CompletableFutures
Write a small GUI program which allows the user to enter a URL and hit a button. The program will then in the background fetch the HTML code from the URL and display it in the GUI. This should not block the user interface. For fetching the page, supplyAsync is an option. thenAccept could be used to update the GUI. For a GUI, Swing is the standard choice for Java. Hint: Have a look at this for issues to watch out for when updating a swing GUI.
You can find a GUI implementation to work with here.
Akka
Following this tutorial, let's try to get Akka installed and working. The easiest way to get akka installed in Eclipse seems to be to create a simple maven project with no prototype and add this dependency to the pom.xml file:
<dependencies>
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-actor_2.12</artifactId>
<version>2.5.11</version>
</dependency>
</dependencies>