Subclasses - Multimedia Introduction to Programming Using Java - page 167

E7. Consider class Counter of Fig. 4.15. Create a subclass in which the incre-
mentation is done by 2 instead of 1. Be sure to specify any methods you write
carefully.

E8. Consider class Counter of Fig. 4.15. Create a subclass that changes the value
of the counter to 0 whenever it reaches 60 (this could be used to count minutes
on a clock). Thus, the range of values of this Counter is 0..59 . We call this
“counting mod 60 ”. Be sure to specify carefuly any methods you write.
E9. Consider class Counter of Fig. 4.15. Create a subclass in which increment-
ing is done “mod n ”, where n≥2 (see the previous exercise). The user should
give n as an argument in a constructor call. Be sure to specify any methods you
write carefully.

E10. Consider class Counter of Fig. 4.15. Create a subclass in which the value
is interpreted as minutes and seconds, e.g. 125 is two minutes and five seconds.
The only change necessary is in method toString , which should return a string
that gives the counter in minutes and seconds.

E11. Design and implement a class Timer , an instance of which contains hours,
seconds, and minutes. It should use three private fields to contain the hours, sec-
onds, and minutes, and each of these field should be some form of Counter (see

/** A counter, which can be incremented */
public class Counter {

private int value= 0; // the current value of the counter

/** Constructor: a counter that starts at 0 */
public Counter (){ }

/** = this counter */
public int getCounter()
{ return value; }

/** Set this counter to c */
public void setCounter( int c)
{ value= c; }

/** increment the counter */
public void click()
{ value= value + 1; }

/** = this counter, as a String */
public String toString()
{ return "" + value }

}
Figure 4.15: Class Counter