Most systems are not built from scratch. Most will use third party libraries, either commercial products or open source utilities.

A good example of this in the Java world is log4J – many projects use it for their logging requirements, and its a good tool. However what most projects will do is use it directly, which means that virtually every class in the system will have a dependency on log4J. If a need arises that log4J can’t handle, or a new version is released that changes some behaviour your project depends on, it can be a huge job to go through all your code making changes.

A useful approach is to wrap all access to 3rd party code yourself, thus giving you a single point of contact between your system’s code and the external library. This wrapper is also a useful place to put any extension functionality that your system needs.