Sunny Number in Java: Meaning, Explanation, Examples, and Practice Problems

A Sunny Number is a number for which the next consecutive number is a perfect square. This is a well-known number theory concept often asked in coding interviews and beginner Java assignments due to its simple and logical approach.

What Is a Sunny Number?

A number N is a Sunny Number if:

N + 1 is a perfect square

That means if you add 1 to the number, the result must be a complete perfect square like 4, 9, 16, 25, and so on.

Examples of Sunny Numbers

• 3 → 3 + 1 = 4 (perfect square)
• 8 → 8 + 1 = 9 (perfect square)
• 15 → 15 + 1 = 16 (perfect square)
• 24 → 24 + 1 = 25 (perfect square)

How to Check Sunny Number?

Steps to determine if a number is Sunny:

1. Add 1 to the given number.
2. Find the square root of the new number.
3. If the square root is an integer → it is a Sunny Number.
4. If not → it is not a Sunny Number.

Java Program to Check Sunny Number

import java.util.*;

public class SunnyNumber {

    static boolean isPerfectSquare(int n) {
        int root = (int)Math.sqrt(n);
        return root * root == n;
    }

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        if (isPerfectSquare(num + 1)) {
            System.out.println("Sunny Number");
        } else {
            System.out.println("Not a Sunny Number");
        }
    }
}

  

Examples Table

Number	N + 1	Perfect Square?	Sunny?
3	4	Yes	Sunny Number
8	9	Yes	Sunny Number
14	15	No	Not Sunny
24	25	Yes	Sunny Number
30	31	No	Not Sunny

3 Practice Challenges

Challenge 1: Print Sunny Numbers in a Range

Write a Java program to display all Sunny Numbers between 1 and 10,000.

Challenge 2: Sunny Numbers in an Array

Accept an array from the user and count how many elements are Sunny Numbers.

Challenge 3: Next Sunny Number

Write a Java program to find the next Sunny Number after a given integer.