Callbacks also handle events in Java GUI programming, in the form of objects implementing the ActionListener interface that are registered with GUI components. The Java code for specifying the behavior of even a simple counter button (like Fig. 2) is notably complex. In contrast, such simple behaviors can be taught in the very first week with App Inventor. In general, App Inventor's paired methods/event handlers hide numerous complexities in the underlying Android operating system and allow complex behaviors, such as web requests, to be taught to students in CS0 or CS1.