Observer Design Pattern In Java

Today, I will discuss simple and very useful behavioral design pattern called — Observer Design Pattern. This design pattern is useful when we want get notified about changes in the object state.

Observer Design Pattern

• The Observer Design Pattern maintains one-to-many dependency between Subject (Observable) and its dependents (Observer) in such a way that whenever state of Subject changes, its dependents get notified.
• The Observer Design Pattern is a design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. 
• The Observer Design Pattern is used when we like to have notification upon changes in the objects state.
• The Observer Design Pattern is one of twenty-three well known Gang of Four design patterns that defines an one-to-many dependency objects so that when one object changes state, all of its dependents get notified and updated automatically.
• The Observer Design Pattern is mainly used to implement distributed event handling , in "event driven" system.
• In such systems, the subject is usually called a "source of events", while the observers are called "sink of events".
• Most modern languages have built-in "event" constructs which implement the observer pattern components.
• Java also offers Observer and Observable to support built-in event constructs for implementing observer pattern. These are available since Java 1. 
• But, in Java 9 these are declared deprecated/obsolete because the event model supported by Observer and Observable is quite limited, the order of notifications delivered by Observable is unspecified, and state changes are not in one-for-one correspondence with notifications. For a richer event model, consider using the java.beans package. For reliable and ordered messaging among threads, consider using one of the concurrent data structures in the java.util.concurrent package. For reactive streams style programming, see the Flow API.  (read more on Deprecate Observer and Observable).