Java label.setText NullPointerException
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/22546028/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
label.setText NullPointerException
提问by twin_suns
Hi first time here but here goes:
嗨，第一次来这里，但这里是：
I have a JavaFX application that changes the FXML UI labels dynamically and the data is pulled from a Player class.
我有一个 JavaFX 应用程序，它动态更改 FXML UI 标签，并且数据是从 Player 类中提取的。
The two classes in question are Player.javaand InterfaceHandler.java.
有问题的两个类是Player.java和InterfaceHandler.java。
The player class stores player details and I want to pass the details to the Interface class which sets the text on the labels.
播放器类存储播放器详细信息，我想将详细信息传递给在标签上设置文本的接口类。
As a test my FXML UI just has a button and two labels.
作为测试，我的 FXML UI 只有一个按钮和两个标签。
If it click the button it calls the handleButtonmethod it sets locationLabelto "Town" fine.
如果它单击按钮，它会调用handleButton它设置locationLabel为“城镇”的方法。
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".
但是，如果我locationLabel()在 Player 类中调用该方法，则在调用时nameLabel.setText(name)会收到 NullPointerException 。通过调试发现Interface类中的name字符串应该是“Dan”。
Can anyone help?
任何人都可以帮忙吗？
Main class:
主要类：
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);
}
}
Player class:
球员等级：
public class Player{
InterfaceHandler ui = new InterfaceHandler();
public void setNameLabel() {
String name = "Dan";
ui.setName(name);
}
InterfaceHandler class:
接口处理器类：
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:
主屏幕.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>
采纳答案by Branislav Lazic
It's because you didn't properly inject your Labelsfrom FXML file.
这是因为您没有正确地Labels从 FXML 文件中注入您的文件。
Annotate your Labelvars. with FXMLannotation:
注释你的Label变量。与FXML注释：
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);
}
}
Also, InterfaceHandler is a controller which you reference with fx:controllerin 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.
此外，InterfaceHandler 是您fx:controller在 FXML 中引用的控制器。这将指示 FXMLLoader 在加载器加载其 FXML 时创建 InterfaceHandler 的新实例。因此，不要在您的 Player 类中创建新的 InterfaceHandler，而是将 InterfaceHandler 设为 Player 的构造函数参数，并使用 loader.getController 从 FXMLLoader 中获取 InterfaceHandler 控制器实例。
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);
}
}
回答by Yash Kalia
I had a similar problem but turns out that I had made the label as a static attribute and when I removed the static part it fixed the nullpointer exeption.
我有一个类似的问题，但事实证明我已将标签作为静态属性，当我删除静态部分时，它修复了空指针异常。
