Tutorial: Keeping Your Java Objects Informed with the Observer Design Pattern:

Keeping Your Java Objects Informed with the Observer Design Pattern

This article, the second in a series on design patterns, introduces the Observer pattern, one of the 23 design patterns defined in the legendary 1995 book Design Patterns—Elements of Reusable Object-Oriented Software. The authors of this book, Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, are known affectionately as the "Gang of Four." (So popular is this book that the Gang of Four has its own acronym—GoF.)

Design Patterns

The GoF book defines 23 design patterns. The patterns fall into three categories:

A creational pattern abstracts the instantiation process.
A structural pattern groups objects into larger structures.
A behavioral pattern defines better communication among objects.

The Observer design pattern fits into the behavioral category. Like the Decorator design pattern, the Observer pattern is one of the most widely used patterns. As you read this article, you may recognize the Observer pattern as an important element of the Java Standard API.

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