CompletableFutures are the rage today. We use them to describe complicated concurrent workflows. For example, HttpClient in Java 11 returns them when we send asynchronous requests. They can also describe parallel divide and conquer workflows. In this talk, we will start by parallelizing a large Fibonacci calculation with Fork/Join. We will then change java.math.BigInteger to multiply in parallel. You will learn how to patch with the new modules system. Then we will show the issues with Fork/Join and show a solution based on CompletableFuture. This talk will be interesting for intermediate to advanced Java programmers.