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 subclass of the “Mom” class as they inherit the “Mom” class components and the “Mom” class 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;
    double Salary;
    public Worker(String name, int age, double salary){
        //Write codes that inherits its key properties
        this.Name = name; 
        this.Age = age;
        this.Salary = salary;
    }
}
public class Bob extends Worker{
    public String Position;
    public Bob(String name, int age, double salary, String position){
        //Write code that maintains bob's information
        super(name, age,salary);
        this.Position = position;
    }
    public void Information(){
        //Write code that writes out Bob's key information
        System.out.println("I am the " + super.Name);
        System.out.println("I am " + super.Age);
        System.out.println("I make " + super.Salary +" social credits a day");
        System.out.println("I am a " + Position);
    }
}
public class Test{
    public static void main(String[] args){
        //Make sure that it runs
        Bob XiaoLing = new Bob("Xiao Ling", 6, 100.0, "sweatshop worker");
        XiaoLing.Information();
    }
}
Test.main(null);

I am the Xiao Ling
I am 6
I make 100.0 social credits a day
I am a sweatshop worker

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:

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? It can allow to have stats for atheletes in general and then have subclasses to specify specific attrubutes in methods.

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(){
        System.out.println("I am a school supply");
    }
}
class Pencil extends SchoolSupply{
    public void Information(){
        System.out.println("I am a pencil");
        super.BasicInfo();
    }
}
class Eraser extends SchoolSupply{
    public void Information(){
        System.out.println("I am an eraser");
        super.BasicInfo();
    }
}
public class Test{
    public static void main(String[] args){
        Pencil pencil = new Pencil();
        Eraser eraser = new Eraser();
        pencil.Information();
        eraser.Information();
    }
}
Test.main(null);

I am a pencil
I am a school supply
I am an eraser
I am a school supply

9.5 Creating References Using Inheritance Hierarchies

Inheritance can be thought as an upside down tree with the branches on the top and the roots on the bottom. The roots is the superclass while the branches 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 __reference_ 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 it’s declared class

A method or operator are 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 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

R_untime____

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

C_ompile____

Also known as static polymorphism. This type is achieved by function overloading or operater overloading
Note: But java doesnt 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()));
    }
}

// Which one of these will run? Which one of these will not? Comment out the line that isn't working and explain why
Entertainer kevin = new Entertainer("Musician");
System.out.println(kevin.getTalent());
// System.out.println(kevin.tellJoke()); kevin is not a comedian so he cannot run the rell a joke method

ArrayList<String> oneLiners = new ArrayList<String>(); 
//Add code which adds jokes to oneLiners like... Why did the programmer quit his job?.. Why did the developer go broke?..
oneLiners.add("why did albert eat for popcorn? for the nile rivers poo");
Entertainer soham = new Comedian("satire", oneLiners); 
System.out.println(soham.getTalent()); 
System.out.println(((Comedian)soham).tellJoke());

Musician


Comedy style: satire
why did albert eat for popcorn? for the nile rivers poo

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: 2094718255

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
FRQ Hacks (0.5): Make a complex FRQ that involves everything we taught. Be sure to have a sample solution along with scoring guidlines and how the solution is scored.
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

MC Questions

Question 1 Question 2 Question 3 Question 4 Question 5 Question 6 Question 7 Question 8 Question 9 Question 10

FRQ

Make an abstract class “Animal” that has with subclasses for mammals, fish, and reptiles and animals for each class and then make functions that display their info of the type of outer thing on their skin, how they travel, sound it makes, and name

class Animal {
    String name;
    String scientificName;
    double population;

    public Animal(String name, String scientificName, double population) {
        this.name = name;
        this.scientificName = scientificName;
        this.population = population;
    }

    public void endangered() {
        if (population <= 2500){
            System.out.println(name + " is endangered.");
        }
        else {
            System.out.println(name + " is not endangered.");
        }
    }

    public void transport(){
        System.out.println(name + " moves.");        
    }

    public void displayInfo() {
        System.out.println("Animal Info:");
        System.out.println("Name: " + name);
        System.out.println("Scientific Name: " + scientificName);
        System.out.println("Population: " + population);
        endangered();
    }
}

Animal animal = new Animal("Great white shark", "Carcharodon carcharias", 3500);
animal.transport();
animal.displayInfo();
System.out.println();

class Mammal extends Animal {
    public Mammal(String name, String scientificName, double population) {
        super(name, scientificName, population);
    }

    @Override
    public void transport() {
        System.out.println(name + " walks.");
    }
}

class Fish extends Animal {
    public Fish(String name, String scientificName, double population) {
        super(name, scientificName, population);
    }

    @Override
    public void transport() {
        System.out.println(name + " swims.");
    }
}

class Bird extends Animal {
    public Bird(String name, String scientificName, double population) {
        super(name, scientificName, population);
    }

    @Override
    public void transport() {
        System.out.println(name + " flies.");
    }
}
Mammal mammal = new Mammal("Grizzly bear", "Ursus arctos horribilis", 119000);
Fish fish = new Fish("Bluefin tuna", "Thunnus thynnus", 1000000);
Bird bird = new Bird("California condor", "Gymnogyps californianus", 559);

mammal.transport();
mammal.displayInfo();
System.out.println();

fish.transport();
fish.displayInfo();
System.out.println();

bird.transport();
bird.displayInfo();
System.out.println();

Great white shark moves.
Animal Info:
Name: Great white shark
Scientific Name: Carcharodon carcharias
Population: 3500.0
Great white shark is not endangered.



Grizzly bear walks.
Animal Info:
Name: Grizzly bear
Scientific Name: Ursus arctos horribilis
Population: 119000.0
Grizzly bear is not endangered.

Bluefin tuna swims.
Animal Info:
Name: Bluefin tuna
Scientific Name: Thunnus thynnus
Population: 1000000.0
Bluefin tuna is not endangered.

California condor flies.
Animal Info:
Name: California condor
Scientific Name: Gymnogyps californianus
Population: 559.0
California condor is endangered.

MCQ

The Car object will be successfully assigned to the reference variable cehicle of type Vehicle.
- This happens because the Parent class can be the subclass
B c = new C();
- The subclass cannot create an instance of the Parent class
The objects of Class G can be treated as objects of Class H and Class J
- Class G inherits all of the properties of Class H, and H inherits J, so G inherits J
C is a subclass of B
- B is parent to D, D is parent to C, so grandparent of C is D
There will be a compile-time error.
- subclass cannot make instance of parent class
“J j = new J();”, “J k = new K();”, “J l = new L();”
- all answers are valid
Inheritance
- bc the subclass inherits
H is an indirect superclass of G
- B is super of G, H is super of B
The root is the superclass, and the branches are the subclasses.
- the subclass stems from the root
The object will be successfully assigned to the reference variable
- the parent can go down.

Unit 9 Lesson • 61 min read

Description

9.1 What is inheritance?

9.2 Using the Super keyword for Constructors

Popcorn Hack:

9.3 Overriding Methods

Why Override Methods:

In this example:

Another Example:

Explanation:

Popcorn Hack:

9.4 Using Super keyword for Methods

9.4 Hack

9.5 Creating References Using Inheritance Hierarchies

Popcorn Hack

9.6 Polymorphism

Example 1

Example 2 & Popcorn Hack

Example 3

9.7 Object Superclass

Example 1

Example 2

Hacks

MC Questions

FRQ

MCQ