Question:
Write a Java servlet, HTMLBank.java that creates the HTML form. Use an instance variable for the balance. IF the user enters a valid amount in the amount text box and then clicks the Deposit button, add the amount to the balance; if the user clicks the Withdrawn button, subtract the amount from the balance. If no amount was entered or if the amount is negative or zero, output a simple HTML page that displays an appropriate error message. Just say then to compile and the url in the Web browser. (second part) to execute a second thread start another instance of our web browser and lead the servlet in it as well. Switch between the 2 instances, perform transactions with each. Write a short paragraph that explains how the displayed balance verifies that the variable, balance, is shared. HINT: use DecimalFormat to format numeric output. Use static method, Double.parseDouble(StringVariable) to convert to String from the text box parameter, amount, to a double. Be sure that an amount was entered before converting to a double.

My other question:
Did I understand correctly and did I do as they ask? PS if they say Switch between the two instances and perform transactions they mean that I should have to web pages open and then test to see if the one's balance is correct to the other one??

My Code: 
import javax.servlet.*;
import javax.servlet.http.*;
 
import java.io.*;
import java.util.*;
import java.net.*;
import java.text.DecimalFormat;
 
public class HTMLBank extends HttpServlet
{
	public DecimalFormat pattern = new DecimalFormat("$#0,000.00");	// To set a patter for display
	public Double actBalance = 0.00;                      		   	// initial display value
	public String displayBalance = pattern.format(actBalance);
	double entered = 0.0;
 
	public void init(ServletConfig conf)throws ServletException
	{
		super.init(conf);
	}
 
	public void  doGet(HttpServletRequest req, HttpServletResponse res)
	throws ServletException, IOException
	{
		res.setContentType("text/html");
		PrintWriter out = res.getWriter();
		res.setHeader("Expires","Tues, 01 Jan 1980 00:00:00 GMT");
		//HttpSession session = req.getSession(true); 				// get current session
 
			out.println("<html>");
			out.println("<head>");
			out.println("<title>Online Bank ATM Simulator</title>");
			out.println("</head>");
			out.println("<body>");
			out.println("<form method=POST action=/servlet/HTMLBank>");
			out.println("<center>");
			out.println("<hr>");
			out.println("<h1>Bank ATM Simulation</h1><br>");
			out.println("Amount: <input type=text name=ENTRY size=20 maxlength=15><br><br>");
			out.println("Balance:"+displayBalance+"<br><br>");
			out.println("<input type=submit name=DEPOSIT value=\"Deposit\">        ");
			out.println("<input type=submit name=WITHDRAW value=\"Withdraw\">        ");
			out.println("<input type=submit name=BALANCE value=\"Balance\">        <br><br>");
			out.println("<hr>");
			out.println("</form>");
			out.println("</table>");
			out.println("</body>");
			out.println("</html>");
	}
 
	public void doPost(HttpServletRequest req, HttpServletResponse res)
	throws ServletException, IOException
	{
		try
		{
			entered = Double.parseDouble(req.getParameter("ENTRY"));
		}
		catch(NumberFormatException e)
		{
			entered = 0.0;
		}
 
		if((req.getParameter("WITHDRAW")!=null) && (entered < actBalance) && (entered > 0))
		{
			actBalance = actBalance - entered;
			doGet(req, res);
		}
		else if((req.getParameter("DEPOSIT")!=null) && (entered > 0))
		{
			actBalance = actBalance + entered;
			doGet(req, res);
		}
		else if(req.getParameter("BALANCE")!=null)
		{
			displayBalance = pattern.format(actBalance);
			doGet(req, res);
		}
		else
		{
			res.setContentType("text/html");
			PrintWriter out = res.getWriter();
			res.setHeader("Expires","Tues, 01 Jan 1980 00:00:00 GMT");
			//HttpSession session = req.getSession(true); 			// get current session
 
			out.println("<HTML>");
			out.println("<HEAD>");
			out.println("<TITLE>Incorrect Input Warning..</TITLE>");
			out.println("</HEAD>");
			out.println("<BODY>");
			out.println("<H1>Invalid Entry</H1>");
			out.println("<H2>You have entered an incorrect amount!!!</H2>");
			out.println("<H3>You must enter a number only and it must be greater than 0.0</H3>");
			out.println("</from>");
			out.println("</BODY>");
			out.println("</HTML>");
		}
	}
}