Lesson 10 of 50 – Variables in Java

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Variables in Java

A variable is a named storage location used to hold data in a Java program. Variables allow us to store information, perform calculations, and use values throughout our programs.

Learning Objective: Learn how to declare, initialize, update, and use variables in Java programs.

1. What Is a Variable?

A variable is a named location that stores a value of a particular data type.

int age = 20;
System.out.println(age);

Output:

In this example:

int is the data type.
age is the variable name.
20 is the stored value.

2. Declaring a Variable

Variable declaration means specifying the variable's data type and name.

int age;
double price;
String name;
char grade;
boolean isPassed;

These statements declare variables without explicitly assigning values to them.

Local variables must be assigned a value before they are read. Fields receive default values automatically.

3. Initializing a Variable

Initialization means assigning an initial value to a variable.

int age = 20;
double price = 99.50;
String name = "Rahul";
char grade = 'A';
boolean isPassed = true;

A variable can be declared and initialized in the same statement.

Example:

int marks = 85;
System.out.println(marks);

Output:

4. Types of Variables by Data Type

Java provides different data types for storing different kinds of information.

Data Type	Example	Purpose
`int`	`int age = 20;`	Whole numbers
`double`	`double price = 99.5;`	Decimal numbers
`char`	`char grade = 'A';`	A single character
`boolean`	`boolean active = true;`	True or false values
`String`	`String name = "Rahul";`	Text

Note that String is a class, not a primitive data type.

5. Changing Variable Values

After declaring a variable, you can assign a new value to it if it is not declared as a constant.

int age = 20;

System.out.println(age);

age = 25;

System.out.println(age);

Output:

20
25

The second assignment replaces the previous value stored in age.

6. Multiple Variables

You can declare multiple variables of the same type in one statement.

int a = 10, b = 20, c = 30;

System.out.println(a);
System.out.println(b);
System.out.println(c);

Output:

10
20
30

You can also declare variables separately when it makes your code easier to understand.

7. Variable Naming Rules

Java identifiers must follow specific naming rules.

Valid variable names:

int age = 20;
String studentName = "Rahul";
int total_marks = 450;
int number1 = 100;
int _count = 5;

Invalid variable names:

int 2number = 10;
int student-name = 20;
int class = 5;

A variable name cannot start with a digit.
Spaces and hyphens are not allowed.
Java keywords cannot be used as ordinary variable names.
Names are case-sensitive.
Names can contain letters, digits, underscores, and dollar signs, subject to identifier rules.

8. Local Variables

A local variable is declared inside a method, constructor, or block. Its scope is limited to the relevant block.

public class Example {
    public static void main(String[] args) {
        int number = 100;

        System.out.println(number);
    }
}

Output:

Local variables must be definitely assigned before they are used.

9. Instance Variables

An instance variable is declared inside a class but outside its methods and constructors. Each object has its own copy of instance variables.

public class Student {
    String name;
    int age;

    public static void main(String[] args) {
        Student s1 = new Student();

        s1.name = "Rahul";
        s1.age = 20;

        System.out.println(s1.name);
        System.out.println(s1.age);
    }
}

Output:

Rahul
20

Instance variables receive default values if no explicit initial value is provided.

10. Static Variables

A static variable belongs to the class rather than to an individual object. It is shared by instances of that class.

public class Student {
    static String institute = "Soopro Pathshala";

    public static void main(String[] args) {
        System.out.println(Student.institute);
    }
}

Output:

Soopro Pathshala

The static keyword declares a class-level field in this example.

11. Constants Using final

The final keyword prevents a variable from being assigned again after it has been initialized.

final double PI = 3.14159;

System.out.println(PI);

Output:

3.14159

This assignment is not allowed after initialization:

PI = 3.14; // Compilation error

By convention, constants are often named using uppercase letters and underscores.

12. Using Variables in Calculations

Variables can store values used in arithmetic calculations.

public class Addition {
    public static void main(String[] args) {
        int a = 10;
        int b = 20;

        int sum = a + b;

        System.out.println("First number: " + a);
        System.out.println("Second number: " + b);
        System.out.println("Sum: " + sum);
    }
}

Output:

First number: 10
Second number: 20
Sum: 30

The expression a + b adds the two integer values and stores the result in sum.

13. Practice Program and Summary

Create a program that stores a student's name, marks, and grade.

public class StudentDetails {
    public static void main(String[] args) {
        String name = "Rahul";
        int marks = 85;
        char grade = 'A';

        System.out.println("Student Name: " + name);
        System.out.println("Marks: " + marks);
        System.out.println("Grade: " + grade);
    }
}

Output:

Student Name: Rahul
Marks: 85
Grade: A

Key Points:

Variables store values in a program.
Declaration specifies a variable's type and name.
Initialization assigns an initial value.
Local, instance, and static variables have different scopes and uses.
The final keyword prevents reassignment.
Choose meaningful variable names.

Congratulations! You have learned how to declare, initialize, update, and use Java variables.

🧠 Quick Quiz

1. Which keyword declares an integer variable?

text
int
number

2. Which keyword prevents a variable from being reassigned?

static
final
public

3. What is the output of this code?

int a = 10;
int b = 20;
System.out.println(a + b);

1020
30
10

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Variables in Java

🧠 Quick Quiz