Want to contribute? Great! First, read this page (including the small print at the end).
Before we can use your code, you must sign the Google Individual Contributor License Agreement (CLA), which you can do online. The CLA is necessary mainly because you own the copyright to your changes, even after your contribution becomes part of our codebase, so we need your permission to use and distribute your code. We also need to be sure of various other things—for instance that you'll tell us if you know that your code infringes on other people's patents. You don't have to sign the CLA until after you've submitted your code for review and a member has approved it, but you must do it before we can put your code into our codebase. Before you start working on a larger contribution, you should get in touch with us first through the issue tracker with your idea so that we can help out and possibly guide you. Coordinating up front makes it much easier to avoid frustration later on.
Contributions made by corporations are covered by a different agreement than the one above, the Software Grant and Corporate Contributor License Agreement.
All submissions, including submissions by project members, require review.
We use Github pull requests. In your request, please give a short explanation of what you are trying to do and why. In the git commit message for the patch, the first line should be a summary of the whole patch. After that, explain the patch in more detail.
For code style, we follow the Google Java Style Guide. We currently use an 80-column limit.
We use Java 8 including all of the new functional fanciness. However, we try to be reasonable and use it only when it actually makes things simpler and clearer.
There is rough documentation of the current design in Design.md.