When to Use the Decorator Pattern?

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The Decorator Pattern is used for adding additional functionality to an existing object (i.e. already instantiated class at runtime), as opposed to object’s class and/or subclass. It is easy to add functionality to an entire class of objects by subclassing an object’s class, but it is impossible to extend a single object this way. With the Decorator Pattern, you can add functionality to a single object and leave others like it unmodified.

In Java, a classical example of the decorator pattern is the Java I/O Streams implementation.

FileReader       frdr = new FileReader(filename);
LineNumberReader lrdr = new LineNumberReader(frdr);

The preceding code creates a reader — lrdr — that reads from a file and tracks line numbers. Line 1 creates a file reader (frdr), and line 2 adds line-number tracking.

Actually, I’d highly encourage you to look at the Java source code for the Java I/O classes.

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