java - JavaFX 8 FXML TextField (maybe other variables) are always null -

i practicing small chat box window , try make user must enter name textfield box (a username) been having problems , sure not first time. keep getting nullpointerexception every time try use 1 of variables, right now, textfield. initialize blank string when type in username.

app.java

public class app extends application {  @override public void start(stage stage) throws exception {     parent root = fxmlloader.load(getclass().getresource("fxmlapp.fxml"));      scene scene = new scene(root);      stage.setscene(scene);     stage.show(); }  public static void main(string[] args) {     launch(); }  } 

fxmlapp.fxml (deleted not needed)

<?xml version="1.0" encoding="utf-8"?>  <?import javafx.scene.text.*?> <?import java.lang.*?> <?import java.util.*?> <?import javafx.scene.*?> <?import javafx.scene.control.*?> <?import javafx.scene.layout.*?>  <anchorpane id="anchorpane" maxheight="900.0" maxwidth="800.0" minheight="300.0" minwidth="200.0" prefheight="600.0" prefwidth="500.0" xmlns="http://javafx.com/javafx/8.0.40" xmlns:fx="http://javafx.com/fxml/1" fx:controller="chatappclient.fxmlchatappclientcontroller">     <children>       <textfield fx:id="username" layoutx="176.0" layouty="488.0" prompttext="enter username" />       <button fx:id="login" layoutx="176.0" layouty="519.0" mnemonicparsing="false" onaction="#login" text="login" />     </children> </anchorpane> 

as can see login button calls

onaction="#login" 

last file: fxmlappcontroller

    package chatappclient;      import java.net.url;     import java.util.resourcebundle;     import javafx.application.platform;     import javafx.event.actionevent;     import javafx.fxml.fxml;     import javafx.fxml.initializable;     import javafx.scene.control.button;     import javafx.scene.control.label;     import javafx.scene.control.textarea;     import javafx.scene.control.textfield;     import javafx.scene.layout.pane;     import javax.swing.joptionpane;      public class fxmlappcontroller implements initializable      {         @fxml         private pane top_container;         private pane bottom_container;         private textfield username ;         private string name;         private button login;          public fxmlchatappclientcontroller ()         {            // without this, nullpointerexception             // it, textfield null             username = new textfield();         }          @override         public void initialize(url url, resourcebundle rb)          {}           @fxml         private void login()          {             // returns empty string, if did not initialize it, exception             system.out.println("debug: " + username.gettext());         }     } 

you need annotate each of fields defined in fxml file @fxml:

private pane top_container; private pane bottom_container; @fxml private textfield username ; private string name; @fxml private button login;