SimulationThread extends Thread

	public void run()

		while (isRunning())

			synchronized (this)

				while (isPaused())

					try this.wait()

			doCalculations()
			doRendering()     // more precisely invoke the repaint() method




******************************************************


............

	public void onUnPauseButtonPress()

		synchronized (Simulation.getSimulationThread())

			Simulation.getSimulationThread().notify();







******************************************************


also be aware that the doCalculations() and the paintComponent() of the panel
need to be synchronized (ideally with the panel object) because paintComponent()
is actually called in the swing thread. you just call repaint() and then swing
will call paintComponent in its own thread. if not synchronized the data could
change half way through the rendering of a frame.