maincontroller
package treetest;
import java.io.IOException;
import java.net.URL;
import java.util.ArrayList;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.control.Button;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.input.MouseEvent;
import javafx.scene.paint.Color;
import javafx.stage.Stage;
/**
*
* @author akhil kumar
*/
public class maincontroller {
@FXML
private GraphicsContext gc;
private Canvas canvas;
private TreeView<String> bro;
TreeItem<String> root=new TreeItem<>("root");
TreeItem<String> child = new TreeItem<>("child");
TreeItem<String> child2 = new TreeItem<>("child");
TreeItem<String> child3 = new TreeItem<>("child");
TreeItem<String> child4 = new TreeItem<>("child");
TreeItem<String> child5 = new TreeItem<>("child");
public void initialize(){
root.getChildren().add(child);
root.getChildren().add(child2);
root.getChildren().add(child3);
root.getChildren().add(child4);
root.getChildren().add(child5);
bro.setRoot(root);
bro.setShowRoot(false);
gc=canvas.getGraphicsContext2D();
gc.setFill(Color.WHITE);
gc.fillRect(0,0,canvas.getWidth(),canvas.getHeight());
}
}
FXML
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.canvas.*?>
<?import javafx.geometry.*?>
<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="600.0" minWidth="800.0" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="treetest.maincontroller">
<top>
<VBox BorderPane.alignment="CENTER">
<children>
<MenuBar>
<menus>
<Menu mnemonicParsing="false" text="File">
<items>
<MenuItem mnemonicParsing="false" text="Close" />
</items>
</Menu>
<Menu mnemonicParsing="false" text="Edit">
<items>
<MenuItem mnemonicParsing="false" text="Delete" />
</items>
</Menu>
<Menu mnemonicParsing="false" text="Help">
<items>
<MenuItem mnemonicParsing="false" text="About" />
</items>
</Menu>
</menus>
</MenuBar>
<HBox>
<children>
<Button fx:id="catchment" mnemonicParsing="false" text="Button" />
<Button fx:id="output" mnemonicParsing="false" text="Button" />
<Button fx:id="junction" mnemonicParsing="false" text="Button" />
<Button fx:id="run" mnemonicParsing="false" text="Button" />
</children>
<padding>
<Insets bottom="10.0" left="10.0" top="10.0" />
</padding>
</HBox>
</children>
</VBox>
</top>
<center>
<Canvas fx:id="canvas" height="533.0" width="592.0" BorderPane.alignment="CENTER">
<cursor>
<Cursor fx:constant="CROSSHAIR" />
</cursor>
</Canvas>
</center>
<left>
<TreeView fx:id="bro" prefHeight="200.0" prefWidth="200.0" BorderPane.alignment="CENTER" />
</left>
</BorderPane>
即使canvas和treeview的fx：id与emain控制器中的id匹配，我也会得到一个空指针异常
在
行bro.setRoot(root)
bro.setShowRoot(false);
gc=canvas.getGraphicsContext2D();
basically in any line itry to use the canvas and treeview i am getting a nullpointer exception
error log
file:/C:/Users/akhil%20kumar/Documents/NetBeansProjects/treetest/dist/run720357897/treetest.jar!/treetest/tree.fxml
at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2601)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2571)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2441)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3214)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3175)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3148)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3124)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3104)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:3097)
at treetest.Treetest.start(Treetest.java:30)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2566)
... 17 more
Caused by: java.lang.NullPointerException
at treetest.maincontroller.initialize(maincontroller.java:52)
... 27 more
我的代码出了什么问题？
第52行
bro.setRoot(root);
奇怪的部分是当我使用graphiccontext gc和初始化函数中的画布删除所有行时，它完全正常工作。
这也特别说明为什么控制器无法在fxml文件中找到对象，因此nullpointer异常的一般答案可能无法帮助我
无法理解为什么会这样？