你好，我有上面的代码
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class CarSessionServlet2 extends HttpServlet {
private String Brands[] = {
"Audi", "Ford", "Toyota"
};
private String Cars[] = {
"A3", "Fiesta", "Yaris"
};
private String screen = "name";
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // reaction
if (screen.equals("name")) { //elegxos apo poia selida kalw tin doPost. Edw apo tin BrandsSelection.html
String cookieName = request.getParameter("name"); // choice made
// will be sent
// back to
// client
String cookieBrand = request.getParameter("brands");
PrintWriter output;
HttpSession session = request.getSession(true);
session.setAttribute("name", cookieName);
session.setAttribute(cookieBrand, getCars(cookieBrand));
response.setContentType("text/html");
output = response.getWriter();
// send HTML page to client
output.println("<HTML><HEAD><meta charset='utf-8'><TITLE>");
output.println("Cookies");
output.println("</TITLE></HEAD><BODY>");
output.println("<P>Welcome Ms./Mr.: ");
// output.println( "<P>" );
output.println(cookieName);
output.println(" <BR> you have chosen ");
output.println(cookieBrand);
output.println("<BR><a href='SecondPage.html'>" + "Click here to continue..." + "</a>");
output.println("</BODY></HTML>");
output.close(); // close stream
screen = "color";
}
if (screen.equals("color")) { //elegxos apo poia selida kalw tin doPost. Edw apo tin SecondPage.html
PrintWriter output;
String color = request.getParameter("color");
HttpSession session = request.getSession(true);
session.setAttribute("color", color);
response.setContentType("text/html");
output = response.getWriter();
// send HTML page to client
output.println("<HTML><HEAD><meta charset='utf-8'><TITLE>");
output.println("Cookies");
output.println("</TITLE></HEAD><BODY>");
output.println("<FORM ACTION="http://localhost:8080/Askisi2Session/CarSessionServlet2" METHOD="GET">");
output.println("<STRONG>To see your selections press the button:<br> </STRONG>");
output.println("<INPUT TYPE="submit" VALUE="Selections">");
output.println("</FORM>");
output.println("</BODY></HTML>");
output.close(); // close stream
screen = "name";
}
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // reaction
// to
// the
// reception
PrintWriter output; // of
// GET
response.setContentType("text/html");
output = response.getWriter();
output.println("<HTML><HEAD><meta charset='utf-8'><TITLE>");
output.println("Cookie with your selections has been read !");
output.println("</TITLE></HEAD><BODY>");
HttpSession session = request.getSession(false); // get client's session;
output.println("<H3>Here is your saved session data:</H3>");
Enumeration e;
if (session != null) {
e = session.getAttributeNames();
} else {
e = null;
}
while (e.hasMoreElements()) {
String name = (String) e.nextElement();
output.println(name + ": " + session.getAttribute(name) + "<BR>");
}
output.println("<a href='BrandsSelection.html'>Click here to Restart...</a>");
output.println("</BODY></HTML>");
output.close(); // close stream
}
private String getCars(String conString) {
for (int i = 0; i < Brands.length; ++i)
if (conString.equals(Brands[i])) {
return Cars[i];
}
return ""; // no matching string found
}
} // END OF CLASS CookieServlet
当我用tomcat 8.0.28运行它时，我得到了一个提到的java.lang.NullPointerException
java.lang.NullPointerException
CarSessionServlet2.getCars(CarSessionServlet2.java:155)
CarSessionServlet2.doPost(CarSessionServlet2.java:43)
javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
第43行是
session.setAttribute(cookieBrand,getCars(cookieBrand));
第155行是
if (conString.equals(Brands[i])) {
首先，我打开一个html页面，填充一个文本框，在单选按钮上做出选择，然后按下一个调用doPost方法的提交按钮。然后出现第二个html页面。我填充了另一个文本框，然后按下另一个提交按钮，我再次调用doPost方法（第二个if语句，screen=color）。然后。。。。。NullPointerException。它应该显示另一个html页面，其中包含一个调用doGet方法的按钮。我不知道为什么我采取的例外
Html第1页（品牌选择）
<HTML>
<HEAD>
<TITLE>Cookie will be written in our disc</TITLE>
</HEAD>
<BODY>
<FORM ACTION="http://localhost:8080/Askisi2Session/CarSessionServlet2" METHOD="POST">
<STRONG>Enter Your Name:<br> </STRONG>
<PRE>
<INPUT TYPE="text" NAME="name"><br><br>
<STRONG>Select the Brand of your Desire:<br> </STRONG>
<PRE>
<INPUT TYPE="radio" NAME="brands" VALUE="Audi">check here for Audi<BR>
<INPUT TYPE="radio" NAME="brands" VALUE="Ford">check here for Ford<BR>
<INPUT TYPE="radio" NAME="brands" VALUE="Toyota" CHECKED>check here for Toyota<BR>
</PRE>
<INPUT TYPE="submit" VALUE="Submit">
<INPUT TYPE="reset"> </P>
</FORM>
</BODY>
</HTML>
HTML第2页（SeondPage）
<HTML>
<HEAD>
<TITLE>Cookie taken into Account</TITLE>
</HEAD>
<BODY>
<FORM ACTION="http://localhost:8080/Askisi2Session/CarSessionServlet2" METHOD="POST">
<STRONG>What is your favorite color?<br> </STRONG>
<PRE>
<INPUT TYPE="text" NAME="color"><br>
</PRE>
<INPUT TYPE="submit" VALUE="Submit">
</FORM>
</BODY>
</HTML>
此方法getCars(...)返回""，因为条件if(conString.equals(Brands[i]))与任何字符串都不匹配：
private String getCars(String conString) {
for (int i = 0; i < Brands.length; ++i)
if (conString.equals(Brands[i])) {
return Cars[i];
}
return "";
}