Exercises for Chapter 8

8.1 It was asserted that the faults in the Dangerous client were caused by two different interruptions. Show that this is the case by implementing each synchronization in turn.


8.2 The correct operation of a TrafficLight instance depends upon its setOtherLight() action being called before its run() action, however it does not enforce this consideration. Re-implement the run() action to throw an exception if theOtherLight instance attribute has not been initialised. Adapt the TrafficDemo client to show the TrafficLight class is now implemented in a secure manner.


8.3 Adapt the holdOn() action to give a different delay depending upon the colour of the light in order to make the simulation more realistic.


8.4 Attempt to demonstrate that a single Java statement can be interrupted. To do this implement a DangerousWriter class which either sets or clears all of its bits in an long attribute in a single action. The action can be shown to be dangerous if the value read from the class is not equal to the hexadecimal values 0x00000000 or 0xFFFFFFFF.


Hint: if the value of the integer attribute, called dangerousInteger, is 0 then all bits are currently clear and can be set by adding 0xFFFFFFFF, adding this a second time will clear all bits. More details can be found in Appendix A1.


8.5 Design and implement a TrafficMonitor class which uses a random number generator to indicate if any traffic is waiting and if traffic is still flowing. Associate a TrafficMonitor instance with each TrafficLight instance. Adapt the TrafficLight class so that, when it is on green, it will continually ask its TrafficMonitor if traffic is still flowing and relinquish control as soon as none is flowing. Demonstrate the effectiveness of the revision using an unchanged TrafficDemo client.


8.6 Further extend the TrafficLight class from exercise 8.5 so that whilst it is on green it asks its own TrafficMonitor for its traffic flow and asks the other TrafficLight for its traffic waiting. It should relinquish control as soon as its traffic flow is significantly lower than the traffic waiting at the other light. Demonstrate the effectiveness of the revision using an unchanged TrafficDemo client.

JFL Contents.

Summary of Chapter 8.