Java TextArea setText() 和 appendText() 返回空指针异常
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Java TextArea setText() and appendText() returning Null Pointer Exception
提问by senex_subconscious
I am using JavaFX 8 and FXML in this project and trying to update my text area with the results from other classes within the program.
我在这个项目中使用 JavaFX 8 和 FXML，并尝试使用程序中其他类的结果更新我的文本区域。
The Text Area is defined in the FXML document as such:
文本区域在 FXML 文档中定义如下：
<TextArea fx:id="feedBack" editable="false" focusTraversable="false"
layoutX="203.0" layoutY="32.0" prefHeight="205.0" prefWidth="308.0"
wrapText="true"/>
It is called in the controller here, note that originally it was a "private" item but to make it work with the classes I made it public:
它在这里的控制器中被调用，请注意，最初它是一个“私有”项目，但为了使其与我公开的类一起使用：
@FXML
public static TextArea feedBack;
EDIT: It is worth noting that when the TextArea is identified as "private" I have no issue getting the set/append text method to work but this does not allow me to use the text area in other classes, which is what I need to do.
编辑：值得注意的是，当 TextArea 被标识为“私有”时，我可以让 set/append 文本方法正常工作，但这不允许我在其他类中使用文本区域，这是我需要的做。
However now when I try to do either appendText() or setText(), such as follows:
但是现在当我尝试执行 appendText() 或 setText() 时，如下所示：
feedBack.setText("Connection Made");
I am given a null point exception. Why is this? I have made this work in JavaFX 7 with FXML by doing the same thing and it works fine. What am I missing to make this work?
我得到了一个空点异常。为什么是这样？我通过做同样的事情在带有 FXML 的 JavaFX 7 中完成了这项工作，并且工作正常。我缺少什么才能完成这项工作？
EDIT, test/proof of brokenness with complete tiny program. I did not make another class for this one as the textarea won't work in the control anyway.
编辑，用完整的小程序测试/证明损坏。我没有为此创建另一个类，因为无论如何 textarea 都不会在控件中工作。
FXML:
文件格式：
<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<AnchorPane id="AnchorPane" prefHeight="200" prefWidth="320" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="textareatester.FXMLDocumentController">
<children>
<Label fx:id="label" layoutX="126" layoutY="120" minHeight="16" minWidth="69" />
<TextArea fx:id="feedback" layoutX="61.0" prefHeight="200.0" prefWidth="200.0" wrapText="true" />
<Button fx:id="dostuff" layoutX="261.0" layoutY="62.0" mnemonicParsing="false" onAction="#handleButtonAction" text="Button" />
</children>
</AnchorPane>
Main Method:
主要方法：
package textareatester;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
public class TextAreaTester extends Application {
@Override
public void start(Stage stage) throws Exception {
Parent root = FXMLLoader.load(getClass().getResource("FXMLDocument.fxml"));
Scene scene = new Scene(root);
stage.setScene(scene);
stage.show();
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
launch(args);
}
}
Controller Class:
控制器类：
package textareatester;
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.TextArea;
public class FXMLDocumentController implements Initializable {
@FXML
private Label label;
@FXML
public static TextArea feedback;
@FXML
private Button dostuff;
@FXML
private void handleButtonAction(ActionEvent event) {
System.out.println("You clicked me!");
feedback.setText("Hello World!"); ///<-- this is what gives me the NPE
}
@Override
public void initialize(URL url, ResourceBundle rb) {
// not touching anything
}
}
Solution for those who are curious:I ended up simply returning my results strings from my methods and then appending them to the TextArea.
对于那些好奇的人的解决方案：我最终只是从我的方法中返回我的结果字符串，然后将它们附加到 TextArea。
采纳答案by Turing85
The problem is the staticat the definition of the TextArea. Since staticattributes are bound to the class and not the object, feedback gets never initiated and is nullwhen accessed in the EventHandler.
问题是static在的定义TextArea。由于static属性绑定到类而不是对象，因此反馈永远不会启动，而是null在 EventHandler 中访问时。
