Questions about the Prototype Pattern
I am learning about the different design patterns and I have a strong feeling I am missing an essential piece (or pieces) in understanding this particular pattern.
In all the websites I have viewed and in the GoF book, I see the clone method. From what I understand, we have some type of object that we can clone when we need varying versions of that object, but we don't want to have to manually create each one using the "new" command (as in Java). This can hide its concrete implementation. So when we clone, we can tweak the clone just a little bit and make it what we need without having to knowing how to originally create that object the hard way. Is this my thinking correct?
I am also told that this can reduce subclassing and subsequently reduce the number of classes you need to make. I don't quite understand this part. Could someone help me grasp this?
My final question is about the abstract factory (or even the factory method) pattern. These factory patterns and the prototype pattern feel like they attempt to hide concrete implementations upon creation of new objects. When is it a good idea to choose one of the other?
Thank you all!
Answer
Prototype pattern
Prototype results in a cloned object which is different from the original object. The state of the original is the same as the clone, at the time of cloning. Thereafter each object may undergo state change. You can think of this as something similar photocopying the original and then modifying the photocopy at a few places.Example
- DVD duplication: Duplication of the master dvd to create several copies
- Reporting object: Consider a report object that contains processed information to be passed to the GUI. The original report contains the data in ascending order. Now, using this pattern one can create a similar report but with data sorted in descending order.
Benefits
- Performance: Cloning (using MemberwiseClone) is considerably less expensive than creating a new object afresh (with new operator). Note that one needs to override the
MemberwiseClose()to perform a deep copy. - Objects can be cloned very dynamically, without any insistence on up-front instantiation. The first created object can be created at any time in the application execution, and further duplication can take place at any time ahead.
When to use it
- When the classes to instantiate are specified at run-time, for example, by dynamic loading.
- 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.
Comparison with Factory Pattern
Prototype pattern allows an object to create customized objects without knowing their class or any details of how to create them. So, it is this aspect it appears to be a lot like the Factory Method pattern. In both these patterns, the client can create any of the derived class objects without knowing anything about their own structure.
But the difference between the two patterns is the fact that the Factory Method concentrates on creating one object of a non existing object type as a fresh creation (by understanding the exact sub-type of the Creator class). The Prototype pattern uses the class itself, especially the derived class for self duplication action.
Factory Method pattern
In this pattern, the client (or consumer) asks the Creator (or factory) for a specific type of object from a class hierarchy. The Creator method of the factory class delegates the creation of the specific object to the derived classes and returns the object of the class of the type asked by client. In essence, you have a single point of contact for the creation of several objects of a class hierarchy.You can think of this as going to an airline ticket counter (controller) and asking for a ticket by giving your preference of the ticket type (first class, executive or economy). The user is not concerned with how the ticket is being generated, even though in an object representation the first class and the economy ticket are both derived from the base ticket class.
When to use
- Flexibility is important (low coupling)
- Objects can be extended in subclasses
- There is a specific reason why one subclass would be chosen over another—this logic forms part of the Factory Method.
- A client delegates responsibilities to subclasses in parallel hierarchies.
Abstract factory pattern
Abstract factory goes a step higher (more abstract) than the factory method pattern. In this case, one can have not just a single, but multiple factories with slight variations. It is responsible for creating objects belonging families of class hierarchies rather than just a single class hierarchy.A specific Factory class already exists. But the Factory will have slightly varying methods. Each method can produce an instance. The client can choose appropriate method and get the instance.
If you take the example of MVC based perfect Architectural Design, the client will be a Business Controller Class while Concrete Products will all be Business Entities. The Factories are Auxiliary ( Helper) Controllers. They work in association with a request from the Business Controller.
When to use
- The system is expected to be independent of how its products are created. It may even expect independence on how the products are composed and represented. The term product applies to the finally resulting object that a client developer would need to make use of by invoking its methods.
- The system that should be configurable with one of the multiple families of products. So the actual selection of the family will not be at coding time but at a later configuration time.
- The family of products is designed to work always together.
- The creation is for a library of products. What is cared more here is the relevant interface and not the implementation.