JPopupMenu is a component in Java Swing that provides a popup menu that can be shown on a mouse click. The popup menu can be customized to contain any number of items such as menu items, check boxes, radio buttons, or custom components.

To create a JPopupMenu and add items to it, you can use the following code:

JPopupMenu popupMenu = new JPopupMenu();
JMenuItem menuItem = new JMenuItem("Menu Item");
popupMenu.add(menuItem);

To show the popup menu on a mouse click, you can use the show() method:

popupMenu.show(component, x, y);

Here, component is the component that should trigger the popup menu when clicked, and x and y are the coordinates where the popup menu should be displayed.

For example, the following code creates a JFrame with a button that shows a popup menu when clicked:

import javax.swing.*;
import java.awt.event.*;

public class PopupMenuExample extends JFrame {
    public PopupMenuExample() {
        JButton button = new JButton("Show Popup Menu");
        add(button);

        JPopupMenu popupMenu = new JPopupMenu();
        JMenuItem menuItem = new JMenuItem("Menu Item");
        popupMenu.add(menuItem);

        button.addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                if (e.isPopupTrigger())
                    popupMenu.show(e.getComponent(), e.getX(), e.getY());
            }

            public void mouseReleased(MouseEvent e) {
                if (e.isPopupTrigger())
                    popupMenu.show(e.getComponent(), e.getX(), e.getY());
            }
        });

        setTitle("Popup Menu Example");
        setSize(300, 200);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

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

In this example, a JButton is added to the JFrame, and a JPopupMenu with a single menu item is created. A MouseListener is added to the button to show the popup menu when the button is right-clicked or the context menu button is pressed.