Scope and Lifetime of a Variable in Java

One of the frequent questions, which any programmer comes across in the programming world is, “What is the scope of this variable?” In this article, our main concern is to shed light upon the scope and lifetime of a variable in Java.
The scope of a variable refers to the areas or the sections of the program in which the variable can be accessed, and the lifetime of a variable indicates how long the variable stays alive in the memory.
A joint statement defining the scope and lifetime of a variable is “how and where the variable is defined.” Let me simplify it further. The comprehensive practice for the scope of a variable is that it is accessible only inside the block it is declared.

Types of Variables and its Scope

There are three types of variables.

Instance Variables
Class Variables
Local Variables

Now, let us dig into the scope and lifetime of each of the above mentioned type.

Instance Variables

A variable which is declared inside a class, but is declared outside any methods and blocks is known as instance variable.
Scope: Throughout the class except in the static methods.
Lifetime: Until the object of the class stays in the memory.

Class Variables

A variable which is declared inside a class, outside all the blocks and is declared as static is known as class variable.
Scope: Throughout the class.
Lifetime: Until the end of the program.

Local Variables

All variables which are not instance or class variables are known as local variables.
Scope: Within the block it is declared.
Lifetime: Until control leaves the block in which it is declared.
Now, let us look at an example code to paint a clear picture and understand the concept of scope and lifetime of variables better.

Example

public class scope_and_lifetime { int num1, num2; //Instance Variables static int result; //Class Variable int add(int a, int b){ //Local Variables num1 = a; num2 = b; return a+b; } public static void main(String args[]){ scope_and_lifetime ob = new scope_and_lifetime(); result = ob.add(10, 20); System.out.println("Sum = " + result); } }

Output

Let us look at the output of the above example code.

Here in the above example code, the variables num1 and num2 are Instance Variables. The variable result is Class Variable. The parameters of the method add, namely, ‘a’ and ‘b’ are Local Variable. Let us try and use the variables outside of their defined scope, and see what happens.

public class scope_and_lifetime { int num1, num2; //Instance Variables int result; int add(int a, int b){ //Local Variables num1 = a; num2 = b; return a+b; } public static void main(String args[]){ scope_and_lifetime ob = new scope_and_lifetime(); result = ob.add(10, 20); num1 = 10; System.out.println("Sum = " + result); } }

Output

Let us look at the output of the above example code.

Accessing variables outside the scope — Fig.2- Accessing variables outside their scope.

See! We get an error. I removed the keyword static from the variable result, which makes it an instance variable. We cannot use instance variable inside a static method, so the usage of num1 inside public static void main(String args[]) gives an error.
Let us look at one more interesting concept.

Nested Scope

We can always use a block of code inside a block of code. This technique of programming is known as nesting of blocks.
Scope: All the variables of the outer block are accessible by the inner block but the variables within inner block are not accessible by the outer block.
Let us look at an example for the above case.

public class scope_and_lifetime { public static void main(String args[]){ int a = 10; //Outer Block { //Inner Block int x = a; { int y = x; } } System.out.println(y); } }

Output

Let us look at the output of the above example code.

As we have tried to access the variable “y” outside the block it is declared, we encounter an error.

Scope and Lifetime of a Variable in Java

Types of Variables and its Scope

Instance Variables

Class Variables

Local Variables

Example

Output

Output

Nested Scope

Output

Summary

Read More

Author : Satyam Kumar -

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