototype
permalink: /patterns/prototype/
categories: Creational
language: en
tags: 
 - Gang Of Four
 - Instantiation
---

## Intent

Specify the kinds of objects to create using a prototypical instance, and create new objects by 
copying this prototype.

## Explanation

First it should be noted that Prototype pattern is not used to gain performance benefits. It's only 
used for creating new objects from prototype instance.

Real world example

> Remember Dolly? The sheep that was cloned! Lets not get into the details but the key point here is 
> that it is all about cloning.

In plain words

> Create object based on an existing object through cloning.

Wikipedia says

> The prototype pattern is a creational design pattern in software development. It is used when the 
> type of objects to create is determined by a prototypical instance, which is cloned to produce new 
> objects.

In short, it allows you to create a copy of an existing object and modify it to your needs, instead 
of going through the trouble of creating an object from scratch and setting it up.

**Programmatic Example**

In Java, it can be easily done by implementing `Cloneable` and overriding `clone` from `Object`

```java
class Sheep implements Cloneable {
  private String name;
  public Sheep(String name) { this.name = name; }
  public void setName(String name) { this.name = name; }
  public String getName() { return name; }
  @Override
  public Sheep clone() {
    try {
      return (Sheep)super.clone();
    } catch(CloneNotSuportedException) {
      throw new InternalError();
    }
  }
}
```

Then it can be cloned like below:

```java
var original = new Sheep("Jolly");
System.out.println(original.getName()); // Jolly

// Clone and modify what is required
var cloned = original.clone();
cloned.setName("Dolly");
System.out.println(cloned.getName()); // Dolly
```

## Class diagram

![alt text](./etc/prototype.urm.png "Prototype pattern class diagram")

## Applicability

Use the Prototype pattern when a system should be independent of how its products are created, 
composed, represented and

* When the classes to instantiate are specified at run-time, for example, by dynamic loading.
* To avoid building a class hierarchy of factories that parallels the class hierarchy of products.
* When instances of a class can have one of only a few different combinations of state. It may be 
more convenient to install a corresponding number of prototypes and clone them rather than 
instantiating the class manually, each time with the appropriate state.
* When object creation is expensive compared to cloning.

## Real world examples

* [java.lang.Object#clone()](http://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#clone%28%29)

## Credits

* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)