java - Thread sleep inside of actionPerformed method

Question

First of all I want to say I'm aware this aproach is wrong so I'm asking this question because of pure curiousity. Lets say I have a swing application like this:

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class ThreadSleeping {
    JFrame frame = new JFrame();
    JPanel panel = new JPanel();
    JButton button = new JButton("Load");
    JLabel label = new JLabel();

    public ThreadSleeping() {
        panel.add(button);

        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                label.setIcon(new ImageIcon(
                        "C:/Users/Public/Pictures/Sample Pictures/Tulips.jpg"));
                System.out.println("Tulips painted");
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                label.setIcon(new ImageIcon(
                        "C:/Users/Public/Pictures/Sample Pictures/Koala.jpg"));
                System.out.println("Koala painted");

            }
        });

        frame.add(panel, BorderLayout.NORTH);
        frame.add(label, BorderLayout.CENTER);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.setSize(1024, 768);
        // frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new ThreadSleeping();
            }
        });
    }
}

Basically when I click a Load button I expect that Tulips.jpg image displays then GUI freezes for a 2 seconds and after that I expect that Koala.jpg image displays. But what happens is that: I click on button, GUI freezes for a 2 seconds and Koala.jpg displays. No Tulips.jpg before that. But thing that confuses me is when I put those System.out.println("Tulips painted"); and System.out.println("Koala painted");. So when I click on button it prints "Tulips painted" and after 2 seconds "Koala painted". Can someone tell me whats going on here? Regards.

Answer 1

1. works in this case, because you programatically freeze ouf Swing GUI, but there is/aren't another update(s), ot another JComponent(s)

2. doens't works in the case that there are a few another updated to the Swing GUI, Thread.sleep(int) freeze Event Dispatch Thread,

3. by default all updates to the JComponents XxxModels never will be visible on the JComponents view

4. example until sleep ended you'll lost all updated to the GUI