Prototype

About 2 min

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 the Prototype pattern is not used to gain performance benefits. It's only
used for creating new objects from prototype instances.

Real-world example

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

In plain words

Create an 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, the prototype pattern is recommended to be implemented as follows. First, create an
interface with a method for cloning objects. In this example, Prototype interface accomplishes
this with its copy method.

public abstract class Prototype<T> implements Cloneable {
    @SneakyThrows
    public T copy() {
        return (T) super.clone();
    }
}

Our example contains a hierarchy of different creatures. For example, let's look at Beast and
OrcBeast classes.

@EqualsAndHashCode(callSuper = false)
@NoArgsConstructor
public abstract class Beast extends Prototype<Beast> {

  public Beast(Beast source) {
  }

}

@EqualsAndHashCode(callSuper = false)
@RequiredArgsConstructor
public class OrcBeast extends Beast {

  private final String weapon;

  public OrcBeast(OrcBeast orcBeast) {
    super(orcBeast);
    this.weapon = orcBeast.weapon;
  }

  @Override
  public String toString() {
    return "Orcish wolf attacks with " + weapon;
  }

}

We don't want to go into too many details, but the full example contains also base classes Mage
and Warlord and there are specialized implementations for those for elves in addition to orcs.

To take full advantage of the prototype pattern, we create HeroFactory and HeroFactoryImpl
classes to produce different kinds of creatures from prototypes.

public interface HeroFactory {
  
  Mage createMage();
  Warlord createWarlord();
  Beast createBeast();
}

@RequiredArgsConstructor
public class HeroFactoryImpl implements HeroFactory {

  private final Mage mage;
  private final Warlord warlord;
  private final Beast beast;

  public Mage createMage() {
    return mage.copy();
  }

  public Warlord createWarlord() {
    return warlord.copy();
  }

  public Beast createBeast() {
    return beast.copy();
  }
}

Now, we are able to show the full prototype pattern in action producing new creatures by cloning
existing instances.

    var factory = new HeroFactoryImpl(
        new ElfMage("cooking"),
        new ElfWarlord("cleaning"),
        new ElfBeast("protecting")
    );
    var mage = factory.createMage();
    var warlord = factory.createWarlord();
    var beast = factory.createBeast();
    LOGGER.info(mage.toString());
    LOGGER.info(warlord.toString());
    LOGGER.info(beast.toString());

    factory = new HeroFactoryImpl(
        new OrcMage("axe"),
        new OrcWarlord("sword"),
        new OrcBeast("laser")
    );
    mage = factory.createMage();
    warlord = factory.createWarlord();
    beast = factory.createBeast();
    LOGGER.info(mage.toString());
    LOGGER.info(warlord.toString());
    LOGGER.info(beast.toString());

Here's the console output from running the example.

Elven mage helps in cooking
Elven warlord helps in cleaning
Elven eagle helps in protecting
Orcish mage attacks with axe
Orcish warlord attacks with sword
Orcish wolf attacks with laser

Class diagram

alt text — 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.

Known uses

java.lang.Object#clone()open in new window