我印象最深刻的error就是JAVA中equals和==的区别。
这是我在写web大作业时出现的错误：
1 public void doPost(HttpServletRequest request, HttpServletResponse response) 2 throws ServletException, IOException { 3 //String checkcode = (String) request.getSession().getAttribute("safecode"); 4 //System.out.println(checkcode); 5 request.setCharacterEncoding("UTF-8"); 6 response.setCharacterEncoding("UTF-8"); 7 response.setContentType("text/html;charset=UTF-8"); String id = request.getParameter("id"); 8 String password = request.getParameter("password"); 9 String password1=request.getParameter("password1"); 10 PrintWriter out = response.getWriter(); 11 MessagerDB messagerDB=new MessagerDB(0); 12 try { 13 if(id=="") 14 { 15 out.write("<html><head><title>两次密码不一致</title></head><body>"); 16 out.write("<h1>对不起，用户名不能为空</h1></body></html>"); 17 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 18 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 19 out.flush(); 20 } 21 else if(password=="") 22 { 23 out.write("<html><head><title>两次密码不一致</title></head><body>"); 24 out.write("<h1>对不起，密码不能为空</h1></body></html>"); 25 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 26 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 27 out.flush(); 28 } 29 else if(!password==password1) 30 { 31 out.write("<html><head><title>两次密码不一致</title></head><body>"); 32 out.write("<h1>对不起，您两次输入的密码不一致，请重新输入</h1></body></html>"); 33 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 34 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 35 out.flush(); 36 } 37 else if(messagerDB.add(id,password,"1")){ 38 response.sendRedirect("/redcross1/messager.jsp"); 39 } 40 else 41 { 42 out.write("<html><head><title>FailedLogin</title></head><body>"); 43 out.write("<h1>对不起，此ID已经被注册，请重新输入</h1></body></html>"); 44 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 45 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 46 out.flush(); 47 } 48 } catch (Exception e) { 49 // TODO Auto-generated catch block 50 e.printStackTrace(); 51 } 52 }
String类的比较，我全部用的是==来比较的，导致密码比较的时候总是错误，即使输入正确的密码，也无法成功进入到程序中。这个错误困扰了我好长时间。最后，我通过在每个if中输出字符，才发现原来是判断String类是否相等时出现了错误，导致程序根本没有进入到任何一个if中。通过百度才知道，原来判断两个String的值是否相等应该用a.equals(b)，这种方法来判断，而==判断的是两个String的地址是否相等，和C++中的判断方法不同。然后，我将代码修改成为以下这样：
1 public void doPost(HttpServletRequest request, HttpServletResponse response) 2 throws ServletException, IOException { 3 //String checkcode = (String) request.getSession().getAttribute("safecode"); 4 //System.out.println(checkcode); 5 request.setCharacterEncoding("UTF-8"); 6 response.setCharacterEncoding("UTF-8"); 7 response.setContentType("text/html;charset=UTF-8"); String id = request.getParameter("id"); 8 String password = request.getParameter("password"); 9 String password1=request.getParameter("password1"); 10 PrintWriter out = response.getWriter(); 11 MessagerDB messagerDB=new MessagerDB(0); 12 try { 13 if(id.equals("")) 14 { 15 out.write("<html><head><title>两次密码不一致</title></head><body>"); 16 out.write("<h1>对不起，用户名不能为空</h1></body></html>"); 17 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 18 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 19 out.flush(); 20 } 21 else if(password.equals("")) 22 { 23 out.write("<html><head><title>两次密码不一致</title></head><body>"); 24 out.write("<h1>对不起，密码不能为空</h1></body></html>"); 25 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 26 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 27 out.flush(); 28 } 29 else if(!password.equals(password1)) 30 { 31 out.write("<html><head><title>两次密码不一致</title></head><body>"); 32 out.write("<h1>对不起，您两次输入的密码不一致，请重新输入</h1></body></html>"); 33 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 34 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 35 out.flush(); 36 } 37 else if(messagerDB.add(id,password,"1")){ 38 response.sendRedirect("/redcross1/messager.jsp"); 39 } 40 else 41 { 42 out.write("<html><head><title>FailedLogin</title></head><body>"); 43 out.write("<h1>对不起，此ID已经被注册，请重新输入</h1></body></html>"); 44 out.write("<meta http-equiv="Content-Language" content="zh-CN">" 45 + "<meta http-equiv="refresh" content="1;url=/redcross1/add.jsp">"); 46 out.flush(); 47 } 48 } catch (Exception e) { 49 // TODO Auto-generated catch block 50 e.printStackTrace(); 51 } 52 }
通过更改使得程序可以正确运行，即输入正确的密码可以进入到系统中。