Unit 9 Lesson

9.1 What is inheritance?

• Inheritance is like a family, except the kids only have one parent instead of two
• For example:

The java code for it:

class Mom{
    // CODE
}
class Son extends Mom{
    // CODE
}
class Daughter extends Mom{
    // CODE
}

In this example, the Son and Daughter inherits the Mom, meaning it inherit the components of the mother. This makes the “Son” and “Daughter” classes the subclasses of the “Mom” class as they inherit the “Mom” class components and the “Mom” class is the superclass.

9.2 Using the Super keyword for Constructors

• One keyword to know is the super keyword
• The super keyword allows the subclass to store key variables in the main class’s constructor (also known as the super class)
• Example below:

public class Vehicle { //This is the Superclass, it inherits the key variables for its subclasses
    public String Name; //They don't have to be public, but I just put public word for fun
    public String Sound;
    public int creation;
    public int Mph;
    public Vehicle(String name, int dateMade, String sound, int mph){ //Similar to the constructor used in Javascript. It maintains values within this superclass
        Name = name; 
        Sound = sound;
        creation = dateMade;
        Mph = mph;
    }
}

public class Car extends Vehicle {
    public int capacity;
    public Car(String name, int dateMade, String sound, int mph, int passangerCapacity){
        super(name, dateMade,sound, mph); //Uses the superclass's constructor to store the key variables for the Car subclass
        capacity = passangerCapacity;
    }
    public void Information(){ //Prints out the values the super class's constructors inherits
        System.out.println(super.Sound + " " + this.Sound);
        System.out.println("I am the " + super.Name);
        System.out.println("I was made in " + super.creation);
        System.out.println("I move at " + super.Mph +" miles per hour");
        System.out.println("I hold at most " + capacity + " people");
    }
}

public class Test {
    public static void main(String[] args){
        Car Tesla = new Car("Tesla", 2003, "VROOM!", 200, 5);
        Tesla.Information();
    }
}

Test.main(null);

VROOM! VROOM!
I am the Tesla
I was made in 2003
I move at 200 miles per hour
I hold at most 5 people

Popcorn Hack:

Make it so that a new instance of Bob runs

public class Worker {
    String Name;
    int Age;
    int Salary;

    public Worker(String name, int age, int salary) {
        this.Name = name;
        this.Age = age;
        this.Salary = salary;
    }
}

public class Bob extends Worker {
    String Position;

    public Bob(String name, int age, int salary, String position) {
        super(name, age, salary);
        this.Position = position;
    }

    public void Information() {
        System.out.println("My name is " + super.Name);
        System.out.println("My age is " + super.Age);
        System.out.println("My salary is " + super.Salary);
        System.out.println("My position is " + Position); // Use 'Position' here, not 'position'
    }
}

public class Test {
    public static void main(String[] args) {
        Bob bob = new Bob("Vivian", 17, 20, "intern");
        bob.Information();
    }
}

Test.main(null);

My name is Vivian
My age is 17
My salary is 20
My position is intern

9.3 Overriding Methods

Method overriding is a concept in object-oriented programming (OOP) that allows a subclass to provide a specific implementation of a method that is already defined in its superclass. This enables a subclass to provide its own behavior for a method while maintaining a relationship with its superclass.

In the context Java, here’s how method overriding works:

Inheritance: Method overriding is closely related to inheritance. You have a superclass (or base class) and a subclass (or derived class). The subclass inherits properties and behaviors from the superclass.

Superclass Method: The superclass defines a method. This method can be overridden by the subclass.

Subclass Overrides: In the subclass, you can provide a new implementation of the same method. This is done by using the same method name, return type, and parameter list.

@Override Annotation (Java): In Java, it’s common to use the @Override annotation to explicitly indicate that a method in the subclass is intended to override a method in the superclass. This helps catch errors during compilation if the method doesn’t correctly match the superclass method signature.

Why Override Methods:

Method overriding is used for several reasons:

Customization: It allows you to customize or extend the behavior of a superclass method in the subclass to meet the specific needs of the subclass.

Polymorphism: Method overriding is a key component of polymorphism. It enables you to treat objects of the subclass as if they were objects of the superclass, promoting flexibility and extensibility.

Consistency: Method overriding helps maintain a consistent interface for classes in an inheritance hierarchy. Code that uses the superclass doesn’t need to be changed when a subclass overrides a method.

Code Reusability: It promotes code reusability by allowing you to build on existing code in the superclass.

class Animal {
    void makeSound() {
        System.out.println("Animals make sounds");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Dog barks");
    }
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Cat meows");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal animal = new Animal();
        Animal dog = new Dog();
        Animal cat = new Cat();

        animal.makeSound(); // Output: Animals make sounds
        dog.makeSound();    // Output: Dog barks
        cat.makeSound();    // Output: Cat meows
    }
}

Main.main(null);

Animals make sounds
Dog barks
Cat meows

In this example:

We have a base class Animal with a method makeSound().

We create two subclasses, Dog and Cat, which inherit from the Animal class.

Both Dog and Cat classes override the makeSound() method with their own implementations.

In the main method, we create instances of the base class and its subclasses.

We demonstrate polymorphism by calling the makeSound() method on objects of the base class and the subclasses. The method called depends on the actual type of the object, not the reference type.

This showcases how method overriding allows you to provide specific implementations for methods in subclasses, promoting polymorphism and custom behavior for each subclass.

Another Example:

<img class=”image” src=”https://github.com/AniCricKet/musical-guacamole/assets/91163802/576237f9-cdc4-409b-84f9-96dffe0cdd5c” width=32%> <img class=”image” src=”https://github.com/AniCricKet/musical-guacamole/assets/91163802/03923e22-2b6e-4e4d-9244-1d5145f6c6d9” width=32%> <img class=”image” src=”https://github.com/AniCricKet/musical-guacamole/assets/91163802/5fe0c72c-c17b-4edb-a567-8c9098998aac” width=32%>

Imagine you’re building a program to manage sports team rosters. You can have a base class ‘Athlete’ representing common attributes and actions of all athletes. Then, create subclasses for specific sports like ‘FootballPlayer’, ‘BasketballPlayer’, and ‘SoccerPlayer’.

// Base Class
class Athlete {
    String name;
    int age;
    int jerseyNumber;
    String position;

    public Athlete(String name, int age, int jerseyNumber, String position) {
        this.name = name;
        this.age = age;
        this.jerseyNumber = jerseyNumber;
        this.position = position;
    }

    public void train() {
        System.out.println(name + " is training.");
    }

    public void displayInfo() {
        System.out.println("Athlete Info:");
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Jersey Number: " + jerseyNumber);
        System.out.println("Position: " + position);
    }
}

Athlete athlete = new Athlete("John Mortensen", 19, 4, "Teacher");
athlete.train();
athlete.displayInfo();

John Mortensen is training.
Athlete Info:
Name: John Mortensen
Age: 19
Jersey Number: 4
Position: Teacher

class FootballPlayer extends Athlete {
    public FootballPlayer(String name, int age, int jerseyNumber, String position) {
        super(name, age, jerseyNumber, position);
    }

    @Override
    public void train() {
        System.out.println(name + " is practicing football drills.");
    }

    @Override
    public void displayInfo() {
        super.displayInfo();
    }
}

class BasketballPlayer extends Athlete {
    public BasketballPlayer(String name, int age, int jerseyNumber, String position) {
        super(name, age, jerseyNumber, position);
    }

    @Override
    public void train() {
        System.out.println(name + " is shooting 3s on the court.");
    }

    @Override
    public void displayInfo() {
        super.displayInfo();
    }
}

class SoccerPlayer extends Athlete {
    public SoccerPlayer(String name, int age, int jerseyNumber, String position) {
        super(name, age, jerseyNumber, position);
    }

    @Override
    public void train() {
        System.out.println(name + " is practicing taking free kicks.");
    }

    @Override
    public void displayInfo() {
        super.displayInfo();
    }
}

FootballPlayer footballPlayer = new FootballPlayer("Tyreek Hill", 28, 10, "Wide Receiver");
BasketballPlayer basketballPlayer = new BasketballPlayer("Jimmy Butler", 32, 22, "Small Forward");
SoccerPlayer soccerPlayer = new SoccerPlayer("Neymar Jr", 31, 10, "Left Winger");

footballPlayer.train();
footballPlayer.displayInfo();
System.out.println();

basketballPlayer.train();
basketballPlayer.displayInfo();
System.out.println();

soccerPlayer.train();
soccerPlayer.displayInfo();
System.out.println();

Tyreek Hill is practicing football drills.
Athlete Info:
Name: Tyreek Hill
Age: 28
Jersey Number: 10
Position: Wide Receiver

Jimmy Butler is shooting 3s on the court.
Athlete Info:
Name: Jimmy Butler
Age: 32
Jersey Number: 22
Position: Small Forward

Neymar Jr is practicing taking free kicks.
Athlete Info:
Name: Neymar Jr
Age: 31
Jersey Number: 10
Position: Left Winger

Explanation:

In this Java code, you have a basic “Athlete” class with information and actions that all athletes share. Then, there are specific types of athletes (football, basketball, and soccer players) that inherit these common traits but also have their unique behaviors, like training routines. Method overriding lets them have their own way of training while keeping the shared information, making the code easy to manage and reuse for different types of athletes.

Popcorn Hack:

Why is it helpful to have a common base class like ‘Athlete’ for all these different types of athletes? How does it make the code more organized?

• every athlete has commonalities so by having a base class ‘Athlete’ helps to allow all sub classes, or specific sport athletes to inherit those features via the extend syntax. However, each type of athlete may have different forms of training, diet, etc which can be changed using override. This way, you don’t need to recreate an Athlete class for each athlete, you only need to override specific methods within the base class. You can have multiple subclasses with unique attributes and methods. This makes the code more concise and organized.

9.4 Using Super keyword for Methods

• Why only use super for constructors when you can use them for methods too?
• With the super key word, not only can you store variables, but also store methods

class Animal{
    public void Introduction(String name){
        System.out.println("I am a " + name);
    }
}
class Dog extends Animal{ 
    public void Woof(){
        super.Introduction("Dog");//Inherits the introduction method in the Animal Class, then introduces itself as a dog
        System.out.println("Woof!"); //Does its own thing
    }
}
class Cow extends Animal{
    public void Moo(){
        super.Introduction("Cow");//Inherits the introduction method in the Animal Class, then introduces itself as a cow
        System.out.println("MOOOO!");//Does its own thing
    }
}
class Test{
    public static void main(String[] args){
        Dog dog = new Dog();
        Cow cow = new Cow();
        dog.Woof();
        cow.Moo();
    }
}
Test.main(null);

I am a Dog
Woof!
I am a Cow
MOOOO!

9.4 Hack

Finish up the code with this criteria: All subclasses must say their origin, the origin can be from SchoolSupply class, and it must run through main.

class SchoolSupply {
    public void BasicInfo(String supply) {
        System.out.println("I am a " + supply);
    }
}

class Pencil extends SchoolSupply {
    public void Information() {
        super.BasicInfo("pencil");  //BasicInfo origin
        System.out.println("I write.");
    }
}

class Eraser extends SchoolSupply {
    public void Information() {
        super.BasicInfo("eraser");   //BasicInfo origin
        System.out.println("I remove mistakes.");
    }
}

public class Test {
    public static void main(String[] args) {
        Pencil pencil = new Pencil();
        Eraser eraser = new Eraser();

        System.out.println("Pencil's Information:");
        pencil.Information();

        System.out.println("\nEraser's Information:");
        eraser.Information();
    }
}

Test.main(null);

Pencil's Information:
I am a pencil
I write.

Eraser's Information:
I am a eraser
I remove mistakes.

9.5 Creating References Using Inheritance Hierarchies

Inheritance can be thought as an upside down tree with the root on the top and the leaves on the bottom. The root is the superclass while the leaves are the subclasses of this superclass. A visual representation of this tree is called a type diagram or hierarchy tree.

A sample structure would be like:

public class A
public class B extends A
public class C extends B
public class D extends C
public class E extends I
public class F extends I
public class G extends H
public class H extends A
public class I extends H

Popcorn Hack

• Draw a hierarchy tree for the above structure and add the picture here

This structure works as C not only inherits properties from B, but it also inherits properties from A. B is like C’s parent and A is like C’s grandparent. Think about it like real life, all families inherit something from their ancestors.

In addition, you could also create objects like this:

C c = new C();
B c = new C();
A c = new C();

// This is the above example in code form

class A {}
class B extends A {}
class C extends B {}

public class Main {
    public static void main(String[] args) {
        C c = new C(); // variable c is of type C
        B b = new C(); // variable b is of type B, but refers to an instance of C
        A a = new C(); // variable a is of type A, but refers to an instance of C
    }
}

9.6 Polymorphism

A polymorphic variable is polymorphic when it can refer to objects from different classes at different points in time.

• A reference variable can store a reference to its declared class or any subclass of its declared class.

A method or operator is considered polymorphic when they are overridden in at least one subclass.

Polymorphism is the act of executing an overridden non-final method from the correct class at runtime based on the actual object type.

Polymorphism allows dynamic binding for a method call to be executed based on the class of the object referenced instead of the declared class.

Example 1

Java polymorphism is mainly split into 2 types

Runtime

• Process in which a function call to the overridden method is resolved at Runtime. This type of polymorphism is achieved by Method Overriding.

Compile Time

• Also known as static polymorphism. This type is achieved by function overloading or operator overloading
• Note: But java doesn’t support Operator Overloading
• When there are multiple functions with the same name but different parameters then these functions are said to be overloaded. Functions can be overloaded by changes in the number of arguments or/and a change in the type of arguments.

Here is an example of compile polymorphism

// Class 1
// Helper class
class Helper {
 
    // Method 1
    // Multiplication of 2 numbers
    static int Multiply(int a, int b)
    {
 
        // Return product
        return a * b;
    }
 
    // Method 2
    // // Multiplication of 3 numbers
    static int Multiply(int a, int b, int c)
    {
 
        // Return product
        return a * b * c;
    }
}
 
// Class 2
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Calling method by passing
        // input as in arguments
        System.out.println(Helper.Multiply(2, 4));
        System.out.println(Helper.Multiply(2, 7, 3));
    }
}
GFG.main(null)

8
42

Example 2 & Popcorn Hack

Before executing cell, look at the example below. Think about which methods compiles? Which methods execute?

import java.util.Random;

public class Entertainer{
    private String talent;
    public Entertainer (String t){
        talent = t;
    }
    public String getTalent(){
        return talent;
    }
}

public class Comedian extends Entertainer{
    private ArrayList<String> jokes;
    public Comedian(String t, ArrayList<String> jks){
        super(t);
        jokes = jks;
    }
    public String getTalent(){
        return "Comedy style: " + super.getTalent();
    }
    public String tellJoke(){
        return jokes.get((int)(Math.random()*jokes.size()));
    }
}

public class Main {
    public static void main(String[] args) {
        // Create an Entertainer object
        Entertainer kevin = new Entertainer("Musician");
        System.out.println(kevin.getTalent());

        // kevin is not a comedian so he can not access that class and thus cannot tell a joke
        // System.out.println(kevin.tellJoke());

        // Create an ArrayList of jokes
        ArrayList<String> oneLiners = new ArrayList<String>();
        oneLiners.add("Why did the programmer quit his job?");
        oneLiners.add("Why did the developer go broke?");

        // Create a Comedian object using the Entertainer reference
        Entertainer soham = new Comedian("Satire", oneLiners);
        System.out.println(soham.getTalent());

        // To call tellJoke, you need to cast soham to a Comedian since the reference type is Entertainer
        System.out.println(((Comedian) soham).tellJoke());
    }
}

Main.main(null);

Musician
Comedy style: Satire
Why did the developer go broke?

Example 3

Here is an example of runtime polymorphism

// Class 1
// Helper class
class Parent {
 
    // Method of parent class
    void Print()
    {
 
        // Print statement
        System.out.println("parent class");
    }
}
// Class 2
// Helper class
class subclass1 extends Parent {
 
    // Method
    void Print() { System.out.println("subclass1"); }
}
// Class 3
// Helper class
class subclass2 extends Parent {
 
    // Method
    void Print()
    {
 
        // Print statement
        System.out.println("subclass2");
    }
}
// Class 4
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating object of class 1
        Parent a;
 
        // Now we will be calling print methods
        // inside main() method
 
        a = new subclass1();
        a.Print();
 
        a = new subclass2();
        a.Print();
    }
}
GFG.main(null)

subclass1
subclass2

9.7 Object Superclass

Now that we have learned about inheritance, what even allows our classes and objects that we have created to work the way they do? Where do the general characteristics of all objects come from? The answer lies in the object class.

The object class is the superclass of all other classes as well as arrays and other data types. The Object class is part of the java.lang package.

When we call a constructor to a “top-level class” that the coder hasn’t declared a superclass for, the Object constructor is implicitly called. In other words, the Object constructor is implicitly called when we call a constructor in a class that doesn’t explicitly extend another class. This will give the object some properties and methods that are common to all classes.

Example 1

public class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public static void main(String[] args) {
        Person person1 = new Person("Jane Doe", 30);
        Person person2 = new Person("Jane Doe", 30);

        System.out.println(person1.equals(person1)); // Since person1 and person1 are the same object, the equals() method will return true
        System.out.println(person1.equals(person2)); // Since person1 and person2 are different objects, the equals() method will return false even though they have the same contents
    }
}

Person.main(null);

// The equals() method is inherited from the Object class
// By default, the equals() method in the Object class checks for object identity, which means it compares memory addresses to see if two references point to the exact same object
// In the code, person1 and person2 are distinct objects, so they have different memory addresses
// When we call person1.equals(person2), it checks if the memory addresses are the same (which they are not), so it returns false.

true
false

Example 2

public class Book {
    String title;
    String author;

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

    public static void main(String[] args) {
        Book book = new Book("The Catcher in the Rye", "J.D. Salinger");
        int hashCode = book.hashCode();
        System.out.println("HashCode: " + hashCode); // The output will be a unique integer value representing the object's hash code. The integer value will be different every time you run it
    }
}

Book.main(null);

// The hashCode() method in the Object class returns a unique integer value for each object
// This value is typically based on the object's memory address.
// In the code, when we call book.hashCode(), it generates a unique integer value representing the book object
// This value can be useful for various purposes, such as organizing objects in collections like HashMaps or HashSet where it helps in efficient retrieval of objects.

HashCode: 1853025748

Hacks

• Popcorn Hacks (0.2): Participate in the discussion and fill in all of the blanks.
• MC Questions (0.1): Answer the 10 MC questions below with short explanations

MC Questions

• 

• the answer is “Car object will be successfully assigned to the reference variable vehicle of type Vehicle.” because when creating the new car object, the code assigns it to a reference variable of type Vehicle. this is called “upcasting”

• 

• “B c = new C();” is not valid because B is not a superclass of C, C is the superclass of B. this will not work because the reference variable’s type (B) should be at the same or higher level in the inheritance hierarchy than the object’s actual type (C).

• 

• the answer is “the objects of Class G can be treated as objects of both Class H and class J” because objects of a derived class can be treated as objects of both the immediate superclass and the entire chain of superclasses up to the root of the inheritance hierarchy.

• 

• “B is a subclass of C” because the order of inheritance goes C -> D -> B thus the objects of C can be referenced by type B.

• 

• the answer is “there will be a compile time error” because Penguin is a subclass of Bird and thus cannot be referenced by bird because its not at the same or higher level of hierarchy.

• 

• An object of a class can be assigned to a reference variable of the same type so “J j = new J();” is valid. Because of polymorphism, which allows objects of subclasses to be assigned to reference variables of their superclass, Class K and Class L objects can be assigned to reference variables of type J as they are subclasses of J. Therefore, all three statements are valid.

• 

• the answer is “polymorphism”. Polymorphism allows a subclass to be used wherever its superclass is expected

• 

• “H is an indirect superclass of G” Since class G extends class B and class B extends class H, class G is a subclass of B, which is a subclass of H.

• 

• the root is the superclass and the branches are the subclasses

• 
• “the object will be successfully assigned to the reference variable” This is allowed due to the principle of polymorphism, which allows objects of subclasses to be treated as objects of their superclass.

FRQ Hacks (0.6): Make a complex FRQ that involves everything we taught. Be sure to have a sample solution along with scoring guidelines and how the solution is scored.

Scenario:

• You are developing a software system for a zoo, which has various types of animals with different characteristics. You have been tasked with implementing a Java program that models the inheritance hierarchy of these animals. The zoo has four types of animals: Mammals, Birds, Reptiles, and Fish.

• Each of these animal types has specific attributes and methods:

• Mammals
  • Attributes: Name (String), Age (int), FurColor (String)
  • Methods: makeSound() (prints “Mammal sound”)
• Birds:
  • Attributes: Name (String), Age (int), FeatherColor (String)
  • Methods: makeSound() (prints “Bird sound”)
• Reptiles:
  • Attributes: Name (String), Age (int), ScaleColor (String)
  • Methods: makeSound() (prints “Reptile sound”)
• Fish:
  • Attributes: Name (String), Age (int), FinColor (String)
  • Methods: makeSound() (prints “Fish sound”)

FRQ Task:

1. Create a Java program that models the inheritance hierarchy of the zoo animals as described above. Implement appropriate classes, constructors, and methods to represent the animals.
2. Create an application that demonstrates the use of these classes, creating instances of each animal type and calling their respective methods.

class Animal {
    protected String name;
    protected int age;

    public Animal(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String makeSound() {
        return "Animal sound";
    }
}

class Mammal extends Animal {
    private String furColor;

    public Mammal(String name, int age, String furColor) {
        super(name, age);
        this.furColor = furColor;
    }

    @Override
    public String makeSound() {
        return "roooarr";
    }
}

class Bird extends Animal {
    private String featherColor;

    public Bird(String name, int age, String featherColor) {
        super(name, age);
        this.featherColor = featherColor;
    }

    @Override
    public String makeSound() {
        return "caw caw";
    }
}

class Reptile extends Animal {
    private String scaleColor;

    public Reptile(String name, int age, String scaleColor) {
        super(name, age);
        this.scaleColor = scaleColor;
    }

    @Override
    public String makeSound() {
        return "hisss";
    }
}

class Fish extends Animal {
    private String finColor;

    public Fish(String name, int age, String finColor) {
        super(name, age);
        this.finColor = finColor;
    }

    @Override
    public String makeSound() {
        return "blub blub";
    }
}

public class Zoo {
    public static void main(String[] args) {
        Mammal lion = new Mammal("Simba", 5, "Golden");
        Bird crow = new Bird("Eddie", 3, "Black");
        Reptile snake = new Reptile("Slytherin", 2, "Green");
        Fish goldfish = new Fish("Nemo", 1, "Orange");

        System.out.println("lion goes " + lion.makeSound());
        System.out.println("crow goes " + crow.makeSound());
        System.out.println("snake goes " + snake.makeSound());
        System.out.println("goldfish goes " + goldfish.makeSound());
    }
}

Zoo.main(null)

lion goes roooarr
crow goes caw caw
snake goes hisss
goldfish goes blub blub

Scoring Guidelines:

1. Proper Inheritance (3 points):
   • Animal class should serve as the superclass.
   • Mammal, Bird, Reptile, and Fish classes should inherit from Animal.
   • Each subclass should have attributes and methods as specified in the scenario.
2. Constructors and Attributes (4 points):
   • Each class should have a constructor that initializes attributes correctly.
   • Attributes (name, age, and type-specific attributes) should be declared and used correctly.
3. Method Overriding (2 points):
   • Each subclass should correctly override the makeSound() method.
   • The overridden method should print the expected sound for each animal type.
4. Application (1 points):
   • The Zoo class should create instances of each animal type.
   • It should call the makeSound() method on each instance.

Total: /10

Challenge (0.1): Make an example that uses everything we taught and it must run through main and uses input and output. Points will be awarded for complexity and creativity

import java.util.Scanner;

class MediaItem {
    protected String title;
    protected int year;
    protected String mediaType;

    public MediaItem(String title, int year, String mediaType) {
        this.title = title;
        this.year = year;
        this.mediaType = mediaType;
    }

    public void displayInfo() {
        System.out.println("Title: " + title);
        System.out.println("Year: " + year);
        System.out.println("Media Type: " + mediaType);
    }

    public void play() {
        System.out.println("Playing the media item.");
    }
}

class Book extends MediaItem {
    private String author;

    public Book(String title, int year, String author) {
        super(title, year, "Book");
        this.author = author;
    }

    @Override
    public void play() {
        System.out.println("Reading the book.");
    }
}

class Movie extends MediaItem {
    private String director;

    public Movie(String title, int year, String director) {
        super(title, year, "Movie");
        this.director = director;
    }

    @Override
    public void play() {
        System.out.println("Watching the movie.");
    }
}

class Music extends MediaItem {
    private String artist;

    public Music(String title, int year, String artist) {
        super(title, year, "Music");
        this.artist = artist;
    }

    @Override
    public void play() {
        System.out.println("Listening to the music.");
    }
}

public class Library {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("Welcome to the Library!");
        System.out.print("Enter the title of the media item: ");
        String title = scanner.nextLine();
        System.out.print("Enter the year of the media item: ");
        int year = scanner.nextInt();

        System.out.println("Choose the media type (Book/Movie/Music): ");
        String mediaType = scanner.next();

        MediaItem mediaItem;

        switch (mediaType.toLowerCase()) {
            case "book":
                System.out.print("Enter the author's name: ");
                String author = scanner.next();
                mediaItem = new Book(title, year, author);
                break;
            case "movie":
                System.out.print("Enter the director's name: ");
                String director = scanner.next();
                mediaItem = new Movie(title, year, director);
                break;
            case "music":
                System.out.print("Enter the artist's name: ");
                String artist = scanner.next();
                mediaItem = new Music(title, year, artist);
                break;
            default:
                System.out.println("Invalid media type.");
                scanner.close();
                return;
        }

        System.out.println("\nMedia Item Information:");
        mediaItem.displayInfo();
        System.out.println("\nPlaying the Media:");
        mediaItem.play();

        scanner.close();
    }
}

Library.main(null)

Welcome to the Library!
Enter the title of the media item: Enter the year of the media item: Choose the media type (Book/Movie/Music): 
Enter the author's name: 
Media Item Information:
Title: Unit 9 Inheritance
Year: 2023
Media Type: Book

Playing the Media:
Reading the book.