Having a login form is arguably one of the most common requirements in web applications. In this chapter, you will learn how to implement a reusable and extensible login form that supports multiple languages, and see the advantages of favoring composition over extension when implementing UI components. Through examples, we'll discuss why extension is not always the best approach, and we'll explore several alternatives for implementing custom server-side UI components with Vaadin.

This chapter covers the following topics:

• Extending layout components
• Using the Composite class
• Externalization of Java Strings