java - why do i get a null pointer exception when i call the constructor of a another class? - 


case "login":         try {             restoreuserinfo();             string email = textf1.gettext();             string password = new string(jpass1.getpassword());             if (hm.containskey(email)) {                 if (hm.get(email).getpassword().equals(password)) {                     showmessagedialog(this, "successfully login");                      ch = new chat_window("lets chat!!");                     system.out.println("after call");                  } else {                     showmessagedialog(this, "invalid password");                 }             } else {                 showmessagedialog(this, "invalid login id , password");             }         } catch (exception exc) {             system.out.println("invalid file");             exc.printstacktrace();         }

why null pointer exception when call "ch = new chat_window("lets chat!!");"? chat_window class available in same package , matching constructor, code below:

public chat_window(string title){     super(title);     setsize(500, 500);     getcontentpane().setbackground(color.light_gray);     setlocationrelativeto(null);     setresizable(false);     c1=new jpanel();     c2=new jpanel();             lbl=new jlabel("type here:");     c1.add(lbl, new gridlayout(1, 2, 50, 50));     ta=new jtextarea(03,30);     c1.add(ta,new gridlayout(1, 2, 50, 50) );     c1.add(send_but,new gridlayout(1, 2, 50, 50));     add(c1, borderlayout.page_end);     send_but=new jbutton("send");             setdefaultcloseoperation(exit_on_close);     setvisible(true);  }

help please!!!

you adding send_but before creating it.

c1.add(send_but,new gridlayout(1, 2, 50, 50));  // send_but null here add(c1, borderlayout.page_end); send_but=new jbutton("send");

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