The super Keyword

Part of the Object Oriented Programming section of Coddy's Java journey — lesson 21 of 87.

The super keyword provides a way for a subclass to reference its parent class. It's essential when you need to access parent class members or constructors from within a child class.

The most common use of super is calling the parent's constructor. This must be the first statement in the subclass constructor:

public class Vehicle {
    private String brand;
    
    public Vehicle(String brand) {
        this.brand = brand;
    }
    
    public String getBrand() {
        return brand;
    }
}

public class Car extends Vehicle {
    private int doors;
    
    public Car(String brand, int doors) {
        super(brand);  // Must be first line
        this.doors = doors;
    }
}

You can also use super to call parent methods, which is particularly useful when the subclass has a method with the same name:

public class Animal {
    public void makeSound() {
        System.out.println("Some sound");
    }
}

public class Cat extends Animal {
    public void makeSound() {
        super.makeSound();  // Calls Animal's version
        System.out.println("Meow!");
    }
}

Think of super as the opposite of this. While this refers to the current object, super refers to the parent class portion of that object. This distinction becomes crucial when building class hierarchies where child classes need to extend rather than completely replace parent behavior.

Challenge

Easy

Let's build an appliance system that demonstrates how child classes can use super to call parent constructors and extend parent method behavior.

You'll create three files to organize your code:

Appliance.java: Create the parent class representing any household appliance. It should have:
- A private field for the appliance's brand (String)
- A private field for wattage (int)
- A constructor that accepts both values and initializes them
- A method getBrand() that returns the brand
- A method getWattage() that returns the wattage
- A method getInfo() that returns: [brand] - [wattage]W
WashingMachine.java: Create a subclass that extends Appliance. A washing machine has everything an appliance has, plus its own special features:
- A private field for capacity (int) representing load capacity in kg
- A constructor that takes brand, wattage, and capacity - use super(brand, wattage) to initialize the parent portion, then set the capacity
- A method getCapacity() that returns the capacity
- A method getInfo() that calls the parent's getInfo() using super.getInfo() and extends it to return: [parent info], Capacity: [capacity]kg
Main.java: Create a WashingMachine and display its complete information. You'll receive three inputs: the brand (String), wattage (int), and capacity (int). Print the result of calling getInfo() on your washing machine.

You will receive three inputs: brand name, wattage, and capacity.

Notice how super serves two purposes here: calling the parent constructor to initialize inherited fields, and calling the parent's method to build upon its behavior rather than replacing it entirely.

Cheat sheet

The super keyword references the parent class from within a subclass.

Use super() to call the parent constructor. This must be the first statement in the subclass constructor:

public class Vehicle {
    private String brand;
    
    public Vehicle(String brand) {
        this.brand = brand;
    }
}

public class Car extends Vehicle {
    private int doors;
    
    public Car(String brand, int doors) {
        super(brand);  // Must be first line
        this.doors = doors;
    }
}

Use super.methodName() to call parent methods, useful when the subclass overrides a method:

public class Animal {
    public void makeSound() {
        System.out.println("Some sound");
    }
}

public class Cat extends Animal {
    public void makeSound() {
        super.makeSound();  // Calls Animal's version
        System.out.println("Meow!");
    }
}

super refers to the parent class portion of an object, while this refers to the current object.

Try it yourself

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // Read inputs
        String brand = scanner.nextLine();
        int wattage = scanner.nextInt();
        int capacity = scanner.nextInt();
        
        // TODO: Create a WashingMachine object with the input values
        
        // TODO: Print the result of calling getInfo() on the washing machine
    }
}