USER-DEFINED CLASSES AND ADTS - Java Programming: From Problem Analysis to Program Design - page 512

public int getHours()
{

//return hr

}

// Similarly document other methods.

}

You can write the Java code of the class Clock using this specification. You can also
specify the design of a class as it is shown in the programming example, Candy Machine,
later in this chapter. To become an effective and good programmer, you must avoid the
temptation to skip the design phase. It is possible that the first few programs that you
write can be coded directly. However, in general, this approach only works for very small
programs. As a matter of fact, you will spend less time implementing, debugging, and
maintaining a code that is properly designed and documented.

Reference this (Optional)
In this chapter, we defined the class Clock . Suppose that myClock is a reference
variable of type Clock . Suppose that the object myClock has been created. Consider
the following statements:

myClock.setTime(5, 6, 59); //Line 1

myClock.incrementSeconds(); //Line 2

The statement in Line 1 uses the method setTime to set the instance variables hr , min ,
and sec of the object myClock to 5 , 6 , and 59 , respectively. The statement in Line 2 uses
the method incrementSeconds to increment the time of the object myClock by one
second. The statement in Line 2 also results in a call to the method incrementMinutes
because, after incrementing the value of sec by 1 , the value of sec becomes 60 , which
then is reset to 0 , and the method incrementMinutes is invoked.
How do you think Java makes sure that the statement in Line 1 sets the instance variables
of the object myClock and not of another Clock object? How does Java make sure that
when the method incrementSeconds calls the method incrementMinutes , the
method incrementMinutes increments the value of the instance variable min of the
object myClock and not of another Clock object?
The answer is that every object has access to a reference of itself. The name of this
reference is this . In Java, this is a reserved word.
Java implicitly uses the reference this to refer to both the instance variables and the
methods of a class. Recall that the definition of the method setTime is: