The Users' Notes for the NAG Library for Java are available here.
There is no Java-specific documentation. You should use the NAG Library documentation (FL Interface) for information relating to a specific routine and its arguments. The NAG Library Manual, Mark 27 is available in the following formats:
The Library is organized into Chapters – each being documented with its own Introduction and Contents list followed by a comprehensive document for each function detailing its purpose, description, list of parameters and possible error exits. Example programs and results are also supplied. All examples are available online to facilitate their use as templates for the users' calling programs.