Where can I find Java projects for collaboration?

Question

Java

I do not have this experience, a cursory search also did not result in concrete success.
"At every corner" tell how great it is to participate in joint OpenSource projects that are interesting for yourself and possibly beneficial.
But apart from beautiful stories, I did not find anything concrete.
Tell a beginner where to look for Java projects in which you can participate for the benefit of society and to gain skills.
Thank you.

Answer 1

github.com
bitbucket.org
is full of open source projects

Answer 2

Is it really so difficult to find popular modules written in java on github?
Look at any project and pay attention to its dependencies