The splunk2.jar on code.google.com is legacy code and no longer available and/or supported. The SDKs available on Developer Platform are the ones that are officially kept up to date with the latest Splunk versions.
Unfortunately there is no documented migration path but we have very rich documentation on the the Splunk SDK for Java. How-To section has sample code for running searches and displaying results. You can use that as a reference and update your code accordingly.