Classes

Key Points:

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

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.

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

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.

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

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.

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.

public class BankAccount {
    private double balance;  // private - encapsulated

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

    public double getBalance() {
        return balance;
    }
}

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.

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.");
    }
}

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.

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

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.