CHAPTER 1 An Initial Tour Java is a language that defines a runtime environment in which user-defined classes execute. Instances of these user-defined classes may represent real-world data that is stored in a database, filesystem, or mainframe transaction processing system. Addi- tionally, small-footprint environments often require a means of managing persistent data in local storage. Because data-access techniques are different for each type of data source, accessing the data presents a challenge to application developers, who need to use a different appli- cation programming interface (API) for each type of data source. This means that you need to know at least two languages to develop business logic for these data sources: