What causes java.lang.IncompatibleClassChangeError?

I'm packaging a Java library as a JAR, and it's throwing many java.lang.IncompatibleClassChangeError s when I try to invoke methods from it. These errors seem to appear at random. What kinds of problems could be causing this error?

Get files from Jar which is on the repository without downloading the whole Jar from Java

I would like to access the jar file on the repository, search inside it for the certain files, retrieve those files and store them on my hard disc. I don't want to download the whole jar and then to search for it.

Unjaring a .jar file in Objective-C

I was wondering if it was possible to extract the contents of a .jar file into a different directory, then add some files to that directory, and then compress the directory back into a .jar file, with the additions. A way of extracting the jar file i found was to use NSTask to execute the terminal command of jar xf (location of jar file) . This worked, but it halted the UI of the program permanently (without crashing it), and unarchived the contents into the directory that the application was stored in, which was a slight mess to clean up. Is there another way of doing this, or am I using NSTask incorrectly? Here is the code I am using: