Java Reference
In-Depth Information
You can download it from the Additional Resources section at
Appendix F shows how to use this documentation.
Variable names such as
number1
,
number2
and
sum
actually correspond to
locations
in the
computer's memory. Every variable has a
name
, a
type
, a
size
(in bytes) and a
value
.
In the addition program of Fig. 2.7, when the following statement (line 18) executes:
number1 = input.nextInt();
// read first number from user
the number typed by the user is placed into a memory location corresponding to the name
number1
. Suppose that the user enters
45
. The computer places that integer value into lo-
cation
number1
(Fig. 2.8), replacing the previous value (if any) in that location. The pre-
vious value is lost, so this process is said to be
destructive
.
number1
45
Fig. 2.8
|
Memory location showing the name and value of variable
number1
.
When the statement (line 21)
number2 = input.nextInt();
// read second number from user
executes, suppose that the user enters
72
. The computer places that integer value into lo-
cation
number2
. The memory now appears as shown in Fig. 2.9.
number1
45
number2
72
Fig. 2.9
|
Memory locations after storing values for
number1
and
number2
.
After the program of Fig. 2.7 obtains values for
number1
and
number2
, it adds the
values and places the total into variable
sum
. The statement (line 23)
sum = number1 + number2;
// add numbers, then store total in sum
performs the addition, then replaces any previous value in
sum
. After
sum
has been calcu-
lated, memory appears as shown in Fig. 2.10. The values of
number1
and
number2
appear
exactly as they did before they were used in the calculation of
sum
. These values were used,
but
not
destroyed, as the computer performed the calculation. When a value is read from
a memory location, the process is
nondestructive
.
