Handling events from a button?

Question

Java

I'm making a javafx application using scene builder
. I want to make a simple button click event handler.
fxml file

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import java.net.*?>
<?import javafx.geometry.Insets?>
<?import javafx.scene.control.*?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.paint.*?>
<?import javafx.scene.text.*?>

<AnchorPane id="AnchorPane" maxHeight="-1.0" maxWidth="-1.0" minHeight="-1.0" minWidth="-1.0" prefHeight="330.0" prefWidth="443.99980000000505" styleClass="background" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/2.2" fx:controller="sample.Controller">
  <children>
    <VBox id="VBox" alignment="CENTER" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="330.0" prefWidth="443.99980000000505" spacing="5.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
      <children>
        <Label id="welcomeLabel" disable="false" prefHeight="35.0" prefWidth="201.0" text="WELCOME" textAlignment="CENTER" textOverrun="CLIP" underline="false" wrapText="false">
          <font>
            <Font size="40.0" />
          </font>
          <textFill>
            <Color blue="1.000" green="0.000" red="0.333" fx:id="x2" />
          </textFill>
        </Label>
        <Label id="userNameLabel" text="User name" textFill="#5200ff">
          <font>
            <Font size="18.0" fx:id="x1" />
          </font>
        </Label>
        <TextField id="userNameTxtF" alignment="CENTER" prefWidth="206.0" />
        <Label id="passwordLabel" font="$x1" text="Password" textFill="$x2" />
        <PasswordField id="passPField" alignment="CENTER" prefWidth="206.0" />
        <Button id="signInBtn" defaultButton="true" mnemonicParsing="false" onAction="#action1" text="Sign In" textFill="$x2">
          <font>
            <Font size="30.0" />
          </font>
        </Button>
      </children>
    </VBox>
  </children>
  <stylesheets>
    <URL value="@../res/DarkTheme.css" />
  </stylesheets>
</AnchorPane>

Controller class

public class Controller {

    private String user;
    private String pass;

    @FXML
    private Label welcomeLabel;
    @FXML
    private Label userNameLabel;
    @FXML
    private Label passwordLabel;
    @FXML
    private TextField userNameTxtF;
    @FXML
    private PasswordField passPField;
    @FXML
    private Button signInBtn;

    public Controller(){
        user = "user";
        pass = "12345678";
    }

@FXML
    private void handleButtonAction(ActionEvent event) {
        signInBtn.addEventHandler(MouseEvent.MOUSE_CLICKED, mouseEvent -> {
            System.out.println("xcvbn");
        });

    }

    @FXML
    private void action(){
        signInBtn.setOnAction((event)->{
            if(userNameTxtF.getText() == user && passPField.getText() == pass){
                welcomeLabel.setText("ALL'S OK");
            }
            else System.out.println("no");
        });
    }
    @FXML
    private void action1(){
        signInBtn.setText("fgsdhjk");
    }
@FXML protected void handleSubmitButtonAction(ActionEvent event) {
        if(userNameTxtF.getText() == user && passPField.getText() == pass)
            System.out.println("fhbskf");
    }
}

I try different options by setting the method name in the onAction="#"button parameter. And in any case it gives the same error:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
  at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1762)
  at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1645)
  at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
  at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
  at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
  at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
  at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
  at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
  at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
  at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
  at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
  at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
  at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
  at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
  at javafx.event.Event.fireEvent(Event.java:198)
  at javafx.scene.Node.fireEvent(Node.java:8216)
  at javafx.scene.control.Button.fire(Button.java:185)
  at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
  at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
  at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
  at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
  at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
  at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
  at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
  at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
  at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
  at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
  at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
  at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
  at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
  at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
  at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
  at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
  at javafx.event.Event.fireEvent(Event.java:198)
  at javafx.scene.Scene$MouseHandler.process(Scene.java:3724)
  at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3452)
  at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1728)
  at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2461)
  at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:348)
  at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:273)
  at java.security.AccessController.doPrivileged(Native Method)
  at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:382)
  at com.sun.glass.ui.View.handleMouseEvent(View.java:553)
  at com.sun.glass.ui.View.notifyMouse(View.java:925)
  at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
  at com.sun.glass.ui.win.WinApplication.lambda$null$141(WinApplication.java:102)
  at com.sun.glass.ui.win.WinApplication$$Lambda$38/374958851.run(Unknown Source)
  at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:483)
  at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
  at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:483)
  at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
  at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1759)
  ... 47 more
Caused by: java.lang.NullPointerException
  at sample.Controller.action1(Controller.java:69)
  ... 57 more

Tell me why the NPE exception is thrown?

Answer 1

See the lowest of the Exceptions:

Caused by: java.lang.NullPointerException
at sample.Controller.action1(Controller.java:69)
... 57 more

As you can see on line 69 in the Controller.java file, an NPE occurs in the action1 method - this
NPE line can only crash here if signInBtn is null.
Let's see what signInBtn is:

@FXML
private Button signInBtn;

As you can see it should be a button defined in fxml file with fx:id equal to signInBtn. We look for this button in xml and find the following:

<Button id="signInBtn" defaultButton="true" mnemonicParsing="false" onAction="#action1" text="Sign In" textFill="$x2">
  <font>
  <Font size="30.0" />
  </font>
</Button>

And this is where the error becomes clear - instead of the fx: id attribute, you just specified id, in other words, you forgot to add a namespace - add it, and the binding will work. Those. correct would be:

<Button fx:id="signInBtn" defaultButton="true" mnemonicParsing="false" onAction="#action1" text="Sign In" textFill="$x2">
  <font>
  <Font size="30.0" />
  </font>
</Button>

I also note that you have the same error with many other attributes. Pay attention to which attributes in which namespace should be used.