Advanced Java · Week 2

Classes

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Java 2 Guided Tutorials - Week 2: Classes and OOP Basics

What You'll Learn

Welcome to Week 2 of Java 2! This week, we'll dive into the world of classes and object-oriented programming (OOP). We'll start with the basics of what a class is and how to create one, then move on to more advanced concepts.

Lesson Overview:

Introduction to Classes
Creating a Class
Constructors
Instance vs Static Methods
OOP Basics (Encapsulation, Inheritance, Polymorphism)
Quiz to test your knowledge
Weekly project to apply what you've learned

By the end of this week, you'll have a solid understanding of classes and basic object-oriented programming concepts in Java. Let's get started!

1. Introduction to Classes

What is a Class?

In Java, a class is a blueprint for creating objects. It defines a data structure that contains data fields (attributes) and methods to work with that data. Think of a class as a template for creating objects of a specific type.

Key Points:

Classes are used to create objects
They encapsulate data and behavior
Classes are the foundation of object-oriented programming

Here's a simple example of a class:

<code>public class Car {
    // Attributes (instance variables)
    String make;
    String model;
    int year;

    // Method
    public void startEngine() {
        System.out.println("The " + make + " " + model + " is starting.");
    }
}</code>

In this example, Car is a class with three attributes (make, model, and year) and one method (startEngine).

2. Creating a Class

How to Create a Class

Let's create a simple class step by step:

1. Start with the class keyword, followed by the class name:

<code>public class Student {
    // Class body goes here
}</code>

2. Add attributes (instance variables):

<code>public class Student {
    String name;
    int age;
    double gpa;
}</code>

3. Add methods:

<code>public class Student {
    String name;
    int age;
    double gpa;

    public void introduce() {
        System.out.println("Hi, I'm " + name + " and I'm " + age + " years old.");
    }

    public void study() {
        System.out.println(name + " is studying hard!");
    }
}</code>

Now you have a complete Student class with attributes and methods!

Try it Yourself

Create a Book class with attributes for title, author, and number of pages. Add a method to display the book's information. Use the compiler below to write your code, then paste the output in the autograder to check your work.

Starter code

public class Book {
    String title;
    String author;
    int pages;

    public void displayInfo() {
        // TODO: print "Title: <title>, Author: <author>, Pages: <pages>"
    }

    public static void main(String[] args) {
        Book book = new Book();
        book.title = "The Great Gatsby";
        book.author = "F. Scott Fitzgerald";
        book.pages = 180;
        book.displayInfo();
    }
}

Write your code here

Make it print Title: The Great Gatsby, Author: F. Scott Fitzgerald, Pages: 180

java

public class Book {
    String title;
    String author;
    int pages;

    public void displayInfo() {
        // TODO: print "Title: <title>, Author: <author>, Pages: <pages>"
    }

    public static void main(String[] args) {
        Book book = new Book();
        book.title = "The Great Gatsby";
        book.author = "F. Scott Fitzgerald";
        book.pages = 180;
        book.displayInfo();
    }
}

3. Constructors

What are Constructors?

A constructor is a special method used to initialize objects. It's called when an object of a class is created and can be used to set initial values for object attributes.

Key Points:

Constructors have the same name as the class
They don't have a return type (not even void)
If you don't create a constructor, Java provides a default no-argument constructor

Let's add a constructor to our Student class:

<code>public class Student {
    String name;
    int age;
    double gpa;

    // Constructor
    public Student(String name, int age, double gpa) {
        this.name = name;
        this.age = age;
        this.gpa = gpa;
    }

    public void introduce() {
        System.out.println("Hi, I'm " + name + " and I'm " + age + " years old.");
    }

    public void study() {
        System.out.println(name + " is studying hard!");
    }
}</code>

Now we can create a Student object like this:

<code>Student alice = new Student("Alice", 20, 3.8);</code>

Try it Yourself

Modify your Book class to include a constructor. Create a Book object and call the method to display its information. Use the compiler to test your code.

Starter code

public class Book {
    String title;
    String author;
    int pages;

    // TODO: set title, author, and pages from the constructor's arguments
    public Book(String title, String author, int pages) {
    }

    public void displayInfo() {
        // TODO: print "Title: <title>, Author: <author>, Pages: <pages>"
    }

    public static void main(String[] args) {
        Book book = new Book("To Kill a Mockingbird", "Harper Lee", 281);
        book.displayInfo();
    }
}

Write your code here

Make it print Title: To Kill a Mockingbird, Author: Harper Lee, Pages: 281

java

public class Book {
    String title;
    String author;
    int pages;

    // TODO: set title, author, and pages from the constructor's arguments
    public Book(String title, String author, int pages) {
    }

    public void displayInfo() {
        // TODO: print "Title: <title>, Author: <author>, Pages: <pages>"
    }

    public static void main(String[] args) {
        Book book = new Book("To Kill a Mockingbird", "Harper Lee", 281);
        book.displayInfo();
    }
}

4. Instance vs Static Methods

Understanding Method Types

In Java, methods can be either instance methods or static methods. Understanding the difference is crucial for proper object-oriented programming.

Instance Methods:

Belong to the instance of the class
Can access instance variables
Called on objects of the class

Static Methods:

Belong to the class itself, not any specific instance
Cannot access instance variables directly
Called on the class itself

Here's an example illustrating both types:

<code>public class MathOperations {
    // Instance variable
    private int value;

    // Constructor
    public MathOperations(int value) {
        this.value = value;
    }

    // Instance method
    public int square() {
        return value * value;
    }

    // Static method
    public static int add(int a, int b) {
        return a + b;
    }
}

// Usage:
MathOperations math = new MathOperations(5);
System.out.println(math.square());  // Outputs: 25

System.out.println(MathOperations.add(3, 4));  // Outputs: 7</code>

Try it Yourself

Modify your Book class to include both an instance method and a static method. The instance method could calculate the reading time based on the number of pages, while the static method could compare two books' page counts. Use the compiler to test your code.

Starter code

public class Book {
    String title;
    String author;
    int pages;

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

    // TODO: instance method — assume 60 pages per hour of reading and
    // print "Reading time: <hours> hours"
    public void printReadingTime() {
    }

    // TODO: static method — print "The longer book is: <title>" for
    // whichever of the two books has more pages
    public static void printLongerBook(Book a, Book b) {
    }

    public static void main(String[] args) {
        Book gatsby = new Book("The Great Gatsby", "F. Scott Fitzgerald", 180);
        Book warAndPeace = new Book("War and Peace", "Leo Tolstoy", 1225);

        gatsby.printReadingTime();
        printLongerBook(gatsby, warAndPeace);
    }
}

Write your code here

Make it print Reading time: 3.0 hours The longer book is: War and Peace

java

public class Book {
    String title;
    String author;
    int pages;

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

    // TODO: instance method — assume 60 pages per hour of reading and
    // print "Reading time: <hours> hours"
    public void printReadingTime() {
    }

    // TODO: static method — print "The longer book is: <title>" for
    // whichever of the two books has more pages
    public static void printLongerBook(Book a, Book b) {
    }

    public static void main(String[] args) {
        Book gatsby = new Book("The Great Gatsby", "F. Scott Fitzgerald", 180);
        Book warAndPeace = new Book("War and Peace", "Leo Tolstoy", 1225);

        gatsby.printReadingTime();
        printLongerBook(gatsby, warAndPeace);
    }
}

5. OOP Basics

Object-Oriented Programming Fundamentals

Object-Oriented Programming (OOP) is a programming paradigm based on the concept of "objects" which can contain data and code. The main principles of OOP are:

1. Encapsulation:

Bundling of data and the methods that operate on that data within a single unit (class). It restricts direct access to some of an object's components, which is a means of preventing accidental interference and misuse of the methods and data.

<code>public class BankAccount {
    private double balance;  // private - encapsulated

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }

    public double getBalance() {
        return balance;
    }
}</code>

2. Inheritance:

A mechanism where you can derive a class from another class for a hierarchy of classes that share a set of attributes and methods.

<code>public class Animal {
    public void eat() {
        System.out.println("This animal eats food.");
    }
}

public class Dog extends Animal {
    public void bark() {
        System.out.println("The dog barks.");
    }
}</code>

3. Polymorphism:

The provision of a single interface to entities of different types or the use of a single symbol to represent multiple different types.

<code>public class Animal {
    public void makeSound() {
        System.out.println("The animal makes a sound");
    }
}

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

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

// Usage:
Animal myDog = new Dog();
Animal myCat = new Cat();
myDog.makeSound();  // Outputs: The dog barks
myCat.makeSound();  // Outputs: The cat meows</code>

These principles form the core of OOP and help in creating more organized, flexible, and maintainable code.

Try it Yourself

Create a simple class hierarchy demonstrating inheritance and polymorphism. For example, create a Shape class with subclasses like Circle and Rectangle. Include a method to calculate area in each class. Use the compiler to test your implementation.

Starter code

class Shape {
    public double area() {
        return 0;
    }
}

class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        // TODO: return the area of the circle (Math.PI * radius * radius)
        return 0;
    }
}

class Rectangle extends Shape {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        // TODO: return the area of the rectangle (width * height)
        return 0;
    }
}

public class ShapeDemo {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rectangle = new Rectangle(6, 4);

        System.out.printf("Circle area: %.2f%n", circle.area());
        System.out.printf("Rectangle area: %.2f%n", rectangle.area());
    }
}

Write your code here

Make it print Circle area: 78.54 Rectangle area: 24.00

java

class Shape {
    public double area() {
        return 0;
    }
}

class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        // TODO: return the area of the circle (Math.PI * radius * radius)
        return 0;
    }
}

class Rectangle extends Shape {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        // TODO: return the area of the rectangle (width * height)
        return 0;
    }
}

public class ShapeDemo {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rectangle = new Rectangle(6, 4);

        System.out.printf("Circle area: %.2f%n", circle.area());
        System.out.printf("Rectangle area: %.2f%n", rectangle.area());
    }
}

7. Weekly Project: Library Management System

Project Description

For this week's project, you'll create a simple Library Management System using the OOP concepts we've learned.

Requirements:

Create a Book class with attributes like title, author, ISBN, and availability status
Create a Library class that manages a collection of Book objects
Implement methods in the Library class to add books, remove books, and check out/return books
Use encapsulation to protect the book's data
Implement a simple menu system for user interaction

Here's a template to get you started:

<code>import java.util.ArrayList;
import java.util.Scanner;

class Book {
    // TODO: Implement Book class
}

class Library {
    // TODO: Implement Library class
}

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

        while (true) {
            System.out.println("
1. Add Book");
            System.out.println("2. Remove Book");
            System.out.println("3. Check Out Book");
            System.out.println("4. Return Book");
            System.out.println("5. Display All Books");
            System.out.println("6. Exit");
            System.out.print("Enter your choice: ");

            int choice = scanner.nextInt();
            scanner.nextLine();  // Consume newline

            switch (choice) {
                case 1:
                    // TODO: Implement add book
                    break;
                case 2:
                    // TODO: Implement remove book
                    break;
                case 3:
                    // TODO: Implement check out book
                    break;
                case 4:
                    // TODO: Implement return book
                    break;
                case 5:
                    // TODO: Implement display all books
                    break;
                case 6:
                    System.out.println("Thank you for using the Library Management System!");
                    return;
                default:
                    System.out.println("Invalid choice. Please try again.");
            }
        }
    }
}</code>

Implement the missing parts of the Book and Library classes, and complete the menu system. Use the online compiler to write and test your code. Once you're satisfied with your program, paste a sample output of adding a book, checking it out, and returning it in the autograder to check your work.

Starter code

import java.util.ArrayList;
import java.util.Scanner;

class Book {
    // TODO: Implement Book class
}

class Library {
    // TODO: Implement Library class
}

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

        while (true) {
            System.out.println("\n1. Add Book");
            System.out.println("2. Remove Book");
            System.out.println("3. Check Out Book");
            System.out.println("4. Return Book");
            System.out.println("5. Display All Books");
            System.out.println("6. Exit");
            System.out.print("Enter your choice: ");

            int choice = scanner.nextInt();
            scanner.nextLine();  // Consume newline

            switch (choice) {
                case 1:
                    // TODO: Implement add book
                    break;
                case 2:
                    // TODO: Implement remove book
                    break;
                case 3:
                    // TODO: Implement check out book
                    break;
                case 4:
                    // TODO: Implement return book
                    break;
                case 5:
                    // TODO: Implement display all books
                    break;
                case 6:
                    System.out.println("Thank you for using the Library Management System!");
                    return;
                default:
                    System.out.println("Invalid choice. Please try again.");
            }
        }
    }
}

Write your code here

Make it print Book added: The Great Gatsby Book checked out: The Great Gatsby Book returned: The Great Gatsby

java

import java.util.ArrayList;
import java.util.Scanner;

class Book {
    // TODO: Implement Book class
}

class Library {
    // TODO: Implement Library class
}

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

        while (true) {
            System.out.println("\n1. Add Book");
            System.out.println("2. Remove Book");
            System.out.println("3. Check Out Book");
            System.out.println("4. Return Book");
            System.out.println("5. Display All Books");
            System.out.println("6. Exit");
            System.out.print("Enter your choice: ");

            int choice = scanner.nextInt();
            scanner.nextLine();  // Consume newline

            switch (choice) {
                case 1:
                    // TODO: Implement add book
                    break;
                case 2:
                    // TODO: Implement remove book
                    break;
                case 3:
                    // TODO: Implement check out book
                    break;
                case 4:
                    // TODO: Implement return book
                    break;
                case 5:
                    // TODO: Implement display all books
                    break;
                case 6:
                    System.out.println("Thank you for using the Library Management System!");
                    return;
                default:
                    System.out.println("Invalid choice. Please try again.");
            }
        }
    }
}

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Classes

Java 2 Guided Tutorials - Week 2: Classes and OOP Basics

What You'll Learn

1. Introduction to Classes

What is a Class?

2. Creating a Class

How to Create a Class

Write your code here

3. Constructors

What are Constructors?

Write your code here

4. Instance vs Static Methods

Understanding Method Types

Write your code here

5. OOP Basics

Object-Oriented Programming Fundamentals

Write your code here

6. Classes and OOP Quiz

Quick check

7. Weekly Project: Library Management System

Project Description

Write your code here