Chapter 3, Problem 59E

Challenge exercise In the current design of ClockDisplay, a ClockDisplay object is responsible for detecting when a NumberDisplay object has rolled over to zero and then telling another NumberDisplay object to increment. In other words, there is no direct link between NumberDisplay objects. Would it be possible to have one NumberDisplay object tell another that it has rolled over, and that the other NumberDisplay object should then increment? For instance, have the minutes object tell the hour object that another hour has passed, or have the seconds object tell the minutes object that another sixty seconds have elapsed. Which of these objects would the timeTick method interact with? What fields would a NumberDisplay object have? What should the hour object do when a whole day has elapsed? Discuss the issues involved in this alternative design and, if you really feel like a challenge, try implementing it! Note that you might need to find out about the Java null keyword, which we don't cover until late in Chapter 4.