What to do when you need to store a (very) large number?

Question

I am trying to do a Project Euler problem but it involves adding the digits of a very large number. (100!)

Using Java int and long are too small.

Thanks for

Answer 1

import java.math.BigInteger;
import java.util.*;

public class Main {
    protected static ArrayList table = new ArrayList();

    static {
        table.add(BigInteger.valueOf(1));
    }

    public static synchronized BigInteger factorial(int x) {
        if (x < 0) throw new IllegalArgumentException("x must be non-negative.");
        for (int size = table.size(); size <= x; size++) {
            BigInteger lastfact = table.get(size - 1);
            BigInteger nextfact = lastfact.multiply(BigInteger.valueOf(size));
            table.add(nextfact);
        }
        return table.get(x);
    }

    public static void main(String[] args) {
        for (int i = 0; i <= 50; i++)
            System.out.println(i + "! = " + factorial(i));
    }
}