U5 Inheritance P1 Student

9.1 Superclasses and Subclasses

A hierarchy of classes is created, extending attributes into subclasses of information (ie. Automobile –> Trucks and Sedans –> Ford, BMW, Nissan, Toyota).

Pre-Requisites

Before we start you need to remember what classes and methods are. Scopes of variables are also important to this section because you need to know which classes can access which variables before extending a variable across classes.

Scope of Variables:

Variables can be declared as private or public.

Popcorn Hack: What are scope do private and public variables allow?

Private	Public
only directly accessible within the class they are declared	accessible anywhere inside and outside their class

These are different types of variables. Only INSTANCE and STATIC variables can be declared as __ or __.

Definition	Scope within Class	Scope to Subclasses
variables within methods	cannot be accessed outside method	cannot be accessed outside class
variables within a class but not inside a method	can be accessed in entire class	can be accessed in subclasses
variables that belong to a class, not instance	can be accessed in entire class	can be accessed in subclasses
variables specific in passing values to the method	cannot be accessed outside method	do not affect inheritance

Now we can go into class hierarchies.

Class Hierarchy

Definitions

Superclasses - a class that contains all the common __ and __ that could be shared among other classes (a blueprint for subclasses)
Subclasses - extends the __ to is specified by a superclass; can also have additional specific attributes
“Is-A” Relationship - the relationship when a subclass __ a superclass (ie. Automobile –> Sedan; a Sedan “is-a” automobile)

`extends` Keyword

extends the __ from the __

class Automobile {
  public String brand; // public instance var
  private String model; // private instance var

  public Automobile(String brand, String model) {
    this.brand = brand;
    this.model = model;
  }

  public void start() {
    System.out.println("Car is starting");
  }
}

class Truck extends Automobile {
  public int cargoCapacity; // subclass specific var

  // instance that is specific to the Truck subclass, with vars from Automobile class
  public Truck(String brand, String model, int cargoCapacity) {
    super(brand, model); // inherited vars
    this.cargoCapacity = cargoCapacity;
  }

  // specific method to Truck
  public void loadCargo() {
    System.out.println("Loading cargo into the truck");
  }
}

class Sedan extends Automobile {
  public boolean isLuxury; // subclass specific var

  // instance that is specific to the Sedan subclass, with vars from Automobile class
  public Sedan(String brand, String model, boolean isLuxury) {
    super(brand, model); // inherited vars
    this.isLuxury = isLuxury;
  }

  // specific method to Sedan
  public void accelerate() {
    System.out.println("Sedan is accelerating");
  }
}

public class Main {
    public static void main(String[] args) {
        Automobile car = new Automobile("Toyota", "Camry");
        Truck truck = new Truck("Ford", "F-150", 1000);
        Sedan sedan = new Sedan("BMW", "328i", true);

        // automobile methods and variables
        System.out.println(car.brand); // Accessing public variable
        // System.out.println(car.model); // compilation error because of the private var
        car.start();


        // truck methods and variables
        System.out.println(truck.brand); // inherited public var
        System.out.println(truck.cargoCapacity); // public var specific to truck
        truck.loadCargo();


        // sedan methods and variables
        System.out.println(sedan.brand); // inherited public var
        System.out.println(sedan.isLuxury); // public var specific to sedan
        sedan.accelerate();
    }
}

Main.main(null);

Toyota
Car is starting
Ford
1000
Loading cargo into the truck
BMW
true
Sedan is accelerating

This example shows how the Automobile class is extended twice, with the Truck and Sedan subclasses.

Popcorn Hack: If I were to declare a variable color that is private in the class Automobile, would I be able to extend and directly access that variable to the subclass Truck or Sedan?

A: No, you would need to encapsulate the private variable as shown above, through a method in the superclass and then inherit that var with super(), which will be explained later.

9.2 Writing Constructors for Subclasses

Learning Objectives

Constructors are not inherited
When a subclass’s constructor doesn’t explicitly call a superclass’s constructor using super, Java inserts a call to the superclass’s no-argument constructor.
The actual parameters passed in the call to the superclass constructor provide values that the constructor can use to initialize the object’s instance variables.
Regardless of whether the superclass constructor is called implicitly or explicitly, the process of calling superclass constructors continues until the Object constructor is called. At this point, all of the constructors within the hierarchy execute beginning with the Object constructor.

Important note: Constructors are NOT inherited by the subclass. See this in action below.

// TO BE INCLUDED EARLIER IN THE LESSON. IT IS NECESSARY FOR THE FUNCTIONALITY OF THIS SECTION.

class Vehicle {
    public int year;
    public String manufacturer;

    public Vehicle(int year, String manufacturer) { // constructor for parent class
        this.year = year;
        this.manufacturer = manufacturer;
    }

    public Vehicle() {
        this.year = 2000;
        this.manufacturer = "Unknown";
    }

    // method to be used later
    public void drive() {
        System.out.println("The driver is driving the car.");
    }
}

class Car extends Vehicle {
    public String model;

    public Car(String model) {
        this.model = model;
    }
}

public class VehicleDemonstration {
    public static void main(String[] args) {
        Car myCar = new Car("Altima");
        System.out.println("Year: " + myCar.year);
        System.out.println("Manufacturer: " + myCar.manufacturer);
        System.out.println("Model: " + myCar.model);
    }
}

VehicleDemonstration.main(null);

Year: 2000
Manufacturer: Unknown
Model: Altima

As you can see, the output uses the no-argument construction info from the base Vehicle constructor.

The super keyword can be used to change parent constructor values.

class NewCar extends Vehicle {
    public String model;

    public NewCar(int year, String manufacturer, String model) {
        super(year, manufacturer); // see the use of super here
        // what happens if you use no arguments with super()? see reminders below
        this.model = model;
    }
}

public class VehicleDemonstration2 {
    public static void main(String[] args) {
        NewCar myCar = new NewCar(2016, "Nissan", "Altima");
        System.out.println("Year: " + myCar.year);
        System.out.println("Manufacturer: " + myCar.manufacturer);
        System.out.println("Model: " + myCar.model);
    }
}

VehicleDemonstration2.main(null);

Year: 2016
Manufacturer: Nissan
Model: Altima

Key Reminders:

If you do call super() in your constructor, it has to be the first line of the constructor.
You cannot assign values to parent attributes/variables without using super().
If you call super() with no arguments, it will use the no-argument parent constructor. This also happens automatically if you don’t include any super() call.

9.3 Overriding Methods

Learning Objectives

Method overriding occurs when a public method in a subclass has the same method signature as a public method in the superclass.
Any method that is called must be defined within its own class or its superclass.
A subclass is usually designed to have modified (overwritten) or additional methods or instance variables.
A subclass will inherit all public methods from the superclass; these methods remain public in the subclass.

There are three options for methods to be used by subclasses:

Methods inherited from the parent class
Unique methods written for the subclass
Override parent methods to modify its implementation

The first two should make sense. Let’s see an example of overriding parent methods below.

public class NuroCar extends Vehicle {
    private String deliveryItem;

    // unique constructor
    public NuroCar(int year, String manufacturer, String deliveryItem) {
        super(year, manufacturer); // another use of super
        this.deliveryItem = deliveryItem;
    }

    // HERE is the overridden function
    public void drive() {
        System.out.println("This car is driving itself!");
    }
}

public class VehicleDemonstration3 {
    public static void main(String[] args) {
        NuroCar pizzaCar = new NuroCar(2023, "Nuro", "Pizza");
        // here's the call to the overridden function
        pizzaCar.drive();
    }
}

VehicleDemonstration3.main(null);

This car is driving itself!

This can be very helpful if you want a certain parent method to function slightly differently for a certain subclass.

Popcorn Hack

A parent class Animal is often used to show how subclasses can differ from their parent classes. An Animal parent class is provided in the cell below. Create a subclass of a certain species that overrides a parent method and uses `super` to call to the parent's constructor while adding its own unique attributes.

Hint:

// parent class
public class Animal {
    private String species;
    private int milesPerHour;

    // no argument constructor
    public Animal() {
        this.species = "Unknown";
        this.milesPerHour = 10;
    }

    // constructor with arguments
    public Animal(String species, int milesPerHour) {
        this.species = species;
        this.milesPerHour = milesPerHour;
    }

    // parent method
    public void move() {
        System.out.println("The " + this.species.toLowerCase() + " runs at " + this.milesPerHour + " miles per hour.");
    }
}

// your subclass goes here

9.4 Super Keyword

Using the super keyword to call a superclass’s method.

public class Performer { //superclass
    public void practice(){
        System.out.println("Honing my craft!");
    }
    public void perform(){
        System.out.println("Performing for an audience!");
    }
}

public class Dancer extends Performer { //subclass
    public void perform(){
        System.out.println("Dancing on the stage!");
    }
}

public class BalletDancer extends Dancer { //subclass
    public void jete(){
        System.out.println("Leaping...");
    }
    public void pirouette(){
        System.out.println("Spinning...");
    }
    public void perform(){
        jete();
        pirouette();
    }
        public static void main(String[] args){
            BalletDancer derrick = new BalletDancer();
            derrick.practice();
            derrick.perform();
        }
}

System.out.println("BalletDancer class: ");
BalletDancer.main(null);

BalletDancer class: 
Honing my craft!
Leaping...
Spinning...

public class Performer { //superclass of Dancer class
    public void practice(){
        System.out.println("Honing my craft!");
    }
    public void perform(){
        System.out.println("Performing for an audience!");
    }
}

public class Dancer extends Performer { //superclass of BalletDancer class
    public void perform(){
        System.out.println("Dancing on the stage!");
    }
}

public class BalletDancer extends Dancer {
    public void jete(){
        System.out.println("Leaping...");
    }
    public void pirouette(){
        System.out.println("Spinning...");
    }
    public void perform(){ 
        perform();//Why is this wrong?
        jete();
        pirouette();
    }
        public static void main(String[] args){
            BalletDancer derrick = new BalletDancer();
            derrick.practice();
            derrick.perform();
        }
}

System.out.println("BalletDancer class: ");
BalletDancer.main(null);

Note:

Super keyword can be placed in any order as it prints out chronologically.

public void perform(){ 
    super.perform();
    jete();
    pirouette();
}

"Honing my craft!
Dancing on the stage!
Leaping...
Spinning..."

However, if we were to write:

public void perform(){ 
    jete();
    pirouette();
    super.perform();
}

"Honing my craft!
Leaping...
Spinning...
Dancing on the stage!"

What if we use 2 super keywords?

public class Performer { //superclass
    public void practice(){
        System.out.println("Honing my craft!");
    }
    public void perform(){
        System.out.println("Performing for an audience!");
    }
}

public class Dancer extends Performer { //subclass
    public void perform(){
        super.perform();
        System.out.println("Dancing on the stage!");
    }
}

public class BalletDancer extends Dancer { //subclass
    public void jete(){
        System.out.println("Leaping...");
    }
    public void pirouette(){
        System.out.println("Spinning...");
    }
    public void perform(){
        super.perform();
        jete();
        pirouette();
    }
        public static void main(String[] args){
            BalletDancer derrick = new BalletDancer();
            derrick.practice();
            derrick.perform();
        }
}

System.out.println("BalletDancer class: ");
BalletDancer.main(null);

BalletDancer class: 
Honing my craft!
Performing for an audience!
Dancing on the stage!
Leaping...
Spinning...

Popcorn Hack

Create a subclass and a superclass by calling the methods from the superclass from the subclass using the keyword super.

// place code here

Hacks

Create a superclass with at least 2 subclasses based on your own topic.

Create a DrawIO diagram for your structure
Create a superclass on your own topic
Create at least two subclasses
Each class must create at least two methods, one private and public variable, and examples of local, static, instance, and parameter variables
Override at least one method
Use the super keyword at least once

U5 Inheritance P1 Student • 30 min read

Description

9.1 Superclasses and Subclasses

Pre-Requisites

Scope of Variables:

Class Hierarchy

`extends` Keyword

9.2 Writing Constructors for Subclasses

Learning Objectives

Key Reminders:

9.3 Overriding Methods

Learning Objectives

Popcorn Hack

9.4 Super Keyword

Using the super keyword to call a superclass’s method.

Note:

However, if we were to write:

What if we use 2 super keywords?

Popcorn Hack

Hacks

9.1 Superclasses and Subclasses

Pre-Requisites

Scope of Variables:

Class Hierarchy

extends Keyword

9.2 Writing Constructors for Subclasses

Learning Objectives

Key Reminders:

9.3 Overriding Methods

Learning Objectives

Popcorn Hack

9.4 Super Keyword

Using the super keyword to call a superclass’s method.

Note:

However, if we were to write:

What if we use 2 super keywords?

Popcorn Hack

Hacks

`extends` Keyword