How to find Factorial of Numbers whose result is way toooo big for the native data structure to accommodate in any platform?

BigFactorial.java

/*
* How to find Factorial of Numbers whose result is way toooo big for the native datastructure to accomodate in any platform?
* Source of Inspiration: CodeChef
* Discussion src: http://discuss.codechef.com/questions/7349/computing-factorials-of-a-huge-number-in-cc-a-tutorial
*/

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class BigFactorial {
    public static void main(String args[] ) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String line = br.readLine();
        int numberOfTestCases = Integer.parseInt(line);
        String result[] = new String[numberOfTestCases];
        for (int i = 0; i < numberOfTestCases; i++) {
            int input = Integer.parseInt(br.readLine());
            result[i] = factorial(input);
        }
        for(int i=0; i<numberOfTestCases; i++) {
            System.out.println(result[i]);
        }
//        System.out.println(Long.MAX_VALUE);
//        System.out.println("20! = 2432902008176640000");
//        System.out.println("20! = " + factorial(20));
//        System.out.println("21! = 51090942171709440000");
//        System.out.println("21! = " + factorial(21));
//        System.out.println("22! = 1124000727777607680000");
//        System.out.println("22! = " + factorial(22));
    }

    private static String factorial(final int input) {
        long fact = 1;
        if(input <= 20) {
            for(int i=2; i<=input; i++) {
                fact *= i;
            }
            return fact + "";
        }

        String result = factorial(20) + "";
        for(int i=21; i<=input; i++) {
            List<String> interimResults = new ArrayList<>();
            String strNum = String.valueOf(i);
            for (int j=strNum.length()-1; j>=0; j--) {
                final String reverse = reverse(multiply(result, strNum.charAt(j)) + zeros(strNum.length() - 1 - j));
                interimResults.add(reverse);
            }
            result = sum(interimResults);
        }
        return result;
    }

    private static String reverse(final String str) {
        return new StringBuilder(str).reverse().toString();
    }

    private static String sum(final List<String> interimResults) {
        StringBuilder result = new StringBuilder("");
        int longestStringLength = interimResults.get(interimResults.size() - 1).length();  //Last row in the interim must be the one..
        int temp = 0, reminder = 0;
        for (int i=0; i<longestStringLength; i++) {
            ListIterator<String> iter = interimResults.listIterator();
            int sum = 0;
            while (iter.hasNext()) {
                sum += intAt(iter.next(), i);
            }
            sum += temp;
            if (sum > 9) {
                temp = sum/10;
                reminder = sum % 10;
            } else {
                temp = 0;
                reminder = sum;
            }
            result.append(reminder);
        }
        if (temp > 0) result.append(temp);
        return result.reverse().toString();
    }

    private static int intAt(final String str, int i) {
        if(i >= str.length()) return 0;
        return Integer.parseInt("" + str.charAt(i));
    }

    private static String zeros(final int j) {
        if (j == 0) return "";
        String result = "";
        for (int i=0; i<j; i++) {
            result += "0";
        }
        return result;
    }

    private static String multiply(final String result, final char cNum) {
        String interimResult = "";
        int temp = 0, reminder = 0;
        for(int i=result.length()-1; i>=0 ; i--) {
            int val1 = char2int(result.charAt(i));
            int val2 = char2int(cNum);
            int mult = (val1 * val2) + temp;
            if (mult > 9) {
                temp = mult/10;
                reminder = mult % 10;
            } else {
                temp = 0;
                reminder = mult;
            }
            interimResult = reminder + interimResult;
        }
        if (temp>0) interimResult = temp + interimResult;
        return interimResult;
    }

    private static int char2int(char c) {
        return Integer.parseInt("" + c);
    }
}