javafx Getting started with javafx Hello World program

Example

The following code creates a simple user interface containing a single Button that prints a String to the console on click.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class HelloWorld extends Application {

    @Override
    public void start(Stage primaryStage) {
        // create a button with specified text
        Button button = new Button("Say 'Hello World'");

        // set a handler that is executed when the user activates the button
        // e.g. by clicking it or pressing enter while it's focused
        button.setOnAction(e -> {
           //Open information dialog that says hello
           Alert alert = new Alert(AlertType.INFORMATION, "Hello World!?");
           alert.showAndWait();
        });

        // the root of the scene shown in the main window
        StackPane root = new StackPane();

        // add button as child of the root
        root.getChildren().add(button);

        // create a scene specifying the root and the size
        Scene scene = new Scene(root, 500, 300);

        // add scene to the stage
        primaryStage.setScene(scene);

        // make the stage visible
        primaryStage.show();
    }

    public static void main(String[] args) {
        // launch the HelloWorld application.

        // Since this method is a member of the HelloWorld class the first
        // parameter is not required
        Application.launch(HelloWorld.class, args);
    }

}

The Application class is the entry point of every JavaFX application. Only one Application can be launched and this is done using

Application.launch(HelloWorld.class, args);

This creates a instance of the Application class passed as parameter and starts up the JavaFX platform.

The following is important for the programmer here:

1. First launch creates a new instance of the Application class (HelloWorld in this case). The Application class therefore needs a no-arg constructor.
2. init() is called on the Application instance created. In this case the default implementation from Application does nothing.
3. start is called for the Appication instance and the primary Stage (= window) is passed to the method. This method is automatically called on the JavaFX Application thread (Platform thread).
4. The application runs until the platform determines it's time to shut down. This is done when the last window is closed in this case.
5. The stop method is invoked on the Application instance. In this case the implementation from Application does nothing. This method is automatically called on the JavaFX Application thread (Platform thread).

In the start method the scene graph is constructed. In this case it contains 2 Nodes: A Button and a StackPane.

The Button represents a button in the UI and the StackPane is a container for the Button that determines it's placement.

A Scene is created to display these Nodes. Finally the Scene is added to the Stage which is the window that shows the whole UI.