Introduction

Every variable in Java has a data type — it tells the computer what kind of value the variable can hold. Whether you're storing a person's age (whole number), a price (decimal), a grade (single character), or a name (text) — you need the right data type.

In this article, we'll explore all 8 primitive data types and the important non-primitive types like String, Array, and Object. By the end, you'll know exactly which type to use for any data!

What are Data Types?

Data types tell the compiler what kind of value a variable can store — numbers, characters, text, or true/false.

Key Points to Remember

Two Categories of Data Types

8 Primitive Data Types

Let's explore each primitive data type in detail.

1. byte

byte b = 100;
System.out.println("Byte value: " + b);
// Output: Byte value: 100

2. short

short s = 5000;
System.out.println("Short value: " + s);
// Output: Short value: 5000

3. int ⭐ (Most Used)

int age = 15;
int marks = 85;
int studentCount = 40;
System.out.println("Age: " + age);
// Output: Age: 15

4. long

long population = 8000000000L;  // Note the 'L'
System.out.println("Population: " + population);
// Output: Population: 8000000000

5. float

float pi = 3.14f;  // Note the 'f'
float price = 99.99f;
System.out.println("Price: " + price);
// Output: Price: 99.99

6. double ⭐ (Most Used for Decimals)

double price = 99.99;
double pi = 3.14159265358979;
double salary = 55000.50;
System.out.println("Salary: " + salary);
// Output: Salary: 55000.5

7. char

char grade = 'A';
char firstLetter = 'R';
char digit = '5';
char symbol = '#';
System.out.println("Grade: " + grade);
// Output: Grade: A

8. boolean

boolean isPass = true;
boolean isSunday = false;
boolean hasPermission = true;
System.out.println("Passed: " + isPass);
// Output: Passed: true

Non-Primitive Data Types

Non-primitive types are built using primitive types. They store references to objects.

1. String

String name = "Ravi";
String city = "Kolkata";
String message = "Hello, World!";
System.out.println("Name: " + name);
// Output: Name: Ravi

2. Array

int[] nums = {1, 2, 3, 4, 5};
String[] names = {"Ravi", "Priya", "Amit"};
System.out.println("First number: " + nums[0]);
// Output: First number: 1

3. Class

class Student {
    String name;
    int age;
}

4. Object

Student s = new Student();
s.name = "Ravi";
s.age = 15;

Complete Example Program

public class DataTypesDemo {
    public static void main(String[] args) {
        int age = 15;
        double price = 99.99;
        char grade = 'A';
        String name = "Ravi";
        boolean isPass = true;

        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Price: " + price);
        System.out.println("Grade: " + grade);
        System.out.println("Passed: " + isPass);
    }
}

Data Type Comparison Table

Real-Life Examples

Tips for Success

Common Mistakes

Did You Know?

When to Use Which Type?

Data Type Conversion (Type Casting)

Sometimes we need to convert one type to another.

Automatic Conversion (Widening)

// Automatic: smaller to larger type
int a = 100;
double b = a;  // int to double (automatic)
System.out.println(b);  // Output: 100.0

Manual Conversion (Narrowing)

// Manual: larger to smaller type
double x = 99.99;
int y = (int) x;  // double to int (need cast)
System.out.println(y);  // Output: 99 (decimal lost!)

Frequently Asked Questions

Q1. Why so many data types?

Different types save memory and represent data efficiently. A byte uses only 1 byte of memory; long uses 8 bytes.

Q2. Which type should I use for age?

Use int. Age is a whole number under 200 (well within int range).

Q3. What's the difference between float and double?

float has 6-7 digit precision (4 bytes). double has 15 digit precision (8 bytes). Use double for better accuracy.

Q4. Is String a primitive type?

No! String is non-primitive (it's a class). But it's used so often, it feels primitive.

Q5. Why can't I use = for comparison?

= is assignment. Use == for comparison (or .equals() for Strings).

Q6. What is default value?

If you don't assign a value, Java gives a default value (0 for numbers, false for boolean, null for objects).

Q7. Can I change data type of a variable?

No! Once declared, data type is fixed. But you can convert values using type casting.

Q8. What's the size of String?

String has variable size — depends on the length of text it contains.

Q9. Which type for boolean values?

Use boolean. It stores only true or false.

Q10. What if my number is too large for int?

Use long. It can hold much larger numbers.