This happens particularly when the first jar file is being recognized in class path and it tries searching the function in 1st jar file and when it does not find it, it throws the error.
You can check which this:
import java.net.;
import org.apache.commons.io.;
String path= org.apache.commons.io.FileUtils.class.getProtectionDomain().getSourceCode().getLocation().getPath();
String decodedPath= URLDecoder.decode(path,“UTF-8”);