Double click also triggering single click in java.awt.event.MouseAdapter

Question

Consider a javax.swing.JPanel with the following as its MouseListener

new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                super.mouseClicked(e);
                var count = e.getClickCount();
                switch (count) {
                    case 1 -> Helper.sop("single count");
                    case 2 -> Helper.sop("double  count");
                    default -> Helper.sop("more than double count %d".formatted(count));
                }
            }
        }

Upon double-clicking , I am getting the following puzzling output

single count
double  count

instead of the expected

double  count

Apparently, during a double click, a single click precedes the said double click. Why is this so ?

---

rough.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.UUID;

public class rough {


    public static void main(String[] args) throws Exception {


        //Helper.sop=System.out.println
        //Helper.launch=launch a JPanel in a JFrame (new,pack,EXIT_ON_CLOSE,<Screen_Size,UUIDTitle,SystemLAF,EDTThread)

        JPanel pn = new JPanel();
        pn.setPreferredSize(new Dimension(500, 500));

        ///////////////////THIS IS ALL THAT MATTERS HERE//////////////////////
        pn.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                super.mouseClicked(e);
                var count = e.getClickCount();

                switch (count) {
                    case 1 -> Helper.sop("single count");
                    case 2 -> Helper.sop("double  count");
                    default -> Helper.sop("more than double count %d".formatted(count));
                }

            }
        });
        /////////////////////////////////////////////////////////////////////


        Helper.launch(pn);


    }


    private static class Helper {

        static public int frameNo = 0;

        public static void sop(Object o) {

            System.out.println(o);
        }

        public static void launch(JPanel pnMain, String frameTitle, String lafClassName) {

            try {
                UIManager.setLookAndFeel(lafClassName);
                f.sop("set laf to " + lafClassName);
            } catch (Exception e) {
                System.err.println("err: couldn't apply laf");
            }

            final var screenSize = Toolkit.getDefaultToolkit().getScreenSize();
            final var pnSize = pnMain.getPreferredSize();


            SwingUtilities.invokeLater(new Runnable() {
                @Override
                public void run() {

                    JFrame frame = new JFrame(frameTitle);

                    if (pnSize.width > screenSize.width || pnSize.getHeight() > screenSize.getHeight())
                        frame.setPreferredSize(new Dimension((int) (screenSize.width * 0.75), (int) (0.75 * screenSize.height)));


                    frame.setContentPane(pnMain);
                    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                    frame.pack();
                    frame.setVisible(true);

                }
            });

        }

        public static void launch(JPanel pnMain, String frameTitle) {

            launch(pnMain, frameTitle, UIManager.getSystemLookAndFeelClassName());

        }


        public static void launch(JPanel pnMain) {

            String title = "Frame no: " + ++frameNo + "\t (" + UUID.randomUUID() + ")";
            launch(pnMain, title);

        }


    }

}

Env:
openjdk 15.0.2 2021-01-19
OpenJDK Runtime Environment (build 15.0.2+7-27)
OpenJDK 64-Bit Server VM (build 15.0.2+7-27, mixed mode, sharing)
Windows 10

Answer 1

Apparently, during a double click, a single click precedes the said double click

This is normal and expected behaviour.

The OS defines the "click interval". If you click multiple times within the interval the click count will increase.

If you click slowly you will get multiple single clicks.

If you click fast, within the "click interval", you will get a single click and a double click.

If you really click fast you could get a single, double and triple click as determined by the click count.

Typically you use the:

1. single click to "select" something and
2. the double click to "process" something.

Think a list of files in Windows explorer. A single click will select the item. A double click will do something with the selected item. If it is a text file it will open it in the default editor. If it is a.mp3 file it will play the song in the default music player.