The main difference between UI and java is that UI should be proficient in artists. If you are interested in graphic color design, you can consider UI.

Java is mainly logical, and you should also know something about UI. For example, some small companies don't have specialized UI and artists, so you need to design it yourself or look for it online.

In fact, the Java you are in contact with is just the tip of the iceberg, and there are still many things, such as some open source frameworks and so on.