FactorialNumber.java



import java.io.IOException;
import java.io.PrintWriter;
import java.math.BigInteger;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class FactorialNumber
 */
@WebServlet("/factorial")
public class FactorialNumber extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		// TODO Auto-generated method stub
		
		//generamos un PrintWriter para escribir en la pagina web
				PrintWriter pr = new PrintWriter(resp.getOutputStream());
				//generamos una cadena que indicara el error en caso de que pase
				String pgError = "<body><h1>El valor de n es invalido, ingrese un numero de 0 a 100</h1></body>";
				try {
					if(req.getParameter("n")!=null ) {
						int n = Integer.parseInt(req.getParameter("n"));
						//si no ocurrio ningun error entonces procedemos a calcular el factorial
						if(n>100 || n<0) {
							pr.write(pgError);
						}else {
							BigInteger factorial = BigInteger.valueOf(1);
							if(n!=0)
							{
								for (int i = 2; i <= n; i++) {									
									BigInteger j = BigInteger.valueOf(i);
									factorial=factorial.multiply(j);
								}
							}
							
						
							//dibujamos el html
							pr.write("<html>"
									+ "<body style=' text-align:center'><h1>El factorial de "+n+" es:</h1>"
									+"<p style='color:red'>"+factorial+"</p>"
									+"</body>"
									+ "</html>");
						}
					}else {
						pr.write(pgError);
					}
					
				} catch (Exception e) {
					pr.write(pgError);
				}
				
				pr.close();
	}

}