Tutorial: Avoid Excessive Subclassing with the Decorator Design Pattern:

Avoid Excessive Subclassing with the Decorator Design Pattern

All developers apply design patterns of one kind or another. Some developers do this consciously; others apply patterns without even knowing it. Used appropriately, design patterns can help you develop software that's robust and easy to maintain. But design patterns aren't panaceas—if you apply a design pattern in the wrong context, you can do more harm than good. The best advice is to learn and understand design patterns before applying them to your software projects.

This article introduces the Decorator design 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).

The GoF book states:

"Design patterns are recurring solutions to software design problems you find again and again in real-world application development. Patterns are about the design and interaction of objects, as well as providing a communication platform concerning elegant, reusable solutions to commonly encountered programming challenges."

It's important to note that design patterns aren't only about design. Implied or not, the interaction of objects is also very important.

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 Decorator design pattern fits into the structural category and is one of the most widely used patterns. As a matter of fact, the Java API makes frequent use of the Decorator pattern.

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