We will use RxJava 2.2.7 (http://reactivex.io) in our examples. It can be added to the project by the following dependency:

<dependency>
    <groupId>io.reactivex.rxjava2</groupId>
    <artifactId>rxjava</artifactId>
    <version>2.2.7</version>
</dependency>

Let's first compare two implementations of the same functionality using the java.util.stream package and the io.reactivex package. The sample program is going to be very simple:

• Create a stream of integers 1,2,3,4,5.
• Filter only even numbers (2 and 4).
• Calculate a square root of each of the filtered numbers.
• Calculate the sum of all the square roots.

Here is how it can be implemented using the java.util.stream package:

double a = IntStream.rangeClosed(1, 5)
                    .filter(i -> i % 2 == 0)
                    .mapToDouble(Double::valueOf)
             ...