QWFIX_J packages all class files into a single jar file, QWFIX_J.jar.
QWFIX_J is implemented in pure Java. It follows exactly the same design as QWFIX.Net. Most classes and methods share the same names. However the naming convention of two libraries are not the same. In .Net, the first character of a method is in upper case, while in Java it is in lower case. For example, the method FIXSession.SendMessage in QWFIX.Net has a corresponding method FIXSession.sendMessage in QWFIX_J.
There is also a slight difference between the Java package name and .Net namespace. For example, the Teraspaces.QWFIX namespace in .Net has a corresponding Java package name of com.teraspaces.qwfix.
Apart from the difference in naming conventions, the QWFIX.Net and QWFIX_J almost share the same set of classes and underlying methods, only with a few exceptions. Please read the API documentation for more details.