Singleton

• This design pattern ensure that a class has only one instance, while providing a global access point to this instance.

Problem

• How to controll access to some shared resource like file or database? May be using single object shared between all & that object locks resource internally.
• How to handle global variables or data? It should not happen that some code is overwriting contents of those variables and crashing app.

Solution

• Don't allow direct creation of object. (Make default constructor private)
• Create a method (static) which acts as a constructor. Under the hood, method calls private constructor to create an object and saves it. All following calls return cached object.