factorial biginteger vector

import java.math.BigInteger;
import java.util.Vector;

public class FactorialBigIntegerVector
{
    protected static Vector<BigInteger> resultstable = new Vector<BigInteger>();

    //guarantee that the value of the first element is always 1
    static
    {   resultstable.addElement( BigInteger.valueOf( 1 ) );
    }

    //multi thread safe
    public static synchronized BigInteger factorial( int x )
    {
        if( x < 0 ) throw new IllegalArgumentException( "x must be non-negative" );

        for( int size = resultstable.size(); size <= x; size++ )
        {
            BigInteger previousfactorial = (BigInteger) resultstable.elementAt( size - 1 );
            BigInteger nextfactorial = previousfactorial.multiply( BigInteger.valueOf(size) );
            resultstable.addElement( nextfactorial );
        }
        return (BigInteger) resultstable.elementAt( x );
    }

    public static void main( String[] args )
    {
        for( int i = 1; i < 50; i++ )
        {
            System.out.println( i + "! = " + factorial( i )  );
        }
    }
}