I've seen this question already on the maling list, but couldn't find any answer to it:
I'm trying to extend a java class with Jython and compile it with Jythonc. Whatever I define as __init__ functions and @sig signature strings, jythonc always generate a call to the parent class's default constructor.... which then raises a java compilation error if the parent default constructor is private !

Is it possible to avoid this without tampering with the parent java class ?

Could this be done when you were doing it in Java?
You might try and call non private constructor, if there is any; jythonc
should then suppress calling the private constructor.

In case you are using java 1.2 or later you can also use a jython
option to allow access to private and protected class members:
(iirc, see Jython's registry).

FYI, the parent class I'm trying to extend is a JUnit TestCase.

You could perhaps revert to PyUnit, which works well in jython.
See module unittest (it is based on JUnit's design).
Python's multiple inheritance is a nice boon when writing test scripts.
Also someone wrote a test runner for swing iirc.

Have fun,