# The Concept of Recursion in JAVA

Whenever there is a task that needs to be done multiple time with different parameters but with the same body recursive methods can deliver a great help .

Mostly algorithms use this way to do smaller subtask that needs to be done repeatedly , this makes the code to more readable and easy to understand .

For loops can perform this tasks that are taken care of by recursive methods but in an old and unpleasant way so to make our code more cleaner and easy to read using recursive methods is advised.

First, consider the simplest instance of our problem. This is usually the case when the inputs to the problem are the simplest or smallest values possible and can generally be solved with a straightforward solution. In our problem, the simplest input possible is when the input num is less than 1. In this case, all we need to do is print a blank line. This case is called a base case, or stopping case—that is, a case containing no recursive calls. As we will see, when this case occurs recursion will end. Additionally, unless this simple case is made to work correctly, no other case will work correctly. Code for a method implementing the base case for our countdown problem is simple:

``````     public static void countDown(int num) {
if (num <= 0) {
System.out.println();
}
}
``````

This method handles the base case, but not the more interesting case when num is 1 or more. To solve this recursively we have to break the problem up into a smaller version of the same problem. This means rephrasing the problem in terms of a method call back to countDown. Let’s say that num equals 3. In other words, we have invoked countDown(3). We want to output “321”.

This is the same as outputting “3” followed by “21”. But outputting “21” is what countDown(2) should output! Our algorithm to solve this becomes

Output 3
countDown(2)—which is responsible for outputting 21
In turn, countDown(2) can repeat the same process:
Output 2
Call countDown(1)—which is responsible for outputting 1
Output 1
Call countDown(0)—which is responsible for outputting a newline
When countDown(0) is invoked, this executes the base case which prints a newline and ends the recursive method calls.
Output num
Call countDown(num-1)—which is responsible for counting from num-1 to 1

``````         public class RecursiveCountdown {
public static void main(String[] args) {
countDown(3);
}
public static void countDown(int num) {
if (num <= 0) {
System.out.println();
}
else {
System.out.print(num);
countDown(num - 1);
}
}

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