Java employs data types to specify the kind of values variables can hold, determining their storage format and valid range. These types are categorized into two main groups: primitive data types and reference data types.

Primitive Data Types

Primitive types store simple values and are divided into four categories:

1. Integer Types

• byte: 8-bit (-128 to 127)
• short: 16-bit (-32,768 to 32,767)
• int: 32-bit (-2³¹ to 2³¹-1)
• long: 64-bit (-2⁶³ to 2⁶³-1)

int count = 100;
long population = 7_900_000_000L;

2. Flaoting-Point Types

• float: 32-bit (~6-7 decimal digits)
• double: 64-bit (~15 decimal digits)

float piApprox = 3.1416f;
double precisePi = 3.141592653589793;

3. Character Type

• char: 16-bit Unicode character

char initial = 'J';

4. Boolean Type

• boolean: true or false

boolean isActive = true;

Reference Data Types

Reference types store memory addresses of objects rather than values directly:

1. Class Types

User-defined types that create objects:

class Vehicle {
    String model;
    int year;
}

Vehicle car = new Vehicle();
car.model = "Tesla";
car.year = 2023;

2. Interface Types

Contracts for implementing classes:

interface Drawable {
    void render();
}

class Square implements Drawable {
    public void render() {
        System.out.println("Drawing square");
    }
}

Drawable shape = new Square();
shape.render();

3. Array Types

Fixed-size collections of elements:

double[] temperatures = {22.5, 19.8, 25.3};
String[] colors = {"Red", "Green", "Blue"};

4. Enum Types

Predefined constant sets:

enum Status { NEW, PROCESSING, COMPLETED }
Status current = Status.PROCESSING;

Type Conversion Features

Autoboxing

Automatic primitive-to-wrapper conversion:

int value = 50;
Integer boxed = value;  // Autoboxing

Unboxing

Automatic wrapper-to-primitive conversion:

Integer wrapped = 75;
int extracted = wrapped;  // Unboxing