To find the next Fibonacci number - Java

FibonacciExample.Java

import java.util.HashMap;
import java.util.Map;

public class FibonacciExample {

    //Check the number is perfect square or not.
    static boolean isPerfectSquare(int x) {
        int s = (int) Math.sqrt(x);
        return (s * s == x);
    }

    // Returns true if n is a Fibonacci Number, else false 
    static boolean isFibonacci(int n) {
        return isPerfectSquare(5 * n * n + 4)
                || isPerfectSquare(5 * n * n - 4);
    }

    // find the index of fibonacci number
    static int findIndex(int n) {
        if (n <= 1) {
            return n;
        }
        int a = 0, b = 1, c = 1;
        int res = 1;
        while (c < n) {
            c = a + b;
            res++;
            a = b;
            b = c;
        }
        return res;
    }

    // find the fibonacci number by Index
    static int fib(int n) {
        if (n <= 1) {
            return n;
        }
        return fib(n - 1) + fib(n - 2);
    }

//    Find the next fibonacci number
    public static int getNextFib(int n) {
        int[] fibs = new int[100];
        int i = 0;
        fibs[0] = 0;
        fibs[1] = 1;
        for (i = 2; i <= n + 1; i++) {
            fibs[i] = fibs[i - 1] + fibs[i - 2];
            if (fibs[i] > n) {
                return fibs[i];
            }
        }
                
        if (fibs[i - 1] < n) {
            return fibs[i - 1] + n;
        }
        
        if(fibs[i-1]== n){
             return fibs[i-1]+ fibs[i-2];
        }

        if (fibs[i] == n - 1 || fibs[i] == n) {
            if (n != 0) {
                if (n == 1) {
                    return 2;
                } else {
                    return fibs[i + 1] + fibs[i + 2];
                }
            } else {
                return 1;
            }
        }

        if (fibs[i - 1] == 1) {
            return fibs[n + n];
        }
        return n;
    }
    
        //Use hashmap
    public static Map<String, String> nextFibonacci(int[] num) {
        int len = num.length;
        Map<String, String> fibNum = new HashMap<>();
        for (int i = 0; i < len; i++) {
            if (isFibonacci(num[i])) {
                // Using a index of given number and find the next fibonacci number.
//                int index = findIndex(num[i])+1;
//                int nextFibNum = fib(index);
                int nextFibNum = getNextFib(num[i]);
                fibNum.put(String.valueOf(i), String.valueOf(nextFibNum));
            } else {
                fibNum.put(String.valueOf(i), String.valueOf(num[i]) + " is not Fibonacci Number");
            }
        }
        return fibNum;
    }

    public static void main(String[] args) {
        int[] numbers = {1, 21, 8};
        for(String nums: nextFibonacci(numbers).values()){
            System.out.println(nums);
        }
    }
}