Java Reference
In-Depth Information
the expression initializes the
Scanner
with
System.in
, which tells the
Scanner
where to
read the input from (i.e., the keyboard).
Calling Class
Account
's
getName
Method
Line 16 displays the
initial
name, which is obtained by calling the object's
getName
meth-
od. Just as we can use object
System.out
to call its methods
print
,
printf
and
println
,
we can use object
myAccount
to call its methods
getName
and
setName
. Line 16 calls
getName
using the
myAccount
object created in line 13, followed by a
dot separator
(
.
),
then the method name
getName
and an
empty
set of parentheses because no arguments are
being passed. When
getName
is called:
1.
The app transfers program execution from the call (line 16 in
main
) to method
get-
Name
's declaration (lines 16-19 of Fig. 3.1). Because
getName
was called via the
my-
Account
object,
getName
“knows” which object's instance variable to manipulate.
2.
Next, method
getName
performs its task—that is, it
returns
the
name
(line 18 of
Fig. 3.1). When the
return
statement executes, program execution continues
where
getName
was called (line 16 in Fig. 3.2).
3.
System.out.printf
displays the
String
returned by method
getName
, then the
program continues executing at line 19 in
main
.
Error-Prevention Tip 3.1
Never use as a format-control a string that was input from the user. When method
System.out.printf
evaluates the format-control string in its first argument, the method
performs tasks based on the conversion specifier(s) in that string. If the format-control string
were obtained from the user, a malicious user could supply conversion specifiers that would
be executed by
System.out.printf
, possibly causing a security breach.
null
—the Default Initial Value for
String
Variables
The first line of the output shows the name “
null
.” Unlike local variables, which are
not
automatically initialized,
every instance variable has a
default initial value
—
a value provid-
ed by Java when you do
not
specify the instance variable's initial value. Thus,
instance vari-
ables
are
not
required to be explicitly initialized before they're used in a program—unless they
must be initialized to values
other than
their default values. The default value for an instance
variable of type
String
(like
name
in this example) is
null
, which we discuss further in
Section 3.3 when we consider
reference types
.
Calling Class
Account
's
setName
Method
Line 21 calls
myAccounts
's
setName
method. A method call can supply
arguments
whose
values
are assigned to the corresponding method parameters. In this case, the value of
main
's local variable
theName
in parentheses is the
argument
that's passed to
setName
so
that the method can perform its task. When
setName
is called:
1.
The app transfers program execution from line 21 in
main
to
setName
method's
declaration (lines 10-13 of Fig. 3.1), and the
argument
value
in the call's paren-
theses (
theName
) is assigned to the corresponding
parameter
(
name
) in the method
header (line 10 of Fig. 3.1). Because
setName
was called via the
myAccount
object,
setName
“knows” which object's instance variable to manipulate.
