If you have an imperative (and probably object-oriented) programming background, this hands-on book will guide you through the alien world of functional programming. Author Joshua Backfield begins slowly by showing you how to apply the most useful implementation concepts before taking you further into functional-style concepts and practices. In each chapter, you'll learn a functional concept and then use it to refactor the fictional XXY company's imperative-style legacy code, writing and testing the functional code yourself. As you progress through the book, you'll migrate from Java 7 to Groovy and finally to Scala as the need for better functional language support gradually increases. Learn why today's finely tuned applications work better with functional code Transform imperative-style patterns into functional code, following basic steps Get up to speed with Groovy and Scala through examples Understand how first-class functions are passed and returned from other functions Convert existing methods into pure functions, and loops into recursive methods Change mutable variables into immutable variables Get hands-on experience with statements and nonstrict evaluations Use functional programming alongside object-oriented design.