Overriding Methods

Last lesson we saw that a child class can replace a parent's method with its own version. That's called overriding. Java has a special annotation for it: @Override

class Criminal {
    void speak() {
        System.out.println("...");
    }
}

class Driver extends Criminal {
    @Override
    void speak() {
        System.out.println("Get in the car!");
    }
}

The @Override annotation isn't technically required, your code compiles without it. But you should always use it. Here's why: if you accidentally misspell the method name, Java will think you're creating a NEW method instead of overriding the old one. With @Override, Java checks that the parent actually has that method and yells at you if it doesn't

class Driver extends Criminal {
    @Override
    void speek() {  // TYPO! Compiler error because Criminal has no speek()
        System.out.println("Get in the car!");
    }
}

Without @Override, this would silently create a useless speek() method and the bug would haunt you for hours. Trust me, Tommy Vercetti doesn't have time for that

Sometimes you don't want to REPLACE the parent's behavior, you want to ADD to it. That's where super.method() comes in

class Vehicle {
    void start() {
        System.out.println("Vehicle starting...");
    }
}

class Car extends Vehicle {
    @Override
    void start() {
        super.start();  // call the parent's version first
        System.out.println("Car engine revving!");
    }
}

public class Main {
    public static void main(String[] args) {
        Car c = new Car();
        c.start();
    }
}

Output

Vehicle starting...
Car engine revving!

super.start() says "run the parent's version of start() first, THEN do my extra stuff." It's like Lance Vance doing everything a regular criminal does, plus his own side missions

Overriding vs Overloading: these sound similar but they're completely different things

Overriding: child class replaces a parent method (same name, same parameters)

class Criminal {
    void speak() { ... }
}
class Driver extends Criminal {
    @Override
    void speak() { ... }  // OVERRIDING: replaces parent's speak()
}

Overloading: same class has multiple methods with the same name but DIFFERENT parameters

class Garage {
    void repair(String car) {
        System.out.println("Repairing " + car);
    }
    void repair(String car, int hours) {  // OVERLOADING: different param count
        System.out.println("Repairing " + car + " in " + hours + " hours");
    }
}

Overriding = between parent and child, same signature. Overloading = same class, different signatures. Don't mix them up on a test :)

Here's a full example with Tommy's businesses

class Business {
    int earnings() {
        return 0;
    }
}

class Club extends Business {
    int customers;

    Club(int customers) {
        this.customers = customers;
    }

    @Override
    int earnings() {
        return customers * 50;
    }
}

class CarWash extends Business {
    int cars;
    int price;

    CarWash(int cars, int price) {
        this.cars = cars;
        this.price = price;
    }

    @Override
    int earnings() {
        return cars * price;
    }
}

public class Main {
    public static void main(String[] args) {
        Club c = new Club(120);
        CarWash w = new CarWash(30, 8);
        System.out.println("Club earnings: " + c.earnings());
        System.out.println("Car wash earnings: " + w.earnings());
    }
}

Output

Club earnings: 6000
Car wash earnings: 240

Mission: Tommy's Business Earnings

Tommy owns several businesses in Vice City, each making money its own way. He needs a program that calculates the earnings of each one.

Create a Business class with an earnings() method that returns 0. Then create two child classes:

1. Club: has an int customers field, overrides earnings() to return customers * 50
2. CarWash: has int cars and int price fields, overrides earnings() to return cars * price

Use @Override on both. In main, store the values in variables: customers for the club, cars and price for the car wash. Create a Club from customers and a CarWash from cars and price, then print each one's earnings with the labels shown below.

Example

Club earnings: 6000
Car wash earnings: 240