J++ did not officially support some parts of the Java standard library, e.g. RMI, JNI. J++ has some extensions not supported in standard Java for Windows specific support and C++-like callbacks. Standard Java uses delegates.

Microsoft concentrates on writing a fast interpreter. Other companies have focussed on JITs and native compilers. If you ask questions in the comp.lang.java.programmer  newsgroup and it comes out you are using J++, be prepared for a shower of tomatoes. They will be annoyed with you for wasting their time with a product that is deliberately not compatible with standard Java and that even Microsoft abandoned years ago. You can’t even download the JVM anymore, much less buy the IDE. The general attitude is, if you have compatibility problems, you more than deserved them for using a primarily Windows-only product that was designed to kill Java.