USER-DEFINED CLASSES AND ADTS - Java Programming: From Problem Analysis to Program Design - page 483

hr 12
min 35
sec 48

hr 12
min 27
sec 15

myClock yourClock

equals

otherClock

FIGURE 8-10 Object myClock and parameter otherClock

Note that otherClock and yourClock refer to the same object. The instance variables
hr , min , and sec of the object otherClock have the values 12 , 27 , and 15 , respectively.
In other words, when the body of the method equals executes, the value of
otherClock.hr is 12 , the value of otherClock.min is 27 , and the value of
otherClock.sec is 15 . The method equals is a member of myClock . When the
method equals executes, the variables hr , min , and sec in the body of the method
equals are the instance variables of the object myClock . Therefore, the instance variable
hr of the object myClock is compared with otherClock.hr , the instance variable min
of the object myClock is compared with otherClock.min , and the instance variable sec
of the object myClock is compared with otherClock.sec .
Once again, in the expression:

8

myClock.equals(yourClock)

the method equals is invoked by myClock and compares the object myClock with the
object yourClock . It follows that the method equals needs only one parameter.
Let us again take a look at the definition of the method equals . Notice that within the
definition of this method, the object otherClock accesses the data members hr , min ,
and sec . However, these data members are private . So is there any violation? The
answer is no. The method equals is a member of the class Clock and hr , min , and
sec are the data members. Moreover, otherClock is an object of the class Clock .
Therefore, the object otherClock can access its private data members within the
definition of the method equals . The same is true for any method of a class.
That is, in general, when you write the definition of a method, say, dummyMethod ,ofa
class , say, DummyClass , and the method uses an object, dummyObject of the class
DummyClass , then within the definition of dummyMethod the object dummyObject can
access its private data members (in fact, any private member of the class).
The method makeCopy copies the instance variables of its parameter, otherClock , into
the corresponding instance variables of the object referenced by the variable using this
method. Its definition is: