Question

# Java

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.stage.Stage;

public class Test extends Application {
  @Override // Override the start method in the Application class
  public void start(Stage primaryStage) {
    // Create a button and place it in the scene
    Button btOK = new Button("OK");
    btOK.setOnAction(e -> System.out.println("OK 1"));
    btOK.setOnAction(e -> System.out.println("OK 2"));

    Scene scene = new Scene(btOK, 200, 250);
    primaryStage.setTitle("MyJavaFX"); // Set the stage title
    primaryStage.setScene(scene); // Place the scene in the stage
    primaryStage.show(); // Display the stage
  }

  /**
   * The main method is only needed for the IDE with limited JavaFX
   * support. Not needed for running from the command line.
   */
  public static void main(String[] args) {
    launch(args);
  }
}

A. When clicking the button, the program displays OK1 OK2.

B. When clicking the button, the program displays OK1.

C. When clicking the button, the program displays OK2.

D. The program has a compile error, because the setOnAction method is invoked twice.

Ans is C

Can anyone explain for me why the answer is C ?? and what the last line: launch(args); means ??

Please be as detail as possible! Thanks!

Answer 1

lauch is a static method in Application class.. When we do launch, it sets up necessary parameters to show the UI and the calls start method, where you have written most of your code..

In the btOK.setOnAction, you are trying to set a action listener method, which will be called when a button is clicked.. So below lines tell:
btOK.setOnAction(e -> System.out.println("OK 1"));
When button is pressed, It will print "OK 1" on screen.

But when you again write btOK.setOnAction(e -> System.out.println("OK 2")); It means, you are creating a new action listener, which will overwrite the older button listener.. so now if you presee the button, you will see "OK 2" printed on the screen.

Hope it helps. Thanks!