Hello, world

In the long tradition of programming languages and libraries, this example is called Hello, World.

Hello World in Java

Create a new file with the following content named Example1.java in the folder src/main/java.

import org.gnome.gtk.*;
import org.gnome.gio.ApplicationFlags;

public class Example1 {

  private static void printHello() {
    System.out.println("Hello World");
  }

  private static void activate(Application app) {
    Window window = new ApplicationWindow(app);
    window.setTitle("Window");
    window.setDefaultSize(200, 200);

    Box box = new Box(Orientation.VERTICAL, 0);
    box.setHalign(Align.CENTER);
    box.setValign(Align.CENTER);

    window.setChild(box);

    Button button = Button.withLabel("Hello World");

    button.onClicked(Example1::printHello);
    button.onClicked(window::destroy);

    box.append(button);

    window.present();
  }

  public static void main(String[] args) {
    Application app = new Application("org.gtk.example", ApplicationFlags.DEFAULT_FLAGS);
    app.onActivate(() -> activate(app));
    app.run(args);
  }
}

Change the final part of the build.gradle file to run Example1.java, and run it with gradle run:

application {
    mainClass = "Example1"
}

As seen above, Example1.java builds further upon Example0.java by adding a button to our window, with the label "Hello World". The handling of the activate event has been moved into a separate method for clarity. Two new GTK widgets are used to accomplish this, button and box. The box variable is created to store a Gtk.Box, which is GTK's way of controlling the size and layout of buttons.

The GtkBox widget is created with Gtk.Grid.Grid(), which takes a Gtk.Orientation enumeration value as parameter. The buttons which this box will contain can either be laid out horizontally or vertically. This does not matter in this particular case, as we are dealing with only one button. After initializing box with the newly created GtkBox, the code adds the box widget to the window widget using Gtk.Window.setChild.

Next the button variable is initialized in similar manner. Button.withLabel is called which returns a Gtk.Button to be stored in button. Afterwards button is added to our box.

Using onClicked(), the button is connected to a method in our app called printHello(), so that when the button is clicked, GTK will call this method. printHello() calls System.out.println() with the string "Hello World" which will print Hello World in a terminal if the GTK application was started from one.

After connecting printHello(), another signal is connected to the "clicked" state of the button using onClicked() as well. In this case the method being called back is Gtk.Window.destroy for the window we created. This has the effect that when the button is clicked, the whole GTK window is destroyed.

More information about creating buttons can be found here.

The rest of the code in Example1.java is identical to Example0.java. The next section will elaborate further on how to add several Widgets to your GTK application.

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