Module 4: Advanced OOP Concepts

Building a Solid Foundation

Welcome to Module 4! You've learned how to create blueprints (Classes) and build from them (Objects). Now it's time to explore the three core principles that give OOP its power and flexibility: **Inheritance**, **Polymorphism**, and **Encapsulation**. Mastering these concepts is what separates a beginner from an intermediate programmer and is essential for building robust, scalable applications.

1Inheritance: Reusing and Extending Code

Inheritance allows a new class to absorb the properties (fields) and behaviors (methods) of an existing class. This promotes code reuse. The existing class is called the **superclass** (or parent), and the new class is the **subclass** (or child).

Imagine we have our `Dog` class. Now we want to create a `Cat` class. Dogs and cats share common traits; they are both animals. Let's create a general `Animal` superclass.

Creating a Superclass and Subclass

The `Animal` class will contain the shared logic. The `Dog` class can then **extend** `Animal` to inherit that logic, while also adding its own specific behaviors.

// Superclass
public class Animal {
    String name;

    public void eat() {
        System.out.println(name + " is eating.");
    }
}

// Subclass
public class Dog extends Animal {
    // The 'name' field and 'eat()' method are automatically inherited!

    // Dog-specific method
    public void bark() {
        System.out.println("Woof Woof!");
    }
}

2Polymorphism: One Action, Many Forms

Polymorphism** (from Greek, meaning "many forms") is the ability of an object to take on many forms. In practice, this often means a subclass can provide its own specific implementation of a method that is already defined in its superclass. This is called **Method Overriding**.

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

public class Dog extends Animal {
    @Override // Annotation to indicate we are overriding
    public void makeSound() {
        System.out.println("Woof Woof!");
    }
}

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

public class BankAccount {
    // This field is private and cannot be accessed directly from outside
    private double balance;

    public BankAccount(double initialBalance) {
        if (initialBalance > 0) {
            this.balance = initialBalance;
        }
    }

    // A "getter" method to safely read the balance
    public double getBalance() {
        return this.balance;
    }

    // A "setter" method with validation
    public void deposit(double amount) {
        if (amount > 0) {
            this.balance += amount;
        }
    }
}