Here's and add-on to Asgeir's answer. Here's the link that I found.
http://www.java2s.com/Code/Jar/s/sequence.htm
Run from the command line ... "java -jar sequence.jar" ... this is a Java application with a GUI.
The help section says:
SEQUENCE is a program for producing UML Sequence Diagrams. In contrast to most similar programs you don't actually draw the diagram. Instead you write a textual description of the method calls you want to diagram and the layout is calculated and drawn automatically.
So this tool doesn't reverse engineer anything, but I can see how it might be helpful if you wanted to quickly diagram things from scratch. Looks like it was built in 2002 and I think there are probably better tools out there now.
Here's another similar tool here:
http://sdedit.sourceforge.net/example/index.html