USER-DEFINED CLASSES AND ADTS - Java Programming: From Problem Analysis to Program Design - page 474

The statement in Line 5 allocates memory space for a Clock object, initializes each instance
variable of the object to 0 , and stores the address of the object into myClock . The statement
in Line 6 allocates memory space for a Clock object; initializes the instance variables hr , min ,
and sec of the object to 9 , 35 ,and 15 , respectively; and stores the address of the object into
yourClock (see Figure 8-2).

Reference
variable

Clock
object

Reference
variable

Clock
object

hr 0 hr 9

min 35
sec 15

myClock yourClock

min 0

sec 0

FIGURE 8-2 Variables myClock and yourClock and associated Clock objects

To be specific, we call the object to which myClock points the object myClock and the
object to which yourClock points the object yourClock (see Figure 8-3).

Object myClock Object yourClock

hr 0 hr 9

myClock yourClock

min 0 min 35

sec 15 sec 0

FIGURE 8-3 Objects myClock and yourClock

Of course, you can combine the statements to declare the variable and instantiate the object
into one statement. For example, the statements in Lines 1 and 5 can be combined as:

Clock myClock = new Clock(); //declare and instantiate myClock

That is, the preceding statement declares myClock to be a reference variable of type Clock
and instantiates the object myClock to store the hours, minutes, and seconds. Each instance
variable of the object myClock is initialized to 0 by the default constructor.
Similarly, the statements in Lines 2 and 6 can be combined as:
Clock yourClock = new Clock(9, 35, 15);

//declare and
//instantiate yourClock