import java.math.BigInteger;
import java.util.ArrayList;
 
public class NewClass {

    protected static ArrayList alist = new ArrayList();

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

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

    public static void main(String[] args) {
        
            System.out.println("100 != " + factorial(100));
       
    }
}