添加到列表时JavaFX ListView为空指针[duplicate]

Question

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


public class MainController extends Application
{
    @Override
    public void start(Stage primaryStage) throws Exception
    {
        Parent root = FXMLLoader.load(getClass().getResource("IntelligentSystems.fxml"));
        primaryStage.setTitle("Intelligent Systems Agent Program");
        primaryStage.setScene(new Scene (root));
        primaryStage.show();

        GUIController GUIList = new GUIController();
        GUIList.DoList.add("agentCtrl");
        GUIList.DoList.add("DeliveryAgent");
        GUIList.PopulateAgentList();
    }

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

第二类是GuiconTroller：

import javafx.fxml.FXML;
import javafx.scene.control.ListView;
import javafx.collections.ObservableList;
import javafx.collections.FXCollections;

public class GUIController {
    //List View variables.
    @FXML
    public ListView AgentsList;

    @FXML
    public ObservableList<String> DoList= FXCollections.observableArrayList();

    @FXML
    public void PopulateAgentList()
    {
        AgentsList.setItems(DoList);
    }
}

目前，我在fxml文件中将Listview的fxId设置为“AgentsList”。

<ListView fx:id="AgentsList" layoutX="15.0" layoutY="39.0" prefHeight="200.0" prefWidth="86.0" />

每当我在IntelliJ中运行该程序时，它总是围绕“agentslist.setitems(_dolist)；”返回一个java.lang.NullPointerException行。

java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:830)
Caused by: java.lang.NullPointerException
    at GUIController.PopulateAgentList(GUIController.java:26)
    at MainController.start(MainController.java:57)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    ... 1 more
Exception running application MainController

Process finished with exit code 1

Answer 1

@fxml注释字段是静态的，JavaFX不支持这一点，因为它的目的是创建/加载同一个fxml文件/控制器类的多个实例。如果您不熟悉‘静态’的实际含义，可以参考静态方法和非静态方法之间的区别是什么？。

至于如何获得正确的控制器实例，您需要加载稍微不同的fxml文件。

FXMLLoader loader = new FXMLLoader(getClass().getResource("IntelligentSystems.fxml"));
Parent root = loader.load(); // must be called before getting the controller!
GUIController controller = loader.getController();

现在可以使用controller变量以非静态方式访问字段。