Java Interview Program - Check Whether a Number is a Sunny Number in Java

Check Whether a Number is a Sunny Number in Java

A sunny number is a number for which n + 1 is a perfect square. In simpler terms, if you add 1 to the number and the result is a perfect square, then the original number is considered a sunny number.

For example, 8 is a sunny number because 8 + 1 = 9, and 9 is a perfect square (3 × 3).

This program helps you check whether a given number is sunny or not by using mathematical operations and a built-in function to find the square root.

Examples

Input: 8
Output: Sunny Number (since 8 + 1 = 9, and √9 = 3)
Input: 10
Output: Not a Sunny Number (since 10 + 1 = 11, and √11 is not an integer)
Input: 0
Output: Not a Sunny Number (since 0 + 1 = 1, and √1 = 1 → Sunny Number)
Input: 15
Output: Sunny Number (since 15 + 1 = 16, and √16 = 4)

Interviewer Expectations

Interviewers are testing:

Your understanding of number properties and perfect squares
Your ability to use Java’s Math library
Logical flow and condition checks
Input validation and edge case handling

Approach

Here's how we can solve the problem step-by-step:

Take an integer n as input.
Check whether n + 1 is a perfect square.
To check if a number is a perfect square, calculate the square root and verify whether squaring it gives back the original number.

Dry Run Example

Input: 24

Step 1: 24 + 1 = 25

Step 2: √25 = 5

Step 3: 5 × 5 = 25 → matches

Output: Sunny Number

Java Program

public class SunnyNumberExample {
    public static void main(String[] args) {
        int n = 15;

        int nextNum = n + 1;
        double sqrt = Math.sqrt(nextNum);

        if ((int)sqrt * (int)sqrt == nextNum) {
            System.out.println(n + " is a Sunny Number.");
        } else {
            System.out.println(n + " is Not a Sunny Number.");
        }
    }
}

15 is a Sunny Number.

Possible Followup Questions with Answers

Q1: What is the difference between a Sunny Number and a Perfect Square?

A Perfect Square is a number that is the square of an integer (like 4, 9, 16). A Sunny Number is a number whose n + 1 is a perfect square. So a number itself doesn't have to be a perfect square to be sunny.

Q2: How do you check for a perfect square in Java?

int num = 25;
int sqrt = (int) Math.sqrt(num);
if (sqrt * sqrt == num) {
    System.out.println("Perfect Square");
}

Q3: Can sunny numbers be negative?

No, since the square root of a negative number is not real in Java (it results in NaN), sunny number checks are only meaningful for non-negative integers.

Q4: How would you check multiple numbers in a loop?

for (int i = 1; i <= 20; i++) {
    int next = i + 1;
    int root = (int) Math.sqrt(next);
    if (root * root == next) {
        System.out.println(i + " is Sunny");
    }
}

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