java.lang.NullPointerException在处理控制在2个控制器之间 - Javafx

问题描述

我在Javafx的两个场景之间尝试了按钮事件。

I've been trying button events between 2 scenes in Javafx.

这是Start.fxml（在视图包中）：

This is Start.fxml (in view package):

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

<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>


<AnchorPane prefHeight="292.0" prefWidth="383.0" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8" fx:controller="controller.StartController">
   <children>
      <Label fx:id="lbl1" layoutX="87.0" layoutY="60.0" prefHeight="17.0" prefWidth="209.0" />
      <TextField fx:id="txt1" layoutX="87.0" layoutY="121.0" prefHeight="25.0" prefWidth="209.0" />
      <Button fx:id="btn1" layoutX="171.0" layoutY="191.0" mnemonicParsing="false" onAction="#btn1Click" text="Click" />
   </children>
</AnchorPane>

这是Preserences.fxml（在视图包中）：

This is Preserences.fxml (in view package):

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

<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>


<AnchorPane prefHeight="292.0" prefWidth="383.0" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8" fx:controller="controller.PreferencesController">
   <children>
      <Label fx:id="lbl2" layoutX="87.0" layoutY="60.0" prefHeight="17.0" prefWidth="209.0" />
      <TextField fx:id="txt2" layoutX="87.0" layoutY="121.0" prefHeight="25.0" prefWidth="209.0" />
      <Button fx:id="btn2" layoutX="171.0" layoutY="191.0" mnemonicParsing="false" onAction="#btn2Click" text="Click" />
   </children>
</AnchorPane>

这是Main.fxml，类似于以前2个文件的容器（在视图包中） ：

This is Main.fxml which is like a container for the previous 2 files.(in view package):

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

<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>


<AnchorPane prefHeight="481.0" prefWidth="468.0" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8" fx:controller="controller.MainController">
   <children>
      <TabPane layoutX="14.0" layoutY="14.0" prefHeight="481.0" prefWidth="468.0" tabClosingPolicy="UNAVAILABLE" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
        <tabs>
          <Tab closable="false" text="Start">
               <content>
                  <fx:include source="Start.fxml" />
               </content>
          </Tab>
          <Tab closable="false" text="Preferences">
               <content>
                  <fx:include source="Preferences.fxml" />
               </content>
          </Tab>
        </tabs>
      </TabPane>
   </children>
</AnchorPane>

这是StartController.java（在控制器包中）：

This is StartController.java (in controller package):

package controller;

import java.net.URL;
import java.util.ResourceBundle;

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.TextField;

public class StartController implements Initializable {

    @FXML public static Label lbl1;
    @FXML public static TextField txt1;
    @FXML public static Button btn1;

    @Override
    public void initialize(URL location, ResourceBundle resources) {

    }

    @FXML private void btn1Click(ActionEvent e){
        System.out.println("Button 1 clicked");
        lbl1.setText(PreferencesController.txt2.getText());
    }

}

这是PreferencesController.java控制器包）：

This is PreferencesController.java (in controller package):

package controller;

import java.net.URL;
import java.util.ResourceBundle;

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.TextField;

public class PreferencesController implements Initializable {

    @FXML public static Label lbl2;
    @FXML public static TextField txt2;
    @FXML public static Button btn2;

    @Override
    public void initialize(URL location, ResourceBundle resources) {

    }

    @FXML private void btn2Click(ActionEvent e){
        System.out.println("Button 2 clicked");
        lbl2.setText(StartController.txt1.getText());
    }

}

这是MainController.java控制器包）：

This is MainController.java (in controller package):

package controller;

import java.net.URL;
import java.util.ResourceBundle;

import javafx.fxml.Initializable;

public class MainController implements Initializable{

    @Override
    public void initialize(URL arg0, ResourceBundle arg1) {

    }

}

最后，这是Main.java在应用程序包中：

Finally, this is Main.java in application package:

package application;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.stage.Stage;
import javafx.scene.Parent;
import javafx.scene.Scene;


public class Main extends Application {
    @Override
    public void start(Stage primaryStage) {
        try {
            Parent root = FXMLLoader.load(getClass().getResource("/view/Main.fxml"));
            Scene scene = new Scene(root);
            scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
            primaryStage.setScene(scene);
            primaryStage.show();
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        launch(args);
    }
}

但是当我点击按钮，错误：

But when I clicked the buttons, it gives me the errors:

"Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException...
...
Caused by: java.lang.NullPointerException
    at controller.StartController.btn1Click(StartController.java:26)
    ... 61 more"

请帮助我！ ：

推荐答案

确定，以便NPE被抛到这一行：

OK so the NPE is being thrown on this line:

    lbl1.setText(PreferencesController.txt2.getText());

$ b b

在基本的Java层面，只有两种方法可以实现：

At the basic Java level, are only two ways that you could get that to happen:

• 当 PreferencesController.txt2 时，> lnl1 是 null 或


• c $ c>是 null 。

• when lnl1 is null, or
• when PreferencesController.txt2 is null.

因为 PreferencesController.txt2 是一个静态字段。

我不熟悉JavaFX或FXML，但我怀疑你的错误是声明字段为 static 。我怀疑这导致FXML机制不注入引用因此这些变量保持为 null ...并且您获得了NPE。

I'm not familiar with JavaFX or FXML, but I suspect that your mistake is declaring the fields as static. I suspect that this causes the FXML mechanisms to not inject the reference to the UI component correctly. So those variables remain as null ... and you get an NPE.

确认是此处：。 （感谢链接@James_D）

Confirmation is here: http://stackoverflow.com/a/23109125/139985. (Thanks for the link @James_D)

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