In summary, open source Java is fine, open source almost-Java is not.
If you make a derivation/fork with the open source Java code Oracle only extends patent grants if your fork passes Oracle's expensive certification tests for Java. This certification is impractical for most people or organizations and is inherently impossible for anybody who is interested in making a new language with the code or adapting some of the code for other purposes.
- an alternative open source Java is quite difficult because the of the costs of certification
- use the open source Java codebase for anything else and Oracle can sue you for patent violations
- contributing to Oracle's existing OpenJDK codebase is fine