Introduction to Inheritance 9.1/9.2 9.3/9.4 9.5 9.6 9.7 Hacks

Hacks

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9.1 Hacks

  • Implement two new subclasses, Circle and Hexagon, extending from the Shape class. Each shape should have a method to calculate its area and should override the print_something() method to print something unique for that shape. Follow the same structure as the Rectangle and Triangle classes!

  • Optional!

  • Create a new subclass called Ellipse that extends Shape. Update Your Driver Code

  • Constructor: Implement a constructor for Ellipse that accepts parameters for name, length, and width. This constructor should call the superclass constructor using super().

  • Test the Ellipse: Instantiate an Ellipse object and print its area. Verify that the constructor correctly initializes the shape and that the super() keyword is used properly. Hints:

  • Ellipse Constructor: Use super(name, length, width) to initialize inherited fields. Check Order: Remember, super() must be the first statement in your subclass constructor.

9.3 Hacks

  • When overriding the area method for both the Ellipse and the Hexagon use the @Override notation.
  • For the area of the hexagon it may be useful to look into the hexagon area formula:

Area = (3√3 / 2) * s²

  • Also override the calc_perimeter() method for both shapes.
  • use the super keyword to have both shaps use both the parent print_something() and child print_something() method Both shapes should print out something along the lines of: “This is a shape and also a hexagon”

9.5 Hacks

Let’s implement the Triangle subclass to deepen your understanding. Below is a half-completed method for the Triangle class. Your task is to complete the draw method. Make sure your implementation returns a unique string for the Triangle class. This exercise will help reinforce how subclasses can extend functionality.

Advanced Challenge: Area Calculation

Now, let’s enhance our Shape class to include an area calculation feature. Modify the Shape class to include an area method, and implement it in your subclasses. Below is a structure to help you get started: Ensure each subclass calculates and returns its area correctly. This will allow you to practice method overriding further and understand how different shapes can extend base functionalities.

Homework Hack

For your homework, create your own class hierarchy for shapes. You should have a base class called Shape with subclasses Triangle, Rectangle, and Hexagon. Each subclass should implement a method called draw(), returning a unique string for each shape type.

- `Triangle`: "Drawing a triangle."

- `Rectangle`: "Drawing a rectangle."

- `Hexagon`: "Drawing a hexagon."

Make sure to demonstrate polymorphism by creating an array of Shape types and iterating through it to call the draw() method. This will reinforce your understanding of class hierarchies and method overriding.

9.6 Hacks

  • using a previous example of inheritance create an example of polymorphsim, or create an example of polymorphic behavhoir between two classes of Shape and Sqaure
  • Using the previous polymorphism popcorn hack, explain which parts are static and dynamic data types and when that is the case
  • Define Down-Casting in your own words
  • using the previous polymorphism example add an example of down-casting.

9.7 Hacks

  • Create an class where you execute an unchanged method from Object, then execute a different method from Object that you changed.

NOTE from me, Aditya Samavedam: All of my popcorn hacks have been done in the individual lesson files. Below are my answers to the FRQ prompt and Multiple Choices.

FRQ Prompt

Consider a program that manages a collection of books, specifically focusing on textbooks. You are required to implement a class named Textbook that extends an existing class called Book. The Textbook class should include the following features:

  1. A private integer field named edition that represents the edition number of the textbook.
  2. A constructor that takes three parameters: a string for the title, a double for the price, and an integer for the edition. This constructor should invoke the superclass constructor to initialize the title and price.
  3. A method getEdition() that returns the edition of the textbook.
  4. A method canSubstituteFor(Textbook other) that determines if the current textbook can be substituted for another textbook. This method should return true if both textbooks have the same title and the current textbook’s edition is equal to or greater than the other textbook’s edition.
  5. An overridden method getBookInfo() that returns a string representation of the textbook information, including the title, price, and edition.
  6. Optional: Include error handling in the constructor to ensure that the edition is a positive integer, and override the toString() method for convenient output of the textbook information.

Write the complete implementation of the Textbook class, including all specified methods and any additional features you believe would be beneficial.

public class Book {
    private String title;
    private double price;

    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }

    public String getTitle() {
        return title;
    }

    public double getPrice() {
        return price;
    }

    public String getBookInfo() {
        return "Title: " + title + ", Price: $" + price;
    }
}

public class Textbook extends Book {
    private int edition;

    public Textbook(String title, double price, int edition) {
        super(title, price);
        if (edition <= 0) {
            throw new IllegalArgumentException("Edition cannot be a negative integer or zero.");
        }
        this.edition = edition;
    }

    public int getEdition() {
        return edition;
    }

    public boolean canSubstituteFor(Textbook other) {
        return this.getTitle().equals(other.getTitle()) && this.edition >= other.getEdition();
    }

    @Override
    public String getBookInfo() {
        return super.getBookInfo() + ", Edition: " + edition;
    }

    @Override
    public String toString() {
        return getBookInfo();
    }
}
public class Main {
    public static void main(String[] args) {
        Textbook myTextbook1 = new Textbook("Biology 1", 13.59, 2);
        Textbook myTextbook2 = new Textbook("Chemistry 1", 17.59, 4);
        Textbook myTextbook3 = new Textbook("Biology 1", 13.59, 1);

        System.out.println(myTextbook1.getTitle());
        System.out.println(myTextbook2.getTitle());

        System.out.println(myTextbook1.getPrice());
        System.out.println(myTextbook2.getPrice());

        System.out.println(myTextbook1.getEdition());
        System.out.println(myTextbook2.getEdition());

        System.out.println(myTextbook1.getEdition());
        System.out.println(myTextbook2.getEdition());

        System.out.println(myTextbook1.getBookInfo());
        System.out.println(myTextbook2.getBookInfo());

        System.out.println(myTextbook1.canSubstituteFor(myTextbook2));
        System.out.println(myTextbook1.canSubstituteFor(myTextbook3));
    }
}
Main.main(null);

Biology 1
Chemistry 1
13.59
17.59
2
4
2
4
Title: Biology 1, Price: $13.59, Edition: 2
Title: Chemistry 1, Price: $17.59, Edition: 4
false
true

Multiple Choice

1. What is wrong with this block of code?

class Shape{
    private double length = 0;
    private double width = 0;
    
    public Shape(double length, double width){
        this.length = length;
        this.width = width;
    }

    public double getArea(){
        return this.length * this.width;
    }

    private String toString(){
        return "Shape length:"+ (new Double(this.length)).toString() + " width:" + (new Double(this.width)).toString();
    }
}

Shape myShape = new Shape(2,3);

System.out.println(myShape.getArea());

a) You can’t use the this keyword in the constructor

b) When passing a double through an argument it must be in the form of 0.0

c) The toString() method must be public

d) The getArea() method doesn’t return a double

Answer) c) The toString() method must be public so that it can be run. When it is private, it cannot be accessed when trying to print the object.

2. Which method cannot be executed in the following example of Polymorphism

class Water{
    public String toString(){
        return "Water";
    }

    private boolean isSalty(){
        return false;
    }

    public String typeOfWater(){
        return "Static";
    }

}

class Lake extends Water{
    public String toString(){
        return "Lake";
    }

    public boolean isSalty(){
        return true;
    }
}

Water myLakeWater = new Lake();

a) typeOfWater()

b) isSalty()

c) toString()

d) getClass()

Answer) b) isSalty() because in the parent class, it is private but in the child class, it is public, which doesn’t make sense as it cannot be overriden in the child class.