Почему выбрасывает Exception при вызове метода из другого пакета?

Question

[thatmaniscool]

Пишу в eclipse.
Имею простой основной класс:

public class Main extends Application {
	private Group root;
	private Button bt1;
	protected Label label;
	private Scene scene;
	private Server server;

	
	@Override
	public void start(Stage primaryStage) {
		server = new Server ();
		bt1 = new Button ("Run Server");
		label = new Label ("This is my label");
		label.setLayoutX(100);
		
		bt1.setOnAction((event) -> server.ChangeLabel());
		
		
		root = new Group (bt1, label);
		scene = new Scene (root, 400, 300);
		primaryStage.setScene(scene);
		primaryStage.show();
		
		/*try {
			BorderPane root = new BorderPane();
			Scene scene = new Scene(root,400,400);
			scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
			primaryStage.setScene(scene);
			primaryStage.show();
		} catch(Exception e) {
			e.printStackTrace();
		} */
	}
	
	public static void main(String[] args) {
		launch(args);
	}
}

И простой класс из другого пакета (пакет Server) c одним методом, который просто меняет текст метки.

public class Server extends Main {
	public void ChangeLabel () {
		super.label.setText("NEW LABEL");
	}
}

При нажатии на кнопку Run Server, выбрасывает Exceptions:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at Server.Server.ChangeLabel(Server.java:6)
at application.Main.lambda$0(Main.java:26)
at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
at javafx.base/javafx.event.Event.fireEvent(Unknown Source)
at javafx.graphics/javafx.scene.Node.fireEvent(Unknown Source)
at javafx.controls/javafx.scene.control.Button.fire(Unknown Source)
at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(Unknown Source)
at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(Unknown Source)
at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
at javafx.base/javafx.event.Event.fireEvent(Unknown Source)
at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.graphics/javafx.scene.Scene$MouseHandler.access$1300(Unknown Source)
at javafx.graphics/javafx.scene.Scene.processMouseEvent(Unknown Source)
at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(Unknown Source)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(Unknown Source)
at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(Unknown Source)
at javafx.graphics/com.sun.glass.ui.View.notifyMouse(Unknown Source)
at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)

Comments

[Сергей Горностаев]

В объекте server ничто не устанавливало поле label, поэтому оно содержит null, а у null нет метода setText()

[thatmaniscool]

Сергей Горностаев,
Спасибо, и что нужно сделать, чтобы исправить эту ошибку?

[Сергей Горностаев]

thatmaniscool, у меня к вам два встречных вопроса:

• По какому учебнику вы учили Java?
  
• Какой смысл кому-либо тратить время и силы на помощь вам, если ответы вы всё равно не принимаете?
  

[thatmaniscool]

Сергей Горностаев,
Thinking in Java, JavaDoc Orange, Tutorials.

[Денис Загаевский]

thatmaniscool, врешь

Answer 1

[alexover]

Другой пакет совершенно не при чем. В стектрейсе явно видно NPE at Server.Server.ChangeLabel(Server.java:6)
после создания new Server (); server.label == null, а вы пытаетесь вызвать метод super.label.setText("NEW LABEL");