Why throws Exception when calling a method from another package?

Question

Java

I am writing in eclipse.
I have a simple main class:

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);
  }
}

And a simple class from another package (Server package) with one method that simply changes the text of the label.

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

When you click on the Run Server button, it throws 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)

Answer 1

The other package is completely useless. The stacktrace clearly shows the NPE at Server.Server.ChangeLabel(Server.java:6)
after creating new Server(); server.label == null and you are trying to call super.label.setText("NEW LABEL");