Creational Patterns

- Factory Pattern


Factory of what? 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.

When to use a Factory Pattern?
The Factory patterns can be used in following cases:
1. When a class does not know which class of objects it must create.
2. A class specifies its sub-classes to specify which objects to create.
3. In programmer’s language (very raw form), you can use factory pattern where you have to create an object of any one of sub-classes depending on the data provided.


Abstract Factory Pattern


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.


When to use Abstract Factory Pattern?
One of the main advantages of Abstract Factory Pattern is that it isolates the concrete classes that are generated. The names of actual implementing classes are not needed to be known at the client side. Because of the isolation, you can change the implementation from one factory to another.


Singleton Pattern:

• Ensure unique instance by defining class final to prevent cloning.
• May be extensible by the subclass by defining subclass final.
• Make a method or a variable public or/and static.
• Access to the instance by the way you provided.
• Well control the instantiation of a class.
• Define one value shared by all instances by making it static.

Difference between static class and static method approaches:
One question which a lot of people have been asking me. What’s the difference between a singleton class and a static class? The answer is static class is one approach to make a class “Singleton”.

We can create a class and declare it as “final” with all the methods “static”. In this case, you can’t create any instance of class and can call the static methods directly.