jaw_value_get_increment doesn't call any method in java, it just calls atk_value_get_increment which is not useful.

What it should do is call a java method to get the minimum increment by which the value of an object can be changed. It returns zero the gdouble is undefined.[1]

Unfortunately, there is no obvious method to make useful in the AccessibleValue class, so the necessary logic will have to be written into the relevant AtkValue class function.[2] The java method (e.g. getIncrement) should ultimately return a double value and the jaw_value_get_increment function which calls it, returns a gdouble. 

[1] https://git.gnome.org/browse/atk/tree/atk/atkvalue.h#n115
[2] http://docs.oracle.com/javase/8/docs/api/javax/accessibility/AccessibleValue.html