Recursion Refreshment in Java

Hello, fellow explorer! Today, we will unravel the mystery of "Recursion" — a concept as enthralling as the patterns formed by two mirrors facing each other. We aim to decipher recursion, understand its inner workings, and master its application in Java.

Consider a stack of books. Want the bottom one? You'll need to remove each book above it, one by one. It's a recurring action — an example of recursion. In programming, recursion encompasses a function calling itself repeatedly until a specific condition is met, similar to descending stairs one step at a time until you reach the ground.

Here's a simple Java function illustrating recursion:

public class Main {
  public static void recursiveFunction(int x) {
    if(x <= 0){ // Termination condition --> base case
      System.out.println("Base case reached");
    } else {
      System.out.print(x);
      recursiveFunction(x - 1); // Recursive function call --> recursive case
    }
  }
  public static void main(String[] args) {
    recursiveFunction(5);
  }
}
/*Output:
5
4
3
2
1
Base case reached
*/

This function keeps calling itself with x getting lower by one until x <= 0, which is our base case. At this point, it stops the recursion.

The base case acts like a friendly signpost, telling the recursion when to stop. In our book stack example, reaching a point where no more books are left to remove serves as the signal. Similarly, x <= 0 is our base case in our function. The base case is crucial as it prevents infinite recursion and related errors.

The recursive case is an essential aspect of recursion — the rule responsible for creating smaller versions of the original problem. Each call brings us a step closer to the base case. Let's use the process of calculating a factorial as an illustrative example.

To find a factorial, we multiply a number by the factorial of the number minus one, and repeat this process until we get to one (our base case):

public class Main {
  static int factorial(int n){
    if(n == 1){ // base case
      return 1; 
    } else {
      return n*factorial(n-1); // recursive case
    }
  }
  public static void main(String[] args) {
    System.out.println(factorial(3)); // we expect 6 (3 * 2 * 1)
  }
}

In this case, we call factorial(3), it returns 3 * factorial(2), where factorial(2) returns 2 * factorial(1). As factorial(1) is a base case, it returns 1. As a result, the whole recursion chain returns 3 * 2 * 1.