Learn Java Tutorial for Beginners, Part 49: Recursion

In this video by Quordnet Academy for the series Learn Java Tutorial for Beginners how to use Recursion have been discussed.
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Java Recursion
Recursion is the technique of making a function call itself. This technique provides a way to break complicated problems down into simple problems which are easier to solve.

Recursion may be a bit difficult to understand. The best way to figure out how it works is to experiment with it.

Recursion Example
Adding two numbers together is easy to do, but adding a range of numbers is more complicated. In the following example, recursion is used to add a range of numbers together by breaking it down into the simple task of adding two numbers:

Example
Use recursion to add all of the numbers up to 10.

public class MyClass {
public static void main(String[] args) {
int result = sum(10);
}
public static int sum(int k) {
if (k greater_than 0) {
return k + sum(k - 1);
} else {
return 0;
}
}
}

Example Explained
When the sum() function is called, it adds parameter k to the sum of all numbers smaller than k and returns the result. When k becomes 0, the function just returns 0. When running, the program follows these steps:

10 + sum(9)
10 + ( 9 + sum(8) )
10 + ( 9 + ( 8 + sum(7) ) )
...
10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + sum(0)
10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + 0
Since the function does not call itself when k is 0, the program stops there and returns the result.

Halting Condition
Just as loops can run into the problem of infinite looping, recursive functions can run into the problem of infinite recursion. Infinite recursion is when the function never stops calling itself. Every recursive function should have a halting condition, which is the condition where the function stops calling itself. In the previous example, the halting condition is when the parameter k becomes 0.

It is helpful to see a variety of different examples to better understand the concept. In this example, the function adds a range of numbers between a start and an end. The halting condition for this recursive function is when end is not greater than start:

Example
Use recursion to add all of the numbers between 5 to 10.

public class MyClass {
public static void main(String[] args) {
int result = sum(5, 10);
}
public static int sum(int start, int end) {
if (end greater_than start) {
return end + sum(start, end - 1);
} else {
return end;
}
}
}

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