Not a direct answer, but you can let the calling microflow determine which page to show next depending on an enumeration value, which was set in the Java action. This lets Mendix deal with the nasty stuff and it makes the usage of the pages visible in the modeler. Great for later changes.
To open a form from java in Mx 5.13.1 you can use:
The first argument is the path to the xml representing the page in your deployment web directory. The second argument is how the form should be opened in this case I'm using the open in content option, other options are MODAL (blocking popup) and WINDOW (non blocking popup). Be aware that this method will give you deprecation warnings in the java code, but as the javadoc does not describe the method I haven't found the alternative yet.
For other options see this page for more detail, although it's the version 4 documentation it should give you more of an idea of what can be done with the addOpenFormFeedback and how to set the function arguments.