constructor: instance of the class.
            private constructor (Driver class)  ==> singleton class
            constructor can only be called within another constructor
               constructor will be executed when the object is created
            can be overloaded cannot be overridden
        
        static block: belongs to the class
                 executed as soon as the class is loaded
                 only gets executed once.
                
        instance block:  belongs to the object
                        gets executed when the object is created
                        executed before constructor