Creational design patterns

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation.

  • Abstract Factory
    Creates an instance of several families of classes This pattern is one level of abstraction higher than factory pattern. This means that the abstract factory returns the factory of classes. Like Factory pattern returned one of the several sub-classes, this returns such factory which later will return one of the sub-classes.
  • Builder
    Separates object construction from its representation . Builder, as the name suggests builds complex objects from simple ones step-by-step. It separates the construction of complex objects from their representation. This is common to all the fast food restaurants and all the children meals. Here, what is important? Every time a children’s meal is ordered, the service boy will take a burger, a fries, a cold drink and a toy. Now suppose, there are 3 types of burgers available. Vegetable, Fish and Chicken, 2 types of cold drinks available. Cola and Orange and 2 types of toys available, a car and a doll. So, the order might be a combination of one of these, but the process will be the same. One burger, one cold drink, one fries and one toy. All these items are placed in a paper bag and is given to the customer. Now let’s see how we can apply software to this above mentioned example. The whole thing is called a children meal. Each of the four burger, cold drink, fries and toy are items in the meal. So, we can say, there is an interface Item having two methods, pack() and price().
  • Factory Method
    Creates an instance of several derived classes Let’s take a non-software example for this. Say, we have to plan for a children meal at a fast food restaurant. What is it comprised of? Well, a burger, a cold drink, a medium fries and a toy. Factory of Of classes. In simple words, if we have a super class and n sub-classes, and based on data provided, we have to return the object of one of the sub-classes, we use a factory pattern.
  • Object Pool
    Avoid expensive acquisition and release of resources by recycling objects that are no longer in use
  • Prototype
    A fully initialized instance to be copied or cloned . The prototype means making a clone. This implies cloning of an object to avoid creation. If the cost of creating a new object is large and creation is resource intensive, we clone the object. We use the interface Cloneable and call its method clone() to clone the object. Again let’s try and understand this with the help of a non-software example.  The example here we will take will be of a plant cell. This example is a bit different from the actual cloning in a way that cloning involves making a copy of the original one. Here, we break the cell in two and make two copies and the original one does not exist. But, this example will serve the purpose. Let’s say the Mitotic Cell Division in plant cells. Let’s take a class PlantCell having a method split(). The plant cell will implement the interface Cloneable.
  • Singleton
    A class of which only a single instance can exist This is one of the most commonly used patterns. There are some instances in the application where we have to use just one instance of a particular class. Let’s take up an example to understand this.A very simple example is say Logger, suppose we need to implement the logger and log it to some file according to date time. In this case, we cannot have more than one instances of Logger in the application otherwise the file in which we need to log will be created with every instance.We use Singleton pattern for this and instantiate the logger when the first request hits or when the server is started.

Rules of thumb

  1. Sometimes creational patterns are competitors: there are cases when either Prototype or Abstract Factory could be used profitably. At other times they are complementory: Abstract Factory might store a set of Prototypes from which to clone and return product objects, Builder can use one of the other patterns to implement which components get built. Abstract Factory, Builder, and Prototype can use Singleton in their implementation.
  2. Abstract Factory, Builder, and Prototype define a factory object that’s responsible for knowing and creating the class of product objects, and make it a parameter of the system. Abstract Factory has the factory object producing objects of several classes. Builder has the factory object building a complex product incrementally using a correspondingly complex protocol. Prototype has the factory object (aka prototype) building a product by copying a prototype object.
  3. Abstract Factory classes are often implemented with Factory Methods, but they can also be implemented using Prototype.
  4. Abstract Factory can be used as an alternative to Facade to hide platform-specific classes.
  5. Builder focuses on constructing a complex object step by step. Abstract Factory emphasizes a family of product objects (either simple or complex). Builder returns the product as a final step, but as far as the Abstract Factory is concerned, the product gets returned immediately.
  6. Builder is to creation as Strategy is to algorithm.
  7. Builder often builds a Composite.
  8. Factory Methods are usually called within Template methods.
  9. Factory Method: creation through inheritance. Prototype: creation through delegation.
  10. Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Prototype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed.
  11. Prototype doesn’t require subclassing, but it does require an Initialize operation. Factory Method requires subclassing, but doesn’t require Initialize.
  12. Designs that make heavy use of the Composite and Decorator patterns often can benefit from Prototype as well.

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