Introduction to Method Overriding and Recursion

Input/Output and Streams in Java

Alex Liu

Software Development Engineer

Defining a base class

The Animal class represents a generic animal

class Animal {
  void makeSound() {
      System.out.println("Animal sound");
  }
  public static void main(String[] args) {
      Animal a = new Animal();
      a.makeSound(); // Output: Animal sound
  }
}

Animal sound

Understanding inheritance-extends

// Use `extends` to create a subclass of `Animal` named `Dog`
class Dog extends Animal {
    void bark() {System.out.println("Bark");}
}
public static void main(String[] args) {
    Dog d = new Dog(); // Create new instance of Dog object which extends Animal
    d.makeSound();
    d.bark();
}

Print both sound line by line:

Animal sound
Bark

Overriding methods in Java

class Cat extends Animal {
    @Override // Use `@Override` to override the behavior of `.makeSound()`
    void makeSound() {
        System.out.println("Meow");
    }
}
public static void main(String[] args) {
    Cat c = new Cat(); // Create a new instance of Cat object which extends Animal
    c.makeSound(); // Call the overrided method `makeSound()`
}

Meow

Understanding recursion

A method calling itself to solve a problem
Requires a base case to prevent infinite recursion

public class RecursionExample {
    static void countdown(int n) {
        // Base case
        if (n == 0) return; 
        System.out.println(n);
        // Recursive call
        countdown(n - 1); 
    }
}

Recursion sample usage

Sample usage

public static void main(String[] args) {
  countdown(5);
}

Recursively call stop when it reach the base case which is 0
```
5
4
3
2
1
```

Summary

extends enables inheritance
@Override modifies inherited behavior
Recursion breaks problems into smaller steps
- Base case prevents infinite recursion

Let's practice!

Input/Output and Streams in Java