A shared counter

The minimum number is 2!

Here is a solution which prints out 2.

An information display

Here is a solution which uses semaphores to protect access to the display.

Ball world

Killing the balls: balls1.zip

Freezing the balls: balls2.zip

Freezing revisited: balls3.zip

Your own CyclicBarrier: balls4-monitor.zip is a solution using monitors, balls4-semaphores.zip a solution using semaphores.

Sieves and bridges

A solution using synchronized: cars.zip

This solution is fair - the monitor methods are careful to change the direction of the traffic flow from time to time. Notice that fairness makes the solution much more complicated.