Method Overloading in Java Explained — How, Why and When to Use It

public class ShippingCostCalculator {

    // Version 1: Calculate cost using just the weight in kilograms
    // Java picks this when you pass a single double argument
    public static double calculateCost(double weightKg) {
        double baseRate = 2.50; // flat rate per kg
        return weightKg * baseRate;
    }

    // Version 2: Calculate cost using weight AND distance in km
    // Java picks this when you pass two double arguments
    public static double calculateCost(double weightKg, double distanceKm) {
        double baseRate = 2.50;
        double distanceRate = 0.10; // extra cost per km
        return (weightKg * baseRate) + (distanceKm * distanceRate);
    }

    // Version 3: Calculate cost for a named express service
    // Java picks this when you pass a double and a String
    public static double calculateCost(double weightKg, String serviceType) {
        double baseCost = calculateCost(weightKg); // reuse Version 1!
        if (serviceType.equals("express")) {
            return baseCost + 15.00; // express surcharge
        }
        return baseCost;
    }

    public static void main(String[] args) {
        // Java compiler matches each call to the right version automatically
        double standardCost    = calculateCost(3.5);               // calls Version 1
        double distanceCost    = calculateCost(3.5, 120.0);        // calls Version 2
        double expressCost     = calculateCost(3.5, "express");    // calls Version 3

        System.out.println("Standard shipping (3.5 kg):          $" + standardCost);
        System.out.println("Distance shipping (3.5 kg, 120 km):  $" + distanceCost);
        System.out.println("Express shipping (3.5 kg):           $" + expressCost);
    }
}

public class RectangleDrawer {

    // ── STRATEGY 1: Different NUMBER of parameters ──────────────────────

    // Draw a square — only one measurement needed (all sides are equal)
    public static void drawRectangle(int sideLength) {
        System.out.println("Drawing a " + sideLength + "x" + sideLength + " square");
    }

    // Draw a rectangle — two measurements needed (width and height differ)
    public static void drawRectangle(int width, int height) {
        System.out.println("Drawing a " + width + "x" + height + " rectangle");
    }


    // ── STRATEGY 2: Different TYPE of parameters ─────────────────────────

    // Calculate area from integer pixel dimensions (whole numbers)
    public static double calculateArea(int widthPixels, int heightPixels) {
        System.out.println("Calculating area from int dimensions");
        return (double) widthPixels * heightPixels;
    }

    // Calculate area from decimal centimetre measurements (precise measurements)
    public static double calculateArea(double widthCm, double heightCm) {
        System.out.println("Calculating area from double dimensions");
        return widthCm * heightCm;
    }


    // ── STRATEGY 3: Different ORDER of parameter types ───────────────────

    // Scale a rectangle: start with the scale factor, then a label
    // e.g. "scale 2x and then name it 'enlarged'"
    public static void scaleRectangle(double scaleFactor, String label) {
        System.out.println("Scaled by " + scaleFactor + ", labelled: " + label);
    }

    // Scale a rectangle: start with a label, then the scale factor
    // e.g. "take the 'thumbnail' and scale it down by 0.5x"
    public static void scaleRectangle(String label, double scaleFactor) {
        System.out.println("Label: " + label + ", then scaled by: " + scaleFactor);
    }


    public static void main(String[] args) {
        // Strategy 1 demos
        drawRectangle(5);        // one int → picks the square version
        drawRectangle(5, 10);    // two ints → picks the rectangle version

        System.out.println();

        // Strategy 2 demos
        double pixelArea = calculateArea(800, 600);      // two ints → integer version
        double cmArea    = calculateArea(29.7, 21.0);    // two doubles → double version
        System.out.println("Pixel area: " + pixelArea);
        System.out.println("Paper area (cm²): " + cmArea);

        System.out.println();

        // Strategy 3 demos
        scaleRectangle(2.0, "enlarged");    // double first → first version
        scaleRectangle("thumbnail", 0.5);   // String first → second version
    }
}

public class TemperatureConverter {

    // Accepts an int temperature
    public static void displayTemperature(int tempCelsius) {
        System.out.println("[int version]    " + tempCelsius + "°C = "
                + (tempCelsius * 9 / 5 + 32) + "°F");
    }

    // Accepts a double temperature (more precise)
    public static void displayTemperature(double tempCelsius) {
        System.out.println("[double version] " + tempCelsius + "°C = "
                + (tempCelsius * 9.0 / 5.0 + 32.0) + "°F");
    }

    // Accepts a long temperature
    public static void displayTemperature(long tempCelsius) {
        System.out.println("[long version]   " + tempCelsius + "°C  (long type)");
    }

    public static void main(String[] args) {
        int    boilingPoint = 100;         // exact int — Java picks int version directly
        double bodyTemp     = 37.5;        // exact double — Java picks double version directly
        byte   freezingByte = 0;           // byte has no matching version!
                                           // Java promotes: byte → short → int
                                           // finds the int version and uses it
        float  warmDay      = 25.0f;       // float has no matching version!
                                           // Java promotes: float → double
                                           // finds the double version and uses it

        System.out.println("--- Exact matches ---");
        displayTemperature(boilingPoint);  // → int version
        displayTemperature(bodyTemp);      // → double version

        System.out.println();
        System.out.println("--- Type promotion at work ---");
        displayTemperature(freezingByte);  // byte promoted to int → int version
        displayTemperature(warmDay);       // float promoted to double → double version
    }
}

// ─────────────────────────────────────────────────────────────────
// OVERLOADING: Same class, same name, different parameter lists
// ─────────────────────────────────────────────────────────────────
class NotificationService {

    // Send a plain text notification
    public void sendNotification(String message) {
        System.out.println("[Text]  Sending: " + message);
    }

    // Send a notification to a specific user (overloaded — extra parameter)
    public void sendNotification(String message, String username) {
        System.out.println("[Text]  Sending to " + username + ": " + message);
    }

    // Send a notification with a priority level (overloaded — different type combo)
    public void sendNotification(String message, int priorityLevel) {
        System.out.println("[Priority " + priorityLevel + "] Sending: " + message);
    }
}


// ─────────────────────────────────────────────────────────────────
// OVERRIDING: Child class replaces a parent class's behaviour
// ─────────────────────────────────────────────────────────────────
class EmailNotificationService extends NotificationService {

    // @Override means: replace the parent's sendNotification(String) entirely
    // Same name, SAME parameter list — this is overriding, not overloading
    @Override
    public void sendNotification(String message) {
        System.out.println("[Email] Sending via email: " + message);
    }
}


public class OverloadingVsOverridingDemo {
    public static void main(String[] args) {
        NotificationService generic = new NotificationService();
        generic.sendNotification("Server is down");           // overloaded v1
        generic.sendNotification("Server is down", "alice"); // overloaded v2
        generic.sendNotification("Server is down", 1);       // overloaded v3

        System.out.println();

        // EmailNotificationService overrides the single-String version
        EmailNotificationService emailService = new EmailNotificationService();
        emailService.sendNotification("Server is down");           // overridden!
        emailService.sendNotification("Server is down", "alice"); // still inherited v2
    }
}