activity_3.md

Activity 3

Introduction to Inheritance

Exercise 3.1 - Introductory Reading - Inheritance

Pillar 3 - What is 'Inheritance'?

When modelling classes using Object-Oriented Programming, we may want to have a class inherit attributes/behaviour from another class. This concept is called inheritance.

Just like bear cubs can inherit traits from their parents; it is the same when modelling systems. We call the (parent) class the base or superclass and any particular (child) class that inherits (i.e. extends) from this base class the subclass. It is often the case that the base class constitutes the most common attributes and behaviours shared by the subclasses.

In the Animal Kingdom, we can have a class hierarchy like the following:

Animal -> Bird -> Pigeon

The Animal class is highest in the hierarchy, so it contains the blueprint for what it means to be an Animal.

The Bird class is a type of Animal, but is more specific and defines the attributes and behaviours most common to Birds. A Pigeon is a type of Bird and it's even more specific.

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Exercise 3.2 - Representing inheritance in class diagrams & the Unified Modelling Language (UML)

• Can you do some research to find out how to represent inheritance in Class Diagrams?

• Draw a diagram to show a Pigeon subclass extending the base class Bird

• The Pigeon class will inherit the following attributes and methods from the Bird class:

• Name

• Color

• Speak

• Fly

• In addition to this, the Pigeon will also have a method called EatPizza, without a return data type as when the method is called, it will write an output string to the console. Only the Pigeon can EatPizza.

You can use pen and paper or any digital drawing tool you prefer.

An example solution to the diagram is available here.

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Exercise 3.3 - Translate the class diagram into code to create a subclass Pigeon class which extends the base class Bird

In your existing Java Console (Command Line) Application Project:

• Add an empty class called Pigeon
• Add the following code to make the Pigeon extend base class Bird
• Add the method EatPizza to Pigeon

//The Pigeon extends the base class Bird
public class Pigeon extends Bird {

    //The Pigeon constructor calls the base class Bird constructor
    //The name and color parameters are passed to the Bird constructor
    public Pigeon(String name, String color) {
        super(name, color);
    }

    public void eatPizza() {
        System.out.println("Delicious pizza!");
    }
}

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Exercise 3.4 - Run your console application

• Add the following code to the main method in the Main class beneath your code from Activity 2.

Pigeon pippa = new Pigeon("Pippa", "Grey");
pippa.speak();
pippa.fly();
pippa.eatPizza();

• Run your console application. You should see the following output to your console.

My name is Tweety and I am a Yellow bird.
I'm Tweety and I can fly high in the blue sky!
My name is Pippa and I am a Grey bird.
I'm Pippa and I can fly high in the blue sky!
Delicious pizza!

Pigeon has inherited methods and properties from Bird!

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Optional Challenge

• Can you add another method on the Pigeon class called Sleep that writes the following string to the console "I'm a sleeping pigeon"?

Click here to see the solution

public class Pigeon extends Bird {

    public Pigeon(String name, String color) {
        super(name, color);
    }

    public void eatPizza() {
        System.out.println("Delicious pizza!");
    }

    public void Sleep() {
        System.out.println("I'm a sleeping pigeon");
    }
}

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What's next?

Wouldn't it be nice to have more flexibility for subclasses to implement methods how they like. Head over to Introduction to Polymorphism in Activity 4 to learn more.

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Key Study Notes

• Inheritance - We call the (parent) class the base or superclass and any particular (child) class that inherits (i.e. extends) from this base class the subclass. This is like Bear cubs inheriting traits from their parents.