Mark Murphy

The ideal type of function in functional programming is a “pure” function. A pure function has no side effects, such as changing the contents of a database. If you call a pure function one million times with the same input, you should get the same output each of those million times.

Functional programming also emphasizes function composition, using “higher-order” functions. For example, you might have a sort() function that can sort some collection of data. However, sort() itself needs be supplied not only with the data but with some other function that knows how to compare two data elements to determine their order. sort(), therefore, might take two parameters:

The data to be sorted
Some identifier or reference to the function to use for order comparison
In this case, sort() is considered to be a “higher-order” function, as it takes another function as a parameter to help define the overall work to be performed.

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”.