JavaFX: Hello World Application

By extending the abstract class 'javafx.application.Application', you can create a JavaFX application. Your JavaFX application must override the start method of Application class.

Find the below example.

HelloWorld.java

package com.sample.demos;

import static javafx.geometry.Pos.CENTER;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.scene.text.Text;
import javafx.stage.Stage;

public class HelloWorld extends Application {
 @Override
 public void start(Stage primaryStage) {

  Button button = new Button();
  button.setText("Click Me");

  Text message = new Text();

  button.setOnAction(event -> {
   message.setText("Hello World!!!!");
  });

  VBox vBox = new VBox(10, button, message);
  vBox.setAlignment(CENTER);

  Scene scene = new Scene(vBox, 300, 300);

  /* Set the scene to primaryStage, and call the show method */
  primaryStage.setTitle("Hello JavaFX Example");
  primaryStage.setScene(scene);
  primaryStage.show();
 }

}

TestFX.java

package com.sample.demos;

import javafx.application.Application;

public class TestFX {
 public static void main(String args[]) {
  Application.launch(HelloWorld.class, args);
 }
}

When you ran above application, you can able to see below window.

Click on the button ‘Click Me’, you can able to see “Hello World!!!!” message.

How it works

Application class is the entry point for all the JavaFX applications. Main class of JavaFX application extends Application class and must provide implementation of 'start' method. 'start' method is the execution entry point for all the JavaFX applications.

When the application is launched, JavaFx application goes through below life cycle.

a. Initialize the instance of specific Application class.

b. Calls the init() method of the application.

c. Calls the start method

d. Wait until the application finishes.

e. Calls the stop method.

let’s analyze each class of HelloWorld application.

Stage and Scene classes

By using Stage and Scene classes we can define user interface container. Stage is the top-level JavaFX container, whereas Scene is the container for all content. Platform constructs the primary stage. Application can construct additional stages.

You can visualise above application like below.

Button button = new Button();

button.setText("Click Me");

Above statements construct a Button instance with text 'Click Me', it defines a node to display button widget.

Text message = new Text();

Above statement construct a Text object, it defines a node to display text.

button.setOnAction(event -> {

         message.setText("Hello World!!!!");

});

Whenever you click on the button, it set the value "Hello World!!!!" to message Text widget. I written this code using lambda expressions. If you would like to know the lambda expressions, I would recommend you to go through my post.

VBox vBox = new VBox(10, button, message);

vBox.setAlignment(CENTER);

VBox class lays out the children in single vertical column. Above statement creates an VBox layout with the horizantal spacing 10, between children (button and message)

Scene scene = new Scene(vBox, 300, 300);

It is the container for all content in a scene graph. By using scene, we can represent all the visual elements of the user interface. A single element in a scene graph is called node.

primaryStage.setTitle("Hello JavaFX Example");

primaryStage.setScene(scene);

primaryStage.show();

Above statements set the title of primaryStage, set the scene and display the stage.

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