With InterClient 7.5.1 and 8.1.5, the creation of a new JDBC connection in Java 8 fails with
java.lang.NoClassDefFoundError: sun/io/ByteToCharConverter
I agree with Sergio, this is now an even older question! :-) But at my workplace, too, we are unfortunately as of 2018 for various reasons still using a super-old Interbase (v9, 2009). So at some point I too was unable to continue using the interclient.jar packaged with IBv9, because I would receive this same sun.io.ByteToCharConverter error. No doubt this was because the Java on my client machine was so much newer now. After all, it is 9 years later. But I did not want in my case to go with the solution of installing a legacy Java on my machine, as has worked for others.
So as a solution, on my client machine (Linux/Debian x64), I installed Interbase 2017, the latest at the time(*). But when installing, I only installed the IB Client (not Server + Client). The /opt/interbase/lib/interclient.jar that is packaged with the IB 2017 client works just fine now (no sun.io error). And yes, this IB 2017 (v13) interclient.jar works just fine connecting to our old IB 2009 server.
(I have to hand it to Embarcadero: They are still 'supporting' being able to connect to older versions -- at least in their interclient.jar and libgds.so. This makes sense as useful of course for those sysadmins who want to migrate from older systems to newer ones.)
(*) You can get a free trial version of Interbase from embarcadero. As far as I can tell, though, when you install it, if you install the Client only and use the interclient.jar, this does not require a license. Only installing the IB server (and depending on number of users/connections, etc) but that is nothing new to IB users/admins.
Works for us, at least, until we upgrade our server's IB version, or move to Firebird.
Hope this helps at least one other person out there.