HashMaps are very common both in Java and the wider programming world as a whole.
You can mostly get away with just knowing how to use them and what they do, but if you want to take a deeper dive this video is a good start.
<iframe width="560" height="315" src="https://www.youtube.com/embed/y11XNXi9dgs?si=mo-elh1WW75-uJv6" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>