The Java language specification is open and there are multiple implementations. OpenJDK is the official open-source reference implementation, and many of the alternative implementations pull from upstream, but OpenJ9 is a different JVM implementation (though does currently use OpenJDK's class libraries to form a complete JDK.)
Before Microsoft opened-up C#, Mono was a completely independent alternative implementation.
Python has CPython (reference open source implementation), but also PyPy, MicroPython and several others.
Before Microsoft opened-up C#, Mono was a completely independent alternative implementation.
Python has CPython (reference open source implementation), but also PyPy, MicroPython and several others.