Modular programming means dividing an application into small parts and then developing it. It is an approach taken by developers to build applications and helps them add efficiency in their development process, thus making it more effective. The book starts with the fundamentals of Modular Programming. Then we move on to the actual implementation, where we teach developers how to divide an application into different modules or layers (such as presentation, execution, security, lifecycle, services, and so on) for better management. Once readers are well-versed in these modules and their development, the book shows how to create bindings in order to join these different modules and form a complete application. Next, the readers will learn how to manage these modules through dependency injection. Later, we move on to testing; readers will learn how to test the different modules of an application. The book ends by teaching readers how to maintain different versions of their application and how to modify it. By the end of the book, readers will have a good understanding of modular programming and will be able to use it to build applications with Java.