Master Core Java Programming From Scratch

What is a Variable?

A variable is a named memory location used to store a value that can be used by a Java program.

Every variable has a data type, a name, and a value.

int age = 21;

Variable Declaration

dataType variableName = value;

For example:

int age = 21;

double salary = 45000.50;

String name = "Samadhan";

boolean active = true;

Primitive Data Types

Java provides eight primitive data types. They represent simple values directly.

Integer Data Types

Java provides four integer types: byte, short, int, and long.

byte smallNumber = 100;

short number = 30000;

int population = 1000000;

long distance = 9876543210L;

Floating-Point Data Types

Java provides float and double for floating-point numbers.

float temperature = 36.5f;

double salary = 55000.75;

A floating-point literal is treated as double by default. Use the f suffix for a float literal.

Character Type

The char type represents a single UTF-16 code unit and uses single quotes.

char grade = 'A';

char symbol = '#';

char digit = '5';

Boolean Type

The boolean type represents one of two logical values: true or false.

boolean isLoggedIn = true;

boolean hasPermission = false;

Reference Data Types

Reference types store references to objects rather than primitive values directly.

Common reference types include classes, arrays, interfaces, enums, and records.

String name = "CIIT Institute";

int[] numbers = {10, 20, 30};

Person person = new Person();

String

String is a class used to represent sequences of characters.

String name = "CIIT Institute";

System.out.println(name);

Strings are objects and are immutable in Java.

Constants with final

The final keyword can be used when a variable should not be reassigned after initialization.

final double PI = 3.141592653589793;

Local Variable Type Inference

Modern Java supports the var keyword for local variables. The compiler infers the variable's type from its initializer.

var name = "Samadhan";

var age = 21;

var salary = 45000.50;

Declaration and Initialization

A variable can be declared first and initialized later when using an explicit type.

int age;

age = 21;

However, var requires an initializer because the compiler needs that value to infer the type.

var age = 21;

Default Values

Instance and static fields receive default values when they are not explicitly initialized. Local variables do not receive automatic default values and must be initialized before use.

Complete Example

public class Program {

    public static void main(String[] args) {

        String name = "CIIT Institute";

        int age = 21;

        double salary = 50000.50;

        char grade = 'A';

        boolean employed = true;

        System.out.println("Name: " + name);

        System.out.println("Age: " + age);

        System.out.println("Salary: " + salary);

        System.out.println("Grade: " + grade);

        System.out.println("Employed: " + employed);

    }

}

Output

Name: CIIT Institute
Age: 21
Salary: 50000.5
Grade: A
Employed: true

Variable Naming Rules

• Variable names are case-sensitive.
• A variable name cannot start with a digit.
• Spaces are not allowed in variable names.
• Java keywords cannot be used as variable names.
• Use meaningful names that describe the stored value.

// Good
int studentAge = 21;

// Avoid
int x = 21;