Here is some information that's good to know when contributing to JavaParser:

• if you're new and like to contribute something, check out the easy issues.
• we work on JavaParser because we like to, not because it earns us money. Please remember that we try to run a professional project in our spare time, on a budget of zero.
• if you know how to fix a problem, please fix it and open a pull request instead of opening an issue.
• if you start working on an issue, please say so with a comment in the issue.
• a pull request should include tests. You can either use BDD (more information here) or JUnit.
• every pull request will automatically be checked by a few tools. Make sure AppVeyor and Travis are green.
• there is some interesting information on the wiki.
• be sure to check the coding guidelines which are easily used by installing the formatting rules as described there.
• pull requests often stay open for at least a few days to give people a chance to review it.
• a pull request is merged when all comments on it have been resolved.
• if you create a pull request for an issue, mention the issue in the format #123 to make github link it automatically.

Thanks for helping!