Mark Murphy

In Android, JavaFx, and other GUI environments, updates to the UI are single-threaded, with a specific thread being responsible for those updates. Typically, we have to do any substantial work on background threads, so we do not tie up the UI thread and prevent it from updating the UI.

Callbacks are simply objects representing chunks of code to be invoked when a certain condition occurs. In the case of asynchronous operations, the “certain condition” often is “when the operation completes, successfully or with an error”.

“Callback hell” occurs when you have lots of nested callbacks, such as this Java snippet: