Book excerpt: This book describes the principles of model building in financial engineering. It explains those models as designs and working implementations for Java-based applications. The book provides software professionals with an accessible source of numerical methods or ready-to-use code for use in business applications. It is the first book to cover the topic of Java implementations for finance/investment applications and is written specifically to be accessible to software practitioners without prior accountancy/finance training. The book develops a series of packaged classes explained and designed to allow the financial engineer complete flexibility.