label.setText NullPointerException [英] label.setText NullPointerException

问题描述

嗨第一次来这里，但是这里：

Hi first time here but here goes:

我有一个JavaFX应用程序，动态更改FXML UI标签，数据从Player类中提取。

I have a JavaFX application that changes the FXML UI labels dynamically and the data is pulled from a Player class.

有问题的两个类是 Player.java 和 InterfaceHandler.java 。

播放器类存储播放器详细信息，我想将详细信息传递给Interface类，该类设置标签上的文本。

The player class stores player details and I want to pass the details to the Interface class which sets the text on the labels.

作为测试，我的FXML用户界面只有一个按钮和两个标签。

As a test my FXML UI just has a button and two labels.

如果点击按钮就会调用 handleButton 方法它将 locationLabel 设置为Town罚款。

If it click the button it calls the handleButton method it sets locationLabel to "Town" fine.

但是如果我在我的Player类中调用 locationLabel（）方法当调用 nameLabel.setText（name）时，我得到NullPointerException。通过调试，我发现Interface类中的名称字符串应该是Dan。

However if I call the locationLabel() method in my Player class I get a NullPointerException when nameLabel.setText(name) is called. Through debugging I find that the name string in the Interface class is what it should be "Dan".

任何人都可以帮忙吗？

主类：

public class Main extends Application {

    public void start(final Stage mainStage) throws Exception {

        Parent root = FXMLLoader.load(getClass().getResource("MainScreen.fxml"));
        Scene scene = new Scene(root);
        mainStage.setTitle("Main Screen");
        mainStage.setScene(scene);
        mainStage.show();    
    }

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

玩家等级：

public class Player{

InterfaceHandler ui = new InterfaceHandler();   

 public void setNameLabel() {

    String name = "Dan";
    ui.setName(name);
}

InterfaceHandler类：

InterfaceHandler class:

    public class InterfaceHandler implements Initializable {

       public Label nameLabel;
       public Label locationLabel;    

public void handleButton(ActionEvent event) throws IOException {        

        locationLabel.setText("Town");
    }

public void setName(String name){       
        nameLabel.setText(name);
    }    
}

MainScreen.fxml：

MainScreen.fxml:

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

    <?import java.lang.*?>
    <?import java.net.*?>
    <?import java.util.*?>
    <?import javafx.scene.*?>
    <?import javafx.scene.control.*?>
    <?import javafx.scene.effect.*?>
    <?import javafx.scene.image.*?>
    <?import javafx.scene.layout.*?>
    <?import javafx.scene.text.*?>

    <AnchorPane id="AnchorPane" prefHeight="629.0" prefWidth="600.0" snapToPixel="true" style="-fx-background-color:  beige;" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/2.2" fx:controller="application.InterfaceHandler">
      <children>            
        <Button fx:id="button1" layoutX="512.0" layoutY="381.0" minWidth="14.0" mnemonicParsing="false" onAction="#handleButton" prefHeight="30.0" prefWidth="51.0" text="Town" visible="true" />        

        <Label fx:id="nameLabel" layoutX="57.0" layoutY="8.0" prefWidth="216.0" text="blank" />
        <Label fx:id="locationLabel" layoutX="68.0" layoutY="27.0" prefWidth="193.0" text="blank" />
      </children>
    </AnchorPane>

推荐答案

这是因为你没有正确注入你的<$来自FXML文件的c $ c>标签。

It's because you didn't properly inject your Labels from FXML file.

注释标签 vars。使用 FXML 注释：

Annotate your Label vars. with FXML annotation:

public class InterfaceHandler implements Initializable {
    @FXML
    public Label nameLabel;
    @FXML
    public Label locationLabel;    

    public void handleButton(ActionEvent event) throws IOException {        
        locationLabel.setText("Town");
    }

    public void setName(String name){       
        nameLabel.setText(name);
    }    
}

此外，InterfaceHandler是您引用的控制器< FXML中code> fx：controller 。这指示FXMLLoader在加载器加载其FXML时创建InterfaceHandler的新实例。所以不要在你的Player类中创建一个新的InterfaceHandler，而是让InterfaceHandler成为Player的构造函数参数，并使用loader.getController从FXMLLoader中获取InterfaceHandler控制器实例。

Also, InterfaceHandler is a controller which you reference with fx:controller in your FXML. This instructs the FXMLLoader to create a new instance of the InterfaceHandler when the loader loads its FXML. So don't create a new InterfaceHandler in your Player class, instead make InterfaceHandler a constructor parameter for Player and use loader.getController to get the InterfaceHandler controller instance out of the FXMLLoader.

FXMLLoader loader = new FXMLLoader();
loader.setLocation(getClass().getResource("MainScreen.fxml"));
Parent root = (Parent)loader.load();
Player player = new Player(loader.getController());
Scene scene = new Scene(root);

. . .

public class Player {
  private InterfaceHandler ui;   

  public Player(InterfaceHandler ui) {
    this.ui = ui;
  }

  public void setNameLabel() {
    String name = "Dan";
    ui.setName(name);
  }
}

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