What open source projects would you recommend to gain experience in C#, Java, AngularJS or Android?

Question

Java

You need to gain experience in the languages, technologies and frameworks that are in demand today (my main language is C ++, in which it is increasingly difficult to find a job). In this connection, in fact, the question.

Answer 1

https://github.com/

Answer 2

hmm, maybe you still decide what exactly you want?)