Decorator/Wrapper Pattern

Decorator Pattern is a very simple pattern in which a decorator object is wrapped around an object, so that new behaviour/responsibility is added to the original object without changing the original object.

Two simple and good sources for decorator pattern are:

http://java-x.blogspot.com/2007/01/implementing-decorator-pattern-in-java.html

http://www.allapplabs.com/java_design_patterns/decorator_pattern.htm

In Liferay, we use the concept of hooks for liferay extensions. We can extend liferay services using hooks with the help of decorator pattern.