JDialog is a class in Java Swing that provides a pop-up window that can be used to get user input or to display some information. It is similar to a JFrame, but it is typically used for more specialized purposes, such as displaying a modal dialog box.

Here's an example of how to create and display a simple JDialog in Java:

import javax.swing.*;

public class MyDialog extends JDialog {
    public MyDialog(JFrame parent, String title, String message) {
        super(parent, title, true); // create a modal dialog
        JPanel panel = new JPanel();
        JLabel label = new JLabel(message);
        panel.add(label);
        getContentPane().add(panel);
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        pack();
        setLocationRelativeTo(null);
        setVisible(true);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setSize(300, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        new MyDialog(frame, "My Dialog", "Hello, world!");
    }
}

In this example, we create a new class MyDialog that extends JDialog. In the constructor, we create a simple JPanel with a JLabel that displays a message. We add the panel to the dialog's content pane, set the default close operation to DISPOSE_ON_CLOSE, and pack and center the dialog. Finally, we make the dialog visible.

In the main method, we create a new JFrame and make it visible. Then we create a new instance of MyDialog, passing the frame as the parent, a title, and a message. The dialog is displayed modally, which means that it will block input to the parent frame until it is closed.