Java Module 006: Week 6 - Inheritance and Polymorphism

This module covers Object-Oriented Programming (OOP) concepts in Java, focusing on inheritance (extends), method overriding (polymorphism), access modifiers (public, private, protected, default), and packages. Designed for beginners building on Weeks 1-5 of the 3-month Java fundamentals plan, it includes detailed explanations, multiple code examples per concept, and practice exercises for each day of Week 6. The module concludes with a console-based Vehicle Showroom mini-project to apply these concepts.

Day 1: Inheritance (`extends`)

Objective: Learn to create parent-child class relationships using inheritance.

Explanation

Inheritance: Allows a class (child/subclass) to inherit attributes and methods from another class (parent/superclass) using the extends keyword.
Syntax: class ChildClass extends ParentClass { // code }
Child classes inherit all non-private members of the parent class.
Use inheritance to reuse code and model "is-a" relationships (e.g., a Car is a Vehicle).
The super keyword can call parent class constructors or methods.

Code Examples

Example 1: Basic Inheritance

class Vehicle {
    String brand;
    int year;
    
    void displayInfo() {
        System.out.println("Brand: " + brand + ", Year: " + year);
    }
}

public class Car extends Vehicle {
    public static void main(String[] args) {
        Car car = new Car();
        car.brand = "Toyota";
        car.year = 2020;
        car.displayInfo();
    }
}

Example 2: Constructor Inheritance

class Animal {
    String name;
    
    Animal(String name) {
        this.name = name;
    }
}

public class Dog extends Animal {
    Dog(String name) {
        super(name); // Call parent constructor
    }
    
    public static void main(String[] args) {
        Dog dog = new Dog("Buddy");
        System.out.println("Dog Name: " + dog.name);
    }
}

Example 3: Adding Child-Specific Attributes

class Person {
    String name;
    
    Person(String name) {
        this.name = name;
    }
}

public class Employee extends Person {
    int id;
    
    Employee(String name, int id) {
        super(name);
        this.id = id;
    }
    
    void display() {
        System.out.println("Name: " + name + ", ID: " + id);
    }
    
    public static void main(String[] args) {
        Employee emp = new Employee("Alice", 101);
        emp.display();
    }
}

Practice

Create a Vehicle parent class with attributes brand and speed; create a Bike child class.
Add a constructor to Bike that calls the parent constructor using super.
Write a program to create a Bike object and print its inherited attributes.

Practice Code:

class Vehicle {
    String brand;
    int speed;
    
    Vehicle(String brand, int speed) {
        this.brand = brand;
        this.speed = speed;
    }
}

public class BikePractice extends Vehicle {
    BikePractice(String brand, int speed) {
        super(brand, speed);
    }
    
    public static void main(String[] args) {
        BikePractice bike = new BikePractice("Yamaha", 80);
        System.out.println("Bike: " + bike.brand + ", Speed: " + bike.speed);
    }
}

Day 2: Method Overriding (Polymorphism)

Objective: Understand method overriding to implement polymorphism.

Explanation

Method Overriding: A child class redefines a parent class method with the same name, parameters, and return type.
Polymorphism: Allows a child class to provide a specific implementation of a parent method.
Use the @Override annotation to ensure correct overriding.
The super keyword can call the parent’s version of the method.

Code Examples

Example 1: Basic Overriding

class Animal {
    void makeSound() {
        System.out.println("Generic animal sound");
    }
}

public class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Meow");
    }
    
    public static void main(String[] args) {
        Cat cat = new Cat();
        cat.makeSound();
    }
}

Example 2: Using super

class Vehicle {
    void describe() {
        System.out.println("This is a vehicle");
    }
}

public class Truck extends Vehicle {
    @Override
    void describe() {
        super.describe(); // Call parent method
        System.out.println("This is a truck");
    }
    
    public static void main(String[] args) {
        Truck truck = new Truck();
        truck.describe();
    }
}

Example 3: Overriding with Parameters

class Shape {
    double getArea() {
        return 0.0;
    }
}

public class Circle extends Shape {
    double radius;
    
    Circle(double radius) {
        this.radius = radius;
    }
    
    @Override
    double getArea() {
        return Math.PI * radius * radius;
    }
    
    public static void main(String[] args) {
        Circle circle = new Circle(5.0);
        System.out.println("Circle Area: " + circle.getArea());
    }
}

Practice

Create a Shape parent class with a getPerimeter method; override it in a Rectangle child class.
Use super to call the parent method in a child class method.
Write a program to create a child class object and call its overridden method.

Practice Code:

class Shape {
    double getPerimeter() {
        return 0.0;
    }
}

public class RectanglePractice extends Shape {
    double length, width;
    
    RectanglePractice(double length, double width) {
        this.length = length;
        this.width = width;
    }
    
    @Override
    double getPerimeter() {
        return 2 * (length + width);
    }
    
    public static void main(String[] args) {
        RectanglePractice rect = new RectanglePractice(4.0, 3.0);
        System.out.println("Perimeter: " + rect.getPerimeter());
    }
}

Day 3: Access Modifiers

Objective: Use access modifiers to control visibility.

Explanation

Access Modifiers: Control access to class members (fields, methods).
- public: Accessible everywhere.
- private: Accessible only within the same class.
- protected: Accessible within the same package and subclasses.
- default (package-private): Accessible within the same package if no modifier is specified.
Use modifiers to enforce encapsulation and protect data.

Code Examples

Example 1: Public and Private

public class Account {
    private double balance;
    public String accountHolder;
    
    public void setBalance(double balance) {
        if (balance >= 0) {
            this.balance = balance;
        }
    }
    
    public double getBalance() {
        return balance;
    }
    
    public static void main(String[] args) {
        Account acc = new Account();
        acc.accountHolder = "Alice";
        acc.setBalance(1000.0);
        // acc.balance = -500; // Error: private field
        System.out.println("Holder: " + acc.accountHolder + ", Balance: " + acc.getBalance());
    }
}

Example 2: Protected in Inheritance

class Vehicle {
    protected String brand = "Generic";
    
    protected void displayBrand() {
        System.out.println("Brand: " + brand);
    }
}

public class Motorcycle extends Vehicle {
    public static void main(String[] args) {
        Motorcycle moto = new Motorcycle();
        moto.brand = "Honda";
        moto.displayBrand();
    }
}

Example 3: Default Access

class Person {
    String name = "Unknown"; // Default access
    
    void display() {
        System.out.println("Name: " + name);
    }
}

public class TestPerson {
    public static void main(String[] args) {
        Person person = new Person();
        person.name = "Bob"; // Accessible in same package
        person.display();
    }
}

Practice

Create a Product class with a private price and public name; add getters/setters.
Create a parent class with a protected method; access it in a child class.
Write a program with a default access field and test access within the same package.

Practice Code:

class Product {
    private double price;
    public String name;
    
    public void setPrice(double price) {
        if (price >= 0) this.price = price;
    }
    
    public double getPrice() {
        return price;
    }
}

public class ProductPractice {
    public static void main(String[] args) {
        Product prod = new Product();
        prod.name = "Phone";
        prod.setPrice(500.0);
        System.out.println("Product: " + prod.name + ", Price: $" + prod.getPrice());
    }
}

Day 4: Packages

Objective: Organize code using packages.

Explanation

Packages: Group related classes to avoid naming conflicts and improve organization.
Syntax: package com.example;
Use import to access classes from other packages (e.g., import com.example.MyClass;).
Common convention: Use reverse domain name (e.g., com.mycompany.project).

Code Examples

Example 1: Basic Package

// File: com/example/Vehicle.java
package com.example;

public class Vehicle {
    public String brand = "Generic";
    
    public void display() {
        System.out.println("Vehicle Brand: " + brand);
    }
}

// File: com/example/TestVehicle.java
package com.example;

public class TestVehicle {
    public static void main(String[] args) {
        Vehicle vehicle = new Vehicle();
        vehicle.display();
    }
}

Example 2: Importing Across Packages

// File: com/example/models/Car.java
package com.example.models;

public class Car {
    public String model = "Sedan";
}

// File: com/example/main/Main.java
package com.example.main;

import com.example.models.Car;

public class Main {
    public static void main(String[] args) {
        Car car = new Car();
        System.out.println("Car Model: " + car.model);
    }
}

Example 3: Package with Inheritance

// File: com/example/vehicles/Vehicle.java
package com.example.vehicles;

public class Vehicle {
    protected String type = "Vehicle";
}

// File: com/example/vehicles/Bus.java
package com.example.vehicles;

public class Bus extends Vehicle {
    public void showType() {
        System.out.println("Type: " + type);
    }
    
    public static void main(String[] args) {
        Bus bus = new Bus();
        bus.showType();
    }
}

Practice

Create a package com.example.animals with an Animal class.
Create another package com.example.main and import the Animal class to use it.
Write a program with a parent and child class in the same package.

Practice Code:

// File: com/example/animals/Animal.java
package com.example.animals;

public class Animal {
    public String name = "Unknown";
}

// File: com/example/main/AnimalTest.java
package com.example.main;

import com.example.animals.Animal;

public class AnimalTest {
    public static void main(String[] args) {
        Animal animal = new Animal();
        animal.name = "Lion";
        System.out.println("Animal: " + animal.name);
    }
}

Day 5: Review and Inheritance/Polymorphism

Objective: Solidify inheritance, polymorphism, access modifiers, and packages.

Explanation

Review best practices:
- Use inheritance for "is-a" relationships.
- Override methods to customize behavior.
- Choose appropriate access modifiers for encapsulation.
- Organize code with packages for clarity.
Combine with Weeks 1-5 concepts (methods, loops, classes).

Code Examples

Example 1: Polymorphism in Action

class Animal {
    void eat() {
        System.out.println("Animal eats food");
    }
}

public class Lion extends Animal {
    @Override
    void eat() {
        System.out.println("Lion eats meat");
    }
    
    public static void main(String[] args) {
        Animal animal = new Lion(); // Polymorphism
        animal.eat();
    }
}

Example 2: Protected Access in Package

// File: com/example/vehicles/Vehicle.java
package com.example.vehicles;

public class Vehicle {
    protected String brand = "Generic";
}

// File: com/example/vehicles/SUV.java
package com.example.vehicles;

public class SUV extends Vehicle {
    void display() {
        System.out.println("SUV Brand: " + brand);
    }
    
    public static void main(String[] args) {
        SUV suv = new SUV();
        suv.display();
    }
}

Example 3: Complex Inheritance

class Device {
    String brand;
    
    Device(String brand) {
        this.brand = brand;
    }
    
    void powerOn() {
        System.out.println(brand + " powers on");
    }
}

public class Smartphone extends Device {
    int storage;
    
    Smartphone(String brand, int storage) {
        super(brand);
        this.storage = storage;
    }
    
    @Override
    void powerOn() {
        System.out.println(brand + " smartphone powers on with " + storage + "GB");
    }
    
    public static void main(String[] args) {
        Smartphone phone = new Smartphone("Samsung", 64);
        phone.powerOn();
    }
}

Practice

Create a Person parent class and a Student child class with an overridden method.
Use protected fields in a parent class and access them in a child class.
Organize a parent and child class in a package and test them.

Practice Code:

class Person {
    protected String name;
    
    Person(String name) {
        this.name = name;
    }
    
    void introduce() {
        System.out.println("I am " + name);
    }
}

public class StudentPractice extends Person {
    StudentPractice(String name) {
        super(name);
    }
    
    @Override
    void introduce() {
        System.out.println("I am student " + name);
    }
    
    public static void main(String[] args) {
        StudentPractice student = new StudentPractice("Eve");
        student.introduce();
    }
}

Weekend: Review and Mini-Project

Objective: Consolidate Week 6 learning with a Vehicle Showroom mini-project.

Review

Revisit:
- Inheritance with extends and super.
- Method overriding for polymorphism.
- Access modifiers for encapsulation.
- Packages for code organization.
Debug common errors (e.g., missing super, incorrect access modifiers).

Mini-Project: Vehicle Showroom

Create a console-based program that:

Defines a Vehicle parent class with attributes brand and price, and a method displayDetails.
Creates child classes Car and Motorcycle with overridden displayDetails methods.
Organizes classes in a package (e.g., com.example.showroom).
Uses a Showroom class to manage multiple vehicles in an array.
Validates inputs (e.g., positive price) and displays all vehicles.

Project Code:

// File: com/example/showroom/Vehicle.java
package com.example.showroom;

public class Vehicle {
    protected String brand;
    protected double price;
    
    Vehicle(String brand, double price) {
        this.brand = brand;
        if (price > 0) this.price = price;
        else this.price = 1000.0;
    }
    
    void displayDetails() {
        System.out.println("Brand: " + brand + ", Price: $" + price);
    }
}

// File: com/example/showroom/Car.java
package com.example.showroom;

public class Car extends Vehicle {
    Car(String brand, double price) {
        super(brand, price);
    }
    
    @Override
    void displayDetails() {
        System.out.println("Car - Brand: " + brand + ", Price: $" + price);
    }
}

// File: com/example/showroom/Motorcycle.java
package com.example.showroom;

public class Motorcycle extends Vehicle {
    Motorcycle(String brand, double price) {
        super(brand, price);
    }
    
    @Override
    void displayDetails() {
        System.out.println("Motorcycle - Brand: " + brand + ", Price: $" + price);
    }
}

// File: com/example/showroom/Showroom.java
package com.example.showroom;

public class Showroom {
    private Vehicle[] vehicles;
    private int vehicleCount;
    
    Showroom(int capacity) {
        vehicles = new Vehicle[capacity];
        vehicleCount = 0;
    }
    
    void addVehicle(Vehicle vehicle) {
        if (vehicleCount < vehicles.length) {
            vehicles[vehicleCount] = vehicle;
            vehicleCount++;
            System.out.println("Added: " + vehicle.brand);
        } else {
            System.out.println("Showroom full");
        }
    }
    
    void displayVehicles() {
        if (vehicleCount == 0) {
            System.out.println("No vehicles in showroom");
            return;
        }
        System.out.println("Showroom Vehicles:");
        for (int i = 0; i < vehicleCount; i++) {
            vehicles[i].displayDetails();
        }
    }
    
    public static void main(String[] args) {
        Showroom showroom = new Showroom(3);
        showroom.addVehicle(new Car("Toyota", 25000.0));
        showroom.addVehicle(new Motorcycle("Honda", 12000.0));
        showroom.displayVehicles();
    }
}

Weekend Practice

Run and test the showroom with different vehicles.
Add a method to the Showroom class to find a vehicle by brand.
Extend the program to calculate the total price of all vehicles.

Extended Practice Code (with find method):

// File: com/example/showroom/ShowroomExtended.java
package com.example.showroom;

public class ShowroomExtended {
    private Vehicle[] vehicles;
    private int vehicleCount;
    
    ShowroomExtended(int capacity) {
        vehicles = new Vehicle[capacity];
        vehicleCount = 0;
    }
    
    void addVehicle(Vehicle vehicle) {
        if (vehicleCount < vehicles.length) {
            vehicles[vehicleCount] = vehicle;
            vehicleCount++;
            System.out.println("Added: " + vehicle.brand);
        } else {
            System.out.println("Showroom full");
        }
    }
    
    void findVehicle(String brand) {
        for (int i = 0; i < vehicleCount; i++) {
            if (vehicles[i].brand.equals(brand)) {
                vehicles[i].displayDetails();
                return;
            }
        }
        System.out.println("Vehicle not found: " + brand);
    }
    
    public static void main(String[] args) {
        ShowroomExtended showroom = new ShowroomExtended(3);
        showroom.addVehicle(new Car("Ford", 30000.0));
        showroom.findVehicle("Ford");
        showroom.findVehicle("BMW");
    }
}

Tips for Week 6

Practice: Test each code example in an IDE or terminal, ensuring proper package structure.
Debug: Check for missing super calls, incorrect overriding, or access modifier issues.
Resources: Refer to Oracle’s Java Tutorials or W3Schools for inheritance and package details.
Experiment: Modify child classes or access modifiers to see their effects.

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Java Module 006: Week 6 - Inheritance and Polymorphism

Day 1: Inheritance (`extends`)

Explanation

Code Examples

Practice

Day 2: Method Overriding (Polymorphism)

Explanation

Code Examples

Practice

Day 3: Access Modifiers

Explanation

Code Examples

Practice

Day 4: Packages

Explanation

Code Examples

Practice

Day 5: Review and Inheritance/Polymorphism

Explanation

Code Examples

Practice

Weekend: Review and Mini-Project

Review

Mini-Project: Vehicle Showroom

Weekend Practice

Tips for Week 6

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Java Module 006: Week 6 - Inheritance and Polymorphism

Day 1: Inheritance (extends)

Explanation

Code Examples

Practice

Day 2: Method Overriding (Polymorphism)

Explanation

Code Examples

Practice

Day 3: Access Modifiers

Explanation

Code Examples

Practice

Day 4: Packages

Explanation

Code Examples

Practice

Day 5: Review and Inheritance/Polymorphism

Explanation

Code Examples

Practice

Weekend: Review and Mini-Project

Review

Mini-Project: Vehicle Showroom

Weekend Practice

Tips for Week 6

Day 1: Inheritance (`extends`)