Java Reference
In-Depth Information
variable's name can be any valid identifier—again, a series of characters consisting of letters,
digits, underscores (
_
) and dollar signs (
$
) that does
not
begin with a digit and does
not
con-
tain spaces. A variable's
type
specifies what kind of information is stored at that location in
memory. Like other statements, declaration statements end with a semicolon (
;
).
Line 11
Scanner input =
new
Scanner(System.in);
is a
variable declaration statement
that specifies the
name
(
input
) and
type
(
Scanner
) of a
variable that's used in this program. A
Scanner
enables a program to read data (e.g., num-
bers and strings) for use in a program. The data can come from many sources, such as the
user at the keyboard or a file on disk. Before using a
Scanner
, you must create it and spec-
ify the
source
of the data.
The
=
in line 11 indicates that
Scanner
variable
input
should be
initialized
(i.e., pre-
pared for use in the program) in its declaration with the result of the expression to the right
of the equals sign—
new Scanner(System.in)
. This expression uses the
new
keyword to
create a
Scanner
object that reads characters typed by the user at the keyboard. The
stan-
dard input object
,
System.in
, enables applications to read
bytes
of data typed by the user.
The
Scanner
translates these bytes into types (like
int
s) that can be used in a program.
The variable declaration statements in lines 13-15
int
number1;
// first number to add
int
number2;
// second number to add
int
sum;
// sum of number1 and number2
declare that variables
number1
,
number2
and
sum
hold data of type
int
—they can hold
in-
teger
values (whole numbers such as
72
,
-1127
and
0
). These variables are
not
yet initial-
ized. The range of values for an
int
is -2,147,483,648 to +2,147,483,647. [
Note:
The
int
values you use in a program may not contain commas.]
Some other types of data are
float
and
double
, for holding real numbers, and
char
,
for holding character data. Real numbers contain decimal points, such as in
3.4
,
0.0
and
-11.19
. Variables of type
char
represent individual characters, such as an uppercase letter
(e.g.,
A
), a digit (e.g.,
7
), a special character (e.g.,
*
or
%
) or an escape sequence (e.g., the
tab character,
\t
). The types
int
,
float
,
double
and
char
are called
primitive types
.
Primitive-type names are keywords and must appear in all lowercase letters. Appendix D
summarizes the characteristics of the eight primitive types (
boolean
,
byte
,
char
,
short
,
int
,
long
,
float
and
double
).
Several variables of the same type may be declared in a single declaration with the vari-
able names separated by commas (i.e., a comma-separated list of variable names). For
example, lines 13-15 can also be written as:
int
number1,
// first number to add
number2,
// second number to add
sum;
// sum of number1 and number2
Good Programming Practice 2.7
Declare each variable in its own declaration. This format allows a descriptive comment
to be inserted next to each variable being declared.
