Insert Contents of 1 Zip into another Zip with java

Question

Okay, So I need a way to take A.zip and insert its contents into b.zip In java. Also I need it to overwrite any files in b.zip that exist in a.zip.

Now, I've been trying for hours to do this with TrueZip. And I've tried a billion different ways only to all end In some sort of logic error. It just doesn't seem possible. But yet I know 7-zip can do it. Any help or hints would be vastly appreciated.