This book describes one of the best technique for code testing. This Technique detects duplicate code and extracts it into one procedure which can be called in another program as per the requirements. Programmers often take this approach because it is quicker and safer, at least in the immediate term, than the other approach of factoring out the code that is common to the old and new features into a separate procedure, and calling this procedure from both places. This is especially true when, due to differences between the existing and new features, it is necessary to restructure the existing code to tease out the part that is needed by the new feature.