Does someone know a good free application/plugin for Eclipse to transform the Java code into UML diagrams and vice versa?
Eclipse itself has a plug-in called Papyrus.
Papyrus is a UML tool that provide very advanced facilities for customizing the tool.
This plug-in supports both generating a UML diagram to Java source code and reversing Java source code to a UML diagram (reverse engineering). There is a well written documentation for end user to do those. Please, take a look the following links:
I don't know what a "good" application/plug-in means. But, since you said a plug-in for Eclipse, I hope this one will satisfy your needs.
Papyrus website itself provides very limited information on how to use it. I would recommend to install Eclipse, go to help, install new software -> Marc repo -> modelling -> Papyrus UML.
As for how to use it check UML Tutorial 3.0 - Basics of Java Class Diagrams in Eclipse with Papyrus (https://www.youtube.com/watch?v=Pim22rLtsW0).
Install, restart Eclipse, go to help -> install new software -> add http://download.eclipse.org/modeling/mdt/papyrus/updates/releases/mars
Remove group by category. Install:
Papyrus Java classes generator
Papyrus Java reverse (incubation)
Papyrus Profile for Java modules (Incubation)
Accept security, restart Eclipse.
Help-> Eclipse marketplace -> search for "QVT", install QVT operational.
As for how to use it check UML Tutorial 3.0 - Basics of Java Class Diagrams in Eclipse with Papyrus.