nullPointerException在处理两个控制器之间的控件时的应用--Javafx

Question

这是start.fxml（在视图包中）：

<?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presences.fxml（在视图包中）：

<?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，它类似于前面2个文件的容器。（视图包）：

<?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>

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());
    }

}

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());
    }

}

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) {

    }

}

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);
    }
}

"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"

请帮帮我！：（

Answer 1

好的，所以NPE被抛出在这一行：

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

在基本Java级别上，只有两种方法可以实现这一点：

• 当lnl1为null或
时 PreferencesController.txt2为 null.时