A method that calls itself is known as a recursive method. And, this technique is known as recursion.
A physical world example would be to place two parallel mirrors facing each other. Any object in between them would be reflected recursively.
How recursion works?
In the above program, recurse() method is called from inside the main method at first (normal method call).
Also, recurse() method is called from inside the same method, recurse(). This is a recursive call.
The recursion continues until some condition is met to prevent it from execution. If not, infinite recursion occurs.
Hence, to prevent infinite recursion, if...else statement (or similar approach) can be used where one branch makes the recursive call and other doesn't.
Example: Factorial of a Number Using Recursion
class Factorial { static int factorial( int n ) { if (n != 0) return n * factorial(n-1); // recursive call else return 1; } public static void main(String[] args) { int number = 5, result; result = factorial(number); System.out.println(number + " factorial = " + result); } }
Output:
5 factorial = 120
Initially, factorial() is called from the main() method with number passed as an argument.
Inside factorial() method, the value of n is 4 initially. During the next recursive call, 3 is passed to the factorial() method. This process continues until n is equal to 0.
When n is equal to 0, if condition fails and the else part is executed which returns 1, and accumulated result is passed to the main() method.
This figure will give you a better idea on how the above program works.
Advantages and Disadvantages of Recursion
When a recursive call is made, new storage location for variables are allocated on the stack. As, each recursive call returns, the old variables and parameters are removed from the stack. Hence, recursion generally use more memory and are generally slow.
On the other hand, recursive solution is much simpler and takes less time to write, debug and maintain.
