It is just a repository to keep some of the code examples Some of the example problems are taken from Codility.com
https://hyperskill.org/study-plan is a great resource for Java learners! (keep in mind, it's paid to use for most of the classes) You can find a subscription for educators and students from IntelliJ (IntelliJ IDEA is an IDE of choice for this and many other projects written in JVM.)