Scope of Variables In Java – GeeksforGeeks
original source : http://www.geeksforgeeks.org/variable-scope-in-java/
Scope of Variables In Java
Introduction
Scope of a variable is the part of the program where the variable is accessible. Like C/C++, in Java, all identifiers are lexically (or statically) scoped, i.e., scope of a variable can determined at compiler time and independent of function call stack.
On a side note, unlike Java/C/C++, Perl supports both dynamic ans static scoping. Perl’s keyword “my” defines a statically scoped local variable, while the keyword “local” defines dynamically scoped local variable i.e., scope depends on function call stack.
Java programs are organized in the form of classes. Every class is part of some package.
Java scope rules can be covered under following categories.
Member Variables (Class Level Scope)
These variables must be declared inside class (outside any function). They can be directly accessed anywhere in class. Let’s take a look at an example:
public class Test
{
// All variables defined directly inside a class
// are member variables
int a;
private String b
void method1() {....}
int method2() {....}
char c;
}
Modifier Package Subclass World public Yes Yes Yes protected Yes Yes No Default (no modifier) Yes No No private No No No
Local Variables (Method Level Scope)
Variables declared inside a method have method level scope and can’t be accessed outside the method.
public class Test
{
void method1()
{
// Local variable (Method level scope)
int x;
}
}
Note : Local variables don’t exist after method’s execution is over.
Here’s another example of method scope, except this time the variable got passed in as a parameter to the method:
class Test
{
private int x;
public void setX(int x)
{
this.x = x;
}
}
The above code uses this keyword to differentiate between the local and class variables.
As an exercise, predict the output of following Java program.
public
class
Test
{
static
int
x =
11
;
private
int
y =
33
;
public
void
method1(
int
x)
{
Test t =
new
Test();
this
.x =
22
;
y =
44
;
System.out.println(
"Test.x: "
+ Test.x);
System.out.println(
"t.x: "
+ t.x);
System.out.println(
"t.y: "
+ t.y);
System.out.println(
"y: "
+ y);
}
public
static
void
main(String args[])
{
Test t =
new
Test();
t.method1(
5
);
}
}
Run on IDE
Output:
Test.x: 22 t.x: 22 t.y: 33 y: 44
Loop Variables (Block Scope)
A variable declared inside pair of brackets “{” and “}” in a method has scope withing the brackets only.
public
class
Test
{
public
static
void
main(String args[])
{
{
// The variable x has scope within
// brackets
int
x =
10
;
System.out.println(x);
}
// Uncommenting below line would produce
// error since variable x is out of scope.
// System.out.println(x);
}
}
Run on IDE
Output:
10
As another example, consider following program with for loop.
class
Test
{
public
static
void
main(String args[])
{
for
(
int
x=
0
; x<
4
; x++)
{
System.out.println(x);
}
// Will produce error
System.out.println(x);
}
}
Run on IDE
Output:
11: error: cannot find symbol System.out.println(x); ^
The right way of doing above is,
// Above program after correcting the error
class
Test
{
public
static
void
main(String args[])
{
int
x;
for
(x=
0
; x<
4
; x++)
{
System.out.println(x);
}
System.out.println(x);
}
}
Run on IDE
Output:
0 1 2 3 4
Let’s look at tricky example of loop scope. Predict the output of following program. You may be surprised if you are regular C/C++ programmer.
class
Test
{
public
static
void
main(String args[])
{
int
x =
5
;
for
(
int
x =
0
; x <
5
; x++)
{
System.out.println(x);
}
}
}
Run on IDE
Output :
6: error: variable a is already defined in method go(int) for (int a = 0; a < 5; a++) ^ 1 error
A similar program in C++ works. See this.
As an exercise, predict the output of following Java program.
class
Test
{
public
static
void
main(String args[])
{
{
int
x =
5
;
{
int
x =
10
;
System.out.println(x);
}
}
}
}
Run on IDE
Some Important Points about Variable scope in Java:
- In general, a set of curly brackets { } defines a scope.
- In Java we can usually access a variable as long as it was defined within the same set of brackets as the code we are writing or within any curly brackets inside of the curly brackets where the variable was defined.
- Any variable defined in a class outside of any method can be used by all member methods.
- When a method has same local variable as a member, this keyword can be used to reference the current class variable.
- For a variable to be read after the termination of a loop, It must be declared before the body of the loop.
This article is contributed by Rishabh Mahrsee. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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