With a message pop-up, there's no return value.
import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class MessagePopUps {
public static void main(String[] a) {
JFrame parent = new JFrame();
JOptionPane.showMessageDialog(parent, "Printing complete");
// JOptionPane.showInternalMessageDialog(parent, "Printing complete");
}
}