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- What is a NullPointerException, and how do I fix it? 12 answers
什么是NullPointerException,我该如何解决? 12个答案
I've got a problem with a NullPointerException.
我遇到了NullPointerException的问题。
This is my Controller:
这是我的控制器:
public class Controller implements Initializable{
public Label lbTitle;
public Label lbAuthor;
public Label lbChapters;
public TextField tfTitle;
public TextField tfAuthor;
public TextArea taChapters;
public ListView<Book> listView;
private void OpenNewWindow(Book book1) throws Exception {
String selected = listView.getSelectionModel().getSelectedItem().toString();
FXMLLoader fxmlLoader = new FXMLLoader();
fxmlLoader.setLocation(getClass().getResource("/zli/BooksFX/NewWindow.fxml"));
Scene scene = new Scene(fxmlLoader.load(), 400, 400);
Stage stage = new Stage();
tfTitle.setText(book1.getTitle());
tfAuthor.setText(book1.getAuthor());
taChapters.setText(book1.toStringChapters());
stage.setTitle(selected);
stage.setScene(scene);
stage.show();
}
@Override
public void initialize(URL location, ResourceBundle resources) {
Chapter chapter1Book1 = new Chapter(1, 100, "First Chapter...");
Chapter chapter2Book1 = new Chapter(2, 400, "Second Chapter...");
ArrayList<Chapter> book1Chapters = new ArrayList<>();
book1Chapters.add(chapter1Book1);
book1Chapters.add(chapter2Book1);
Book book1 = new Book("IT: Not the one with the clown", "Stephen ITBerg", book1Chapters);
Chapter chapter1Book2 = new Chapter(1, 100, "First Chapter...");
Chapter chapter2Book2 = new Chapter(2, 400, "Second Chapter...");
ArrayList<Chapter> book2Chapters = new ArrayList<>();
book1Chapters.add(chapter1Book2);
book1Chapters.add(chapter2Book2);
Book book2 = new Book("Z Men", "Stan Lee", book2Chapters);
Chapter chapter1Book3 = new Chapter(1, 100, "First Chapter...");
Chapter chapter2Book3 = new Chapter(2, 400, "Second Chapter...");
ArrayList<Chapter> book3Chapters = new ArrayList<>();
book1Chapters.add(chapter1Book3);
book1Chapters.add(chapter2Book3);
Book book3 = new Book("Captain AlivePool", "Undead Zombie", book3Chapters);
ObservableList<Book> items = FXCollections.observableArrayList (book1, book2, book3);
listView.setItems(items);
listView.setOnMouseClicked(e -> {
try {
OpenNewWindow(book1);
} catch (Exception e1) {
e1.printStackTrace();
}
});
}
}
And this is my .fxml:
这是我的.fxml:
<Pane prefHeight="400.0" prefWidth="400.0" style="-fx-background-color: #1ba77f;" xmlns="http://javafx.com/javafx/8.0.121" xmlns:fx="http://javafx.com/fxml/1" fx:controller="zli.BooksFX.Controller">
<ListView fx:id="listView" disable="true" opacity="0.0" prefHeight="400.0" prefWidth="400.0" />
<Label fx:id="lbTitle" layoutX="8.0" layoutY="20.0" prefHeight="27.0" prefWidth="57.0" text="Title: " />
<Label fx:id="lbAuthor" layoutX="8.0" layoutY="90.0" text="Author: " />
<TextField text="ert" fx:id="tfTitle" editable="false" layoutX="72.0" layoutY="14.0" prefHeight="39.0" prefWidth="316.0" />
<TextField text="sdf" fx:id="tfAuthor" editable="false" layoutX="72.0" layoutY="84.0" prefHeight="39.0" prefWidth="316.0" />
<Label fx:id="lbChapters" layoutX="8.0" layoutY="160.0" text="Chapters: " />
<TextArea text="sdfs" fx:id="taChapters" editable="false" layoutX="7.0" layoutY="187.0" prefHeight="200.0" prefWidth="383.0" />
</Pane>
And this is the Error message:
这是错误消息:
java.lang.NullPointerException
at zli.BooksFX.Controller.OpenNewWindow(Controller.java:37)
at zli.BooksFX.Controller.lambda$initialize$0(Controller.java:73)
at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
at javafx.graphics/javafx.scene.Scene$ClickGenerator.postProcess(Scene.java:3589)
at javafx.graphics/javafx.scene.Scene$ClickGenerator.access$8300(Scene.java:3517)
at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:3885)
at javafx.graphics/javafx.scene.Scene$MouseHandler.access$1300(Scene.java:3604)
at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1874)
at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2613)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:397)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:434)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:433)
at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)
at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:175)
at java.base/java.lang.Thread.run(Thread.java:844)
Now, when I want to set the Text of my TextFields and TextAreas (tfTitle, tfAuthor, taChapters) to the assigned value, it gives me a "java.lang.NullPointerException" on the Line 37 in Controller, which is this:
现在,当我想将TextFields和TextAreas的文本(tfTitle,tfAuthor,taChapters)设置为指定的值时,它在Controller的第37行上给出了一个“java.lang.NullPointerException”,它是这样的:
tfTitle.setText(book1.getTitle());
And for your information: "Book" is a storage Class for Chapters.
并且为了您的信息:“Book”是章节的存储类。
Because it was asked for, my other .fxml:
因为它被要求,我的另一个.fxml:
<GridPane xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/2.2"
fx:controller="zli.BooksFX.Controller">
<ListView fx:id="listView" prefWidth="400" />
</GridPane>
2 个解决方案
#1
0
You use the class Controller as the controller of two different fxml-files.
您使用类Controller作为两个不同fxml文件的控制器。
The first only declares listView but not tfTitle, tfAuthor and taChapters therefore you get a NullPointerException at tfTitle.setText(book1.getTitle());
第一个只声明listView而不是tfTitle,tfAuthor和taChapters因此你在tfTitle.setText(book1.getTitle())得到一个NullPointerException;
When you load the second fxml-file in OpenNewWindow you also creates a new Controller object.
在OpenNewWindow中加载第二个fxml文件时,还会创建一个新的Controller对象。
Now you can reuse the first controller with fxmlLoader.setController(this); and remove fx:controller="zli.BooksFX.Controller" from NewWindow.fxml (Don't do this since you will run into similar problems like this).
现在你可以使用fxmlLoader.setController(this)重用第一个控制器;并从NewWindow.fxml中删除fx:controller =“zli.BooksFX.Controller”(不要这样做,因为你会遇到类似这样的问题)。
Or create two independent controller for each fxml-file and pass the book between the controllers.
或者为每个fxml文件创建两个独立的控制器,并在控制器之间传递该书。
For the second controller you need something like:
对于第二个控制器,您需要以下内容:
public void setBook(Book book) {
tfTitle.setText(book.getTitle());
tfAuthor.setText(book.getAuthor());
taChapters.setText(book.toStringChapters());
}
And pass the book with ((BookDetailController)fxmlLoader.getController()).setBook(book1); after fxmlLoader.load()
并用((BookDetailController)fxmlLoader.getController())传递这本书.setBook(book1);在fxmlLoader.load()之后
#2
1
May be you have not initialized your Texfield. use @FXML Annotation over tfTitle declaration
可能是你没有初始化你的Texfield。在tfTitle声明上使用@FXML Annotation
@FXML
public TextField tfTitle;
Same goes for other fields too.
其他领域也是如此。
#1
0
You use the class Controller as the controller of two different fxml-files.
您使用类Controller作为两个不同fxml文件的控制器。
The first only declares listView but not tfTitle, tfAuthor and taChapters therefore you get a NullPointerException at tfTitle.setText(book1.getTitle());
第一个只声明listView而不是tfTitle,tfAuthor和taChapters因此你在tfTitle.setText(book1.getTitle())得到一个NullPointerException;
When you load the second fxml-file in OpenNewWindow you also creates a new Controller object.
在OpenNewWindow中加载第二个fxml文件时,还会创建一个新的Controller对象。
Now you can reuse the first controller with fxmlLoader.setController(this); and remove fx:controller="zli.BooksFX.Controller" from NewWindow.fxml (Don't do this since you will run into similar problems like this).
现在你可以使用fxmlLoader.setController(this)重用第一个控制器;并从NewWindow.fxml中删除fx:controller =“zli.BooksFX.Controller”(不要这样做,因为你会遇到类似这样的问题)。
Or create two independent controller for each fxml-file and pass the book between the controllers.
或者为每个fxml文件创建两个独立的控制器,并在控制器之间传递该书。
For the second controller you need something like:
对于第二个控制器,您需要以下内容:
public void setBook(Book book) {
tfTitle.setText(book.getTitle());
tfAuthor.setText(book.getAuthor());
taChapters.setText(book.toStringChapters());
}
And pass the book with ((BookDetailController)fxmlLoader.getController()).setBook(book1); after fxmlLoader.load()
并用((BookDetailController)fxmlLoader.getController())传递这本书.setBook(book1);在fxmlLoader.load()之后
#2
1
May be you have not initialized your Texfield. use @FXML Annotation over tfTitle declaration
可能是你没有初始化你的Texfield。在tfTitle声明上使用@FXML Annotation
@FXML
public TextField tfTitle;
Same goes for other fields too.
其他领域也是如此。