Define a simple class, say Position, that holds three numbers, e.g., representing the cooordinates of an airplane in the sky. Define a method to set a position and methods to get its coordinates. Set up two threads, say Reader and Writer, that respectively get (read) and set (write) a position.Since the threads may read and write a position concurrently, without some care, it may be possible to get positions that were never set. To verify this possibility, design the writer thread so that the three coordinates of a position are the same. In the reader thread, if after reading a position the coordinates are not the same, issue a message.
You can fix the problem, by synchronizing read and write accesses to a position. An object with synchronized read/write access is called a Protected Variable.