Scope of Variables In Java

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.
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;
}

We can declare class variables anywhere in class, but outside methods.

Access specified of member variables doesn’t effect scope of them within a class.

Member variables can be accessed outside a class with following rules

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);     } }

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);     } }

Output:

10

As another example, consider following program with a 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);     } }

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);     } }

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 a = 5;         for (int a = 0; a < 5; a++)         {             System.out.println(a);         }     } }

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);             }         }     } } NEXT>>