An Observer Pattern says that "just define a one-to-one dependency so that when one object changes state, all its dependents are notified and updated automatically".
The observer pattern is also known as Dependents or Publish-Subscribe.
UML for Observer Pattern:
Implementation of Observer Pattern
Create a ResponseHandler1 class the will implement the java.util.Observer interface.
Create a ResponseHandler2 class the will implement the java.util.Observer interface.
Create an EventSource class that will extend the java.util.Observable class .