Core Java Design Pattern : Behavioral Pattern : Observer Design Pattern

• Observer Design Pattern
• Examples

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Also Known As : Dependents, Publish-Subscribe

This kind of interaction is also known as publish-subscribe. The subject is the publisher of notifications. It sends out these notifications without having to know who its observers are. Any number of observers can subscribe to receive notifications.

Use the Observer pattern in any of the following situations:

• When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently.
• When a change to one object requires changing others, and you don’t know how many objects need to be changed.
• When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don’t want these objects tightly coupled.

The spreadsheet and the bar chart don’t know about each other, thereby letting you reuse only the one you need. But they behave as though they do. When the user changes the information in the spreadsheet, the bar chart reflects the changes immediately, and vice versa.

The Observer defines a one-to-many relationship so that when one object changes state, the others are notified and updated automatically. Some auctions demonstrate this pattern. Each bidder possesses a numbered paddle that is used to indicate a bid. The auctioneer starts the bidding, and “observes” when a paddle is raised to accept the bid. The acceptance of the bid changes the bid price which is broadcast to all of the bidders in the form of a new bid.

UML Diagram for Interest update Observer Application having Implementation of Observer Design Pattern

Download:

Observer Design Pattern Code (13 KB)