In jGrasp is there some sort of method of the views the javadoc documentation of a file?
Hit the documentation button on the toolbar (looks like an open book), or from menus "File" > "Generate Documentation". There are similar features to generate/view full project documentation if you use projects. Projectless documentation is created in your temp directory, while project-based documentation is stored permanently with the project.
You can't hover the mouse over a javadoc comment and view documentation for that comment immediately - a function that some IDEs provide.