What is the disadvantage of using the Singleton pattern? It is enticing to use this pattern for all the classes as it makes it easy to get the reference of the singleton object.

The intent of the Singleton pattern is to ensure a class has only one instance and to provide a global point of access to it. True, the second part about providing a global point of access is enticing, but the primary disadvantage of using the Singleton pattern for all classes is related to the first part of the intent; i.e., that it allows only one instance of the class. For most classes in an application, you will need to create multiple instances. What purpose would a Customer class serve if you could create only one Customer object?