Data Types and Variables

1. Primitive Data Types

Java has 8 primitive data types:

Type	Size	Description	Example
byte	1 byte	Whole number (small range)	byte a = 100;
short	2 bytes	Whole number	short b = 500;
int	4 bytes	Whole number (default)	int x = 1000;
long	8 bytes	Very large whole number	long l = 900000000L;
float	4 bytes	Decimal number (less precise)	float f = 3.14f;
double	8 bytes	Decimal number (more precise)	double d = 3.14159;
char	2 bytes	A single character	char c = 'A';
boolean	1 bit	true or false	boolean b = true;

2. Reference Data Types

These store references to objects:

• Arrays
• Strings
• Classes
• Interfaces

String name = "John";
int[] scores = {90, 85, 78};

3. Variable Declaration

You must declare a variable before using it:

// <dataType> <variableName> [assign = <value>];
int age;          // Declaration
age = 25;         // Initialization

You can also do both at once:

int age = 25;

4. Naming Rules <variableName>

• Must start with a letter, _ or $.
• Can contain letters and digits.
• Case-sensitive (age ≠ Age).
• Use camelCase for convention: studentScore.

Code Demo

public class DataTypeDemo {
    public static void main(String[] args) {
        // Primitive types
        int age = 30;
        double salary = 45000.50;
        char grade = 'A';
        boolean isEmployed = true;

        // Reference types
        String name = "Alice";
        int[] scores = {80, 85, 90};

        // Output
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Salary: $" + salary);
        System.out.println("Grade: " + grade);
        System.out.println("Is Employed: " + isEmployed);
        System.out.println("Scores: " + scores[0] + ", " + scores[1] + ", " + scores[2]);
    }
}

Output:

Name: Alice
Age: 30
Salary: $45000.5
Grade: A
Is Employed: true
Scores: 80, 85, 90