Define a simple class, say Position, that holds three numbers,
e.g., representing the coordinates 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.

This implementation also shows:
  - how to synchronize accesses that span more than one method call

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On the instructor's system, the message the coordinates of a
position are not the same occurs only early in the execution of
the program and only if some tracing messages (which widen the
critical regions) are printed during execution.

Writing and reading a more complex structure instead of a few
numbers should make the problem more conspicuous.