Check out Kent C. Dodds excellent article on the subject. He mentions a few ways on how you can find a project for your first contribution.

For example, you can search for first-timers-only tag on github.

As a Java Developer of 7 years (though I am still in high school), I can tell you that the process isn't easy. I don't know of any particular projects, but you do have several options.

The first option is to search for projects on GitHub, and if you don't understand something in the project, google it. Learn why it's useful, and understand the concept. Then just repeat the process.

For looking for Java related GitHub projects, I recommend installing the Githunt Google Chrome Extension. Every time you open a new tab, it will show you the most popular projects of the week. You can filter the search results for Java.

Another option I highly recommend is learning Android app development, since a lot of projects related to Java are also related to this topic. Learning Android app development can also teach you a lot of high level concepts. This approach is heavy on the learn by doing side, but it's a lot of fun.

I assume you're using github. So I'd suggest that pick a simple repo and then if you don't understand a method or function or something, check the repo wiki/any supporting documentation and then if that fails then you should check google, search for it. That's all the part of learning :)