Java Reference
In-Depth Information
The basic scope rules are as follows:
1.
The scope of a parameter declaration is the body of the method in which the dec-
laration appears.
2.
The scope of a local-variable declaration is from the point at which the declara-
tion appears to the end of that block.
3.
The scope of a local-variable declaration that appears in the initialization section
of a
for
statement's header is the body of the
for
statement and the other expres-
sions in the header.
4.
A method or field's scope is the entire body of the class. This enables a class's in-
stance methods to use the fields and other methods of the class.
Any
block may contain variable declarations. If a local variable or parameter in a
method has the same name as a field of the class, the field is
hidden
until the block termi-
nates execution—this is called
shadowing
. To access a shadowed field in a block:
• If the field is an instance variable, precede its name with the
this
keyword and a
dot (
.
), as in
this.x
.
• If the field is a
static
class variable, precede its name with the class's name and
a dot (
.
), as in
ClassName
.x
.
Figure 6.9 demonstrates scoping issues with fields and local variables. Line 7 declares
and initializes the field
x
to
1
. This field is
shadowed
in any block (or method) that declares
a local variable named
x
. Method
main
(lines 11-23) declares a local variable
x
(line 13)
and initializes it to
5
. This local variable's value is output to show that the field
x
(whose
value is
1
) is
shadowed
in
main
. The program declares two other methods—
useLocalVa-
riable
(lines 26-35) and
useField
(lines 38-45)—that each take no arguments and
return no results. Method
main
calls each method twice (lines 17-20). Method
useLocal-
Variable
declares local variable
x
(line 28). When
useLocalVariable
is first called (line
17), it creates local variable
x
and initializes it to
25
(line 28), outputs the value of
x
(lines
30-31), increments
x
(line 32) and outputs the value of
x
again (lines 33-34). When
uselLocalVariable
is called a second time (line 19), it
recreates
local variable
x
and
rein-
itializes
it to
25
, so the output of each
useLocalVariable
call is identical.
1
// Fig. 6.9: Scope.java
2
// Scope class demonstrates field and local variable scopes.
3
4
public class
Scope
5
{
6
// field that is accessible to all methods of this class
private static int
x =
1
;
7
8
9
// method main creates and initializes local variable x
10
// and calls methods useLocalVariable and useField
11
public static void
main(String[] args)
12
{
13
int
x =
5
;
// method's local variable x shadows field x
14
Fig. 6.9
|
Scope
class demonstrates field and local-variable scopes. (Part 1 of 2.)
