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Observer Pattern

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.


  • It describes the coupling between the objects and the observer.
  • It provides the support for broadcast-type communication.


  • When the change of a state in one object must be reflected in another object without keeping the objects tight coupled.
  • When the framework we writes and needs to be enhanced in future with new observers with minimal chamges.

UML for Observer Pattern:

Observer Pattern

Implementation of Observer Pattern

Step 1:

Create a ResponseHandler1 class the will implement the java.util.Observer interface.

Step 2:

Create a ResponseHandler2 class the will implement the java.util.Observer interface.

Step 3:

Create an EventSource class that will extend the java.util.Observable class .


Observer Pattern

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