Write a simple JavaFX application that opens a window. The window contains a button that's labeled "Hello, World!"
To create a simple JavaFX application with a window containing a "Hello, World!" button, follow these steps:

1. First, make sure you have the JavaFX library installed. You can download it from the official website: https://gluonhq.com/products/javafx/

2. Set up your JavaFX project in your favorite IDE (e.g., IntelliJ IDEA or Eclipse).

3. Create a new Java class, for example, HelloWorld.java, and replace its contents with the following code:
```java
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class HelloWorld extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {
        Button helloButton = new Button("Hello, World!");

        StackPane root = new StackPane();
        root.getChildren().add(helloButton);

        Scene scene = new Scene(root, 300, 200);

        primaryStage.setTitle("Hello World JavaFX");
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}
```

4. Save your changes and run the HelloWorld.java class. You should see a window with a "Hello, World!" button in the center.

This code creates a basic JavaFX application with a StackPane as the root container. It adds a Button with the label "Hello, World!" to the StackPane, sets the Scene size to 300x200 pixels, and sets the window title to "Hello World JavaFX." When you run the application, you'll see a window with the "Hello, World!" button.