Java Data Types and Variables Explained

In Java, data types specify the kind of data a variable can store, and variables are containers that hold values during program execution. Understanding both is fundamental for writing effective Java code.

1. What Is a Variable?

A variable is a name given to a memory location where data is stored.

Example

int age = 25;

String name = "John";

Here:

age is a variable of type int

name is a variable of type String

2. Types of Variables in Java

Java supports three kinds of variables:

1. Local Variables

Declared inside a method or block.

void test() {

    int count = 10; // local variable

}

2. Instance Variables

Belong to an object (non-static).

class Person {

    String name; // instance variable

}

3. Static Variables

Shared among all objects (declared with static).

static int totalUsers = 0;

3. Java Data Types

Java has two main categories of data types:

A. Primitive Data Types

These are basic built-in types.

Data Type Size Example Description

byte 1 byte byte a = 10; Small integer range

short 2 bytes short s = 200; Larger than byte

int 4 bytes int age = 25; Most common integer type

long 8 bytes long distance = 123456L; Very large integers

float 4 bytes float price = 10.5f; Decimal values (less precision)

double 8 bytes double salary = 45678.90; More precise decimal values

char 2 bytes char grade = 'A'; Single character

boolean 1 bit boolean isLogin = true; True/false values

B. Non-Primitive (Reference) Data Types

Used for objects or collections.

Examples:

String

Arrays

Classes & Objects

Interfaces

Example:

String city = "Hyderabad";

int[] numbers = {1, 2, 3};

4. Declaring and Initializing Variables

Declaration

int age;

Initialization

age = 30;

Declaration + Initialization

int age = 30;

5. Naming Rules for Variables (Important)

✔ Must start with a letter, _, or $

✔ Cannot start with a number

✔ No spaces allowed

✔ Case-sensitive

✔ Cannot use Java keywords (class, int, etc.)

Examples

Valid:

int employeeCount;

String _name;

double $salary;

Invalid:

int 1age;

String employee name;

6. Type Casting in Java

A. Widening (Implicit)

Small → large (safe conversion)

int x = 10;

double y = x; // OK

B. Narrowing (Explicit)

Large → small (risk of data loss)

double a = 10.5;

int b = (int) a;  // 10

7. Constants in Java

Use final keyword to make a variable unchangeable.

final int MAX_USERS = 100;

8. Example Program Using Data Types and Variables

public class DataTypeExample {

    public static void main(String[] args) {

        int id = 101;

        String name = "Alice";

        double salary = 50000.75;

        boolean isActive = true;

        System.out.println(id);

        System.out.println(name);

        System.out.println(salary);

        System.out.println(isActive);

    }

}

Summary Table

Category Types

Primitive Types byte, short, int, long, float, double, char, boolean

Non-Primitive Types String, Arrays, Classes, Interfaces

Variable types Local, Instance, Static

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November 26, 2025

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