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:

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

3. Variable Declaration

You must declare a variable before using it:

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

You can also do both at once:

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int age = 25;

4. Naming Rules <variableName>

Code Demo

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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:

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