I can compile this JNA example code (from step 2 of https://github.com/twall/jna/#getting_started):
package com.sun.jna.examples;
import com.sun.jna.Library
Either just remove this line and recompile (which is fine in this case as you just try out some sample)
package com.sun.jna.examples;
or read up on what packages in java are and how they have to be handled (ChssPly76s Posts as a starter).
Better choose the second option as sooner or later (probably sooner) you will have to deal with packages anyway. So just take the time now to read up on it.