java - Mistake when create 2 thread files in one program -

i have 2 thread file in project of server: threadsocket , threadvalidate. threadsocket used sending , receiving information between client , server threadvalidate used testing account in signin , signup form.

this threadvalidate:

public class threadvalidate extends thread{ socket socket; string username = null, password = null; bufferedreader infromclient = null; printwriter outtoclient = null;  string driver = "sun.jdbc.odbc.jdbcodbcdriver"; string databaseurl = "jdbc:odbc:driver={sql server};server=abc-pc;database=abc;uid=sa;pwd=";  public threadvalidate(socket psocket){     this.socket=psocket;  }   public synchronized void run(){     try{          class.forname(driver);         connection connection;         connection = drivermanager.getconnection(databaseurl);           infromclient = new bufferedreader(new inputstreamreader(socket.getinputstream()));         outtoclient = new printwriter(socket.getoutputstream(), true);           username = infromclient.readline();         password = infromclient.readline();         system.out.println(username + " " + password);         //Đăng nhập         string sqlsignin = "select * userdangnhap username = ? , password = ?";         // preparedstatement         preparedstatement statementsignin = connection.preparestatement(sqlsignin);          statementsignin.setstring(1, username);         statementsignin.setstring(2, password);          resultset rssignin = statementsignin.executequery();            if (rssignin != null)         {             username = rssignin.getstring("username");             password = rssignin.getstring("password");              outtoclient.println("exist");         }         else {                outtoclient.println("not exist");         }     } catch (classnotfoundexception | sqlexception | ioexception ex){      } } }  

and code in signin form:

public class signin extends javax.swing.jframe {      socket socketvalidate = null;     private bufferedreader invalidate = null;     private printwriter outvalidate = null;     public static string username = null;     public static string password = null;     public static string kq = "";       public signin (java.awt.frame parent, boolean modal) {         initcomponents();         this.setresizable(false);     }      public signin (socket client){         this.socketvalidate = client;     }      public void listensocket() throws unknownhostexception {         string serveraddress = "localhost";          try {              socketvalidate = new socket(serveraddress, 4444);              invalidate = new bufferedreader(new inputstreamreader(socketvalidate.getinputstream()));              outvalidate = new printwriter(socketvalidate.getoutputstream(), true);           }         catch  (ioexception e)  {             system.out.println("no i/o" + e.getmessage());             system.exit(1);         }         catch (exception ex) {             system.out.println("unknown host: " + serveraddress);             system.exit(1);         }      }       private void bntdnactionperformed(java.awt.event.actionevent evt) {          this.username = txtusername.gettext();         this.password = txtpassword.gettext();         joptionpane.showmessagedialog(this, username + "   " + password);          outvalidate.println(username);         outvalidate.println(password);          try {             kq = invalidate.readline();         } catch (ioexception ex) {             logger.getlogger(dangnhap.class.getname()).log(level.severe, null, ex);         }          switch (kq) {             case "exist":                 this.dispose();                 break;             case "not exist":                 joptionpane.showmessagedialog(this, "retype again, please");                 break;         }      }   

it shows error on these lines: outvalidate.println(username); outvalidate.println(password); error

exception in thread "awt-eventqueue-0" java.lang.nullpointerexception @ client.signin .bntdnactionperformed(signin .java:188) @ client.signin .access$000(signin .java:22) @ client.signin $1.actionperformed(signin .java:102) 

please me fix it, don't understand reason why. can't send information username , password threadvalidate process it. threadsocket.java in project of server , gameclient.form in project of client run ok.

note: threadsocket , threadvalidate in same project of server, gameclient , signin in same project of client.