Inheritance

Watch this video as support 👇

https://youtu.be/yXzoxxIYfgQ

4 pillars of OOP inheritance

4 pillars of abstraction

Inheritance concept

Why does inheritance make sense

Inheritance Problem

Because we dont want to repeat ourselves. D.R.Y. - Dont repeat Yourself

• All users should have a username and a password and all users should be able to generate a new password.

• Only the Admin has a email attribute and can show statistics

Inheritance implemented

Lets try and create this without inheritance

User.java

public class User {
    public String username;
    public String password;

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public String generateNewPassword() {
        return "asd123";
    }
}

PremiumUser.java

public class PremiumUser {
    public String username;
    public String password;

    public PremiumUser(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public String generateNewPassword() {
        return "asd123";
    }

    public boolean putInBasket(int discount) {
        return true;
    }
}

AdminUser.java

public class AdminUser {
    public String username;
    public String password;
    public String email;

    public AdminUser(String username, String password, String email) {
        this.username = username;
        this.password = password;
        this.email = email;
    }

    public String generateNewPassword() {
        return "asd123";
    }

    public String showStatistics() {
        return "Fancy statistics";
    }
}

This code should not be to foreign for you. We create 3 classes that have different attributes. But we are repeating ourselves quite a lot. That is always a bad sign when programming!

Now lets try with inheritance

lets try and simplify the code using inheritance

User.java

public class User {
    public String username;
    public String password;

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public String generateNewPassword() {
        return "asd123";
    }
}

PremiumUser.java

public class PremiumUser extends User {
    public PremiumUser(String username, String password) {
        super(username, password);
    }

    public boolean putInBasket(int discount) {
        return true;
    }
}

AdminUser.java

public class AdminUser extends User {
    public String email;

    public AdminUser(String username, String password, String email) {
        super(username, password);
        this.email = email;
    }

    public String showStatistics() {
        return "Fancy statistics";
    }
}

There are two key parts to this!

• extends - Tells Java that the PremiumUser and AdminUser should extend the functionality of User. In other words they will inherit the functionality of User!

• super see below

• The last thing we do is to take the email from the constructor and set that to this.email.

super

super refers to the instance of the parent class. It works like this. The difference is that this referes to the object of the class where super refers to the object of the parent class!

Lets look at the example above again

In order to get the attributes and methods of the User class we have to initialize the object using the constructor! This we do by writing

super(username, password);

This is the same as doing this: new User("benjamin", "asd123")!

So when we have called super(username, password); the AdminUser now has access to the attributes and method of User!

It is possible to write super.generateNewPassword() because super simply refers to the parent class which is User.

Let's use the new inherited classes!

public class Main {
    public static void main(String[] args) {
        User mieParker = new User("Mie Parker", "fairydust");
        System.out.println(mieParker.generateNewPassword()); // asd123
        PremiumUser jonPedersen = new PremiumUser("Jon pedersen", "shhh!");
        jonPedersen.putInBasket(200); // true
        AdminUser jensFraIt = new AdminUser("Jens pedersen", "password123", "[e-mail]");
        jensFraIt.showStatistics(); // Fancy statistics
    }
}

Relationships

There are different relationships between objects. The replationships can help decide how we should structure our application.

Is a

The is a relationships is fx

• An Apple is a fruit. So we could maybe have base class fruit and apple as child class

• A Volvo is a car. So again a class volvo inherits from Car

Has a

This meant that a class references another class. Fx what if the a Cat has a favourite food that is another class?

class Cat {
    public String name;
    public int mood;
    public boolean isHungry;
    public int energy;
  	public Food favoriteFood;

  public Cat(String name, int mood, boolean isHungry, int energy, Food favoriteFood) {
      this.name = name;
      this.mood = mood;
      this.isHungry = isHungry;
      this.energy = energy;
    	this.favoriteFood = favoriteFood;
  }

  public void meow() {
      System.out.println("meow");
  }
}

Now the cat has a favorite food.

Override

In the example above there is a method called generateNewPassword it simply returns the same password for each user objects. So if a user, PremiumUser or AdminUser gets a new password they would get the same password. This is off course a problem. In a real world scenario the generateNewPassword off course would generate a new password. But let's use this as an opportunity to talk about overriding methods.

In the generateNewPassword what if the PremiumUser and the AdminUser could override the generateNewPassword method. Let's do that for the AdminUser 👇

public class AdminUser extends User {
    public String email;

    public AdminUser(String username, String password, String email) {
        super(username, password);
        this.email = email;
    }

    public String showStatistics() {
        return "Fancy statistics";
    }

    @Override
    public String generateNewPassword() {
        return "new admin password";
    }
}

There are two relevant things here

1. The @override annotation tells Java that the child class method (AdminUser is a child of User) overrides the parent class method (User)

2. public String generateNewPassword() { - simply writing the same method overrides the method from the parent class. @override is not necessary, but it's good practice to have it!

Now at least the individual child classes have their own generateNewPassword method

Exercises

Exercise 1 - level 1

Create a Garment class. A Garment can have a price and a name.

Add some specific garment classes like fx Trousers. Trousers can fx have a length, color etc.

In a main method create a couple of garments and print their value

Exercise 2 - level 1

Create a new class called Computer. Before you add any more code, know that you will need to add two additional classes: Laptop and SmartPhone.

1. For a parent class add 3 properties, 2 methods, and a constructor.

2. For a child class add at least 1 additional property and 1 additional method.

Exercise 3 - level 2

Car

Create a super class called Car. The Car class has the following fields and methods.

• speed

• regularPrice

• color

• getSalePrice()

Truck

Create a sub class of Car class and name it as Truck. The Truck class has the following fields and methods.

• weight

• getSalePrice()

If the weight of a Truck is more than 2000 kg then there is a discount of 10% otherwise 20%

Ford

Create a subclass of Car class and name it as Ford. The Ford class has the following fields and methods

• year

• manufacturerDiscount

• getSalePrice()

If a manufacturerDiscount is set then the salesPrice will be that much cheaper

Sedan

Create a subclass of Car class and name it as Sedan. The Sedan class has the following fields and methods.

• length

• getSalePrice()

If the length is more than 20 meters then 5% discount otherwise 10% discount

AutoShop

Create AutoShop class which contains the main() method. Perform the following within the main() method.

• Create an instance of Sedan class and initialize all the fields with appropriate values. Use super(...) method in the constructor for initializing the fields of the superclass.

• Create two instances of the Ford class and initialize all the fields with appropriate values. Use super(...) method in the constructor for initializing the fields of the super class.

• Create a Truck instance

• Create an instance of Car class and initialize all the fields with appropriate values. Display the sale prices of all instances.

Exercise 4 - level 3

We have to create the classes for a School.

This school have different employees 👇

• Teacher

• Janitor

• Headmaster

• It admin

There are also classes (as in a school class not a java class). A class can have a list of students and a name.

A school can have a list of classes and a list of employees.

Try and think about the different classes you would need to solve this problem. First write the plan down.

When you have a plan start writing the actual classes to solve this problem.

The functionality insinde the methods is not important, but the classes and the class structure is!

For the employees, make a parent class. Think about what sepcific attributes you would have for the different roles