Simply awesome! I tried out several argument parsing libraries before I found argparse4j. It is just so much more elegant and easy to use. I will never use anything else for Java again.
Fantastic. Very good use of dynamic features of Java. Method chaining is also a great solution to Java's lack of kwargs. One improvement that would be nice, though, is the ability to specify multiple types in the .Type() method in the same way it can be done in the .setDefault() method. Not crucial by any means, but would make some tasks easier.