Filtering data

In the first chapter's reactive sum example, we were filtering the user input, based on a special pattern. The pattern was, for example, a: <number>. It is common to filter only interesting bits of data from the data stream. For example, it's useful to filter out <enter> key-down events only from all key-down events, or only lines containing a given expression from a file. That's why it is important to not only be able to transform our data but also to learn how to filter it.

There are many filtering operators in RxJava. The most important of these operators is filter(). Its marble diagram is very simple and is shown here:

It shows that the filter() operator filters the data by some property. In the diagram, it's the form of the elements: it filters only circles. Like all the other operators, filter() creates a new Observable instance from the source. This Observable instance emits only items that comply to the condition, defined by the filter...