Singleton is a most widely used design pattern. If a class has and only has one instance at every moment, we call this design as singleton. For example, for class Mouse (not a animal mouse), we should design it in singleton.
You job is to implement a
getInstance() method for given class, return the same instance of this class every time you call this method.
A a = A.getInstance(); A b = A.getInstance(); a == b // returns true