Java Recursion

Recursion in Java is a programming technique where a method calls itself to solve a problem. It is useful for problems that can be broken down into smaller, similar subproblems.

1. Structure of a Recursive Method

A recursive method generally has two parts:

Base Case → Condition to stop recursion (prevents infinite loop).
Recursive Case → Part where the method calls itself.

2. Example 1: Factorial of a Number

The factorial of n is n! = n * (n-1) * ... * 1.

public class Main {
static int factorial(int n) {
if (n == 0 || n == 1) { // base case
return 1;
} else {
return n * factorial(n - 1); // recursive case
}
}public static void main(String[] args) {
int num = 5;
System.out.println("Factorial of " + num + " = " + factorial(num));
}
}

public class Main {

static int factorial(int n) {

if (n == 0 || n == 1) { // base case

return 1;

} else {

return n * factorial(n - 1); // recursive case

}

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

int num = 5;

System.out.println("Factorial of " + num + " = " + factorial(num));

}

📌 Output:

3. Example 2: Fibonacci Series

Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8…
F(n) = F(n-1) + F(n-2), F(0)=0, F(1)=1

public class Main {
static int fibonacci(int n) {
if (n == 0) return 0;
if (n == 1) return 1;
return fibonacci(n - 1) + fibonacci(n - 2);
}public static void main(String[] args) {
int n = 10;
for (int i = 0; i < n; i++) {
System.out.print(fibonacci(i) + " ");
}
}
}

public class Main {

static int fibonacci(int n) {

if (n == 0) return 0;

if (n == 1) return 1;

return fibonacci(n - 1) + fibonacci(n - 2);

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

int n = 10;

for (int i = 0; i < n; i++) {

System.out.print(fibonacci(i) + " ");

}

📌 Output:

4. Example 3: Sum of First N Natural Numbers

public class Main {
static int sum(int n) {
if (n == 1) return 1; // base case
return n + sum(n - 1); // recursive case
}public static void main(String[] args) {
int n = 5;
System.out.println("Sum of first " + n + " numbers = " + sum(n));
}
}

public class Main {

static int sum(int n) {

if (n == 1) return 1; // base case

return n + sum(n - 1); // recursive case

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

int n = 5;

System.out.println("Sum of first " + n + " numbers = " + sum(n));

}

📌 Output:

5. Example 4: Reverse a String Using Recursion

public class Main {
static String reverse(String str) {
if (str.isEmpty()) return ""; // base case
return reverse(str.substring(1)) + str.charAt(0);
}public static void main(String[] args) {
String text = "Java";
System.out.println("Reversed: " + reverse(text));
}
}

public class Main {

static String reverse(String str) {

if (str.isEmpty()) return ""; // base case

return reverse(str.substring(1)) + str.charAt(0);

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

String text = "Java";

System.out.println("Reversed: " + reverse(text));

}

📌 Output:

6. Key Points About Recursion

Always define a base case to stop recursion.
Each recursive call consumes stack memory, deep recursion may cause StackOverflowError.
Recursion is often elegant, but loops may be more memory-efficient.
Useful in factorials, Fibonacci series, tree traversal, sorting algorithms (QuickSort, MergeSort).

Java Recursion

Java Recursion

1. Structure of a Recursive Method

2. Example 1: Factorial of a Number

3. Example 2: Fibonacci Series

4. Example 3: Sum of First N Natural Numbers

5. Example 4: Reverse a String Using Recursion

6. Key Points About Recursion

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