I don't define anything, there are Java standards which define source code, binary code and runtime behaviour compatibility. That makes it possible to run Java apps on non-Oracle JVMs, use non-Oracle tools, etc. Android doesn't have anything Java outside of source code. And even Java source code is not 100% compatible. It's just not Java at all and never was. You can't even use many open source Java libraries on Android because they are not Android compatible at the source level.