The sum of each digit when raised to its position value equals the former number

Variable-Narcissistic-Number

/*
89 is a unique number and searching similar others
The number 89 is the first integer with more than one digit that fulfills the property partially introduced below:
example
    89 = 8^1 + 9^2
    135 = 1^1 + 3^2 + 5^3
The sum of each digit when raised to its position value equals the former number
A range is given and all those numbers with above property is collected.
*/

public class SumDigPower {
    public static List<Long> sumDigPow(long a, long b){
        List<Long> numberArList = new ArrayList<>();        //returning ArrayList of Long
        while (a<=b){                                       //inbetween the range
            String stringFromInt = new StringBuilder().append(a).toString();    //converting every number to string
            int[] intArFromString = new int[stringFromInt.length()];
            double sum = 0;
            for (int i = 0; i < stringFromInt.length(); i++) {
                intArFromString[i] = stringFromInt.charAt(i) - '0';     //getting into int array by substracting 0 ASCI code
                sum = sum + Math.pow(intArFromString[i],i+1);}           //math
            int sumInt = (int) sum;     //type casting to make it equal to long
            if (a == sumInt){
                numberArList.add(a);}
            a++;}                 //increasing the input number
        return numberArList;
    }
}