组件仅在悬停时绘制

Question

Can someone explain why my components only get drawn when I hover over where they are supposed to be?

I setup a borderless frame than can be dragged anywhere and I'm trying to create an exit button at the top right but it doesn't get drawn until I hover over it. I paint to the JFrame a background image, then draw my button and set the whole thing visible.

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

public class GUI extends JFrame
{
    private Image Background = null;
    private static Point Offset = new Point();

    public GUI() {
        this.setUndecorated(true);
        this.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        AddListeners();
        SetCustomTheme();
        LoadBackground();
        Layout();
        pack();
        this.setSize(300, 300);
        this.setVisible(true);
    }

    private void Layout() {
        GroupLayout Info = new GroupLayout(this.getContentPane());
        this.getContentPane().setLayout(Info);
        JButton Button = new JButton();

        Info.setHorizontalGroup(
            Info.createSequentialGroup()
               .addComponent(Button)
         );

        Info.setVerticalGroup(
            Info.createParallelGroup()
                .addComponent(Button)
        );
    }

    private void SetCustomTheme() {
        try {
            UIManager.setLookAndFeel("javax.swing.plaf.nimbus.NimbusLookAndFeel");
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
        }
    }

    private void LoadBackground() {
        try {
            Background = ImageIO.read(getClass().getResource("Images/meh.png"));
        } catch (Exception Ex) {

        }
    }

    private void SetCustomIcon() {
        Image Icon = Toolkit.getDefaultToolkit().getImage("Images/lol.jpg");
        setIconImage(Icon);
    }

    private void AddListeners() {
        this.addMouseListener(new MouseAdapter() {
            @Override public void mousePressed(MouseEvent e) {
              Offset.x = e.getX();
              Offset.y = e.getY();
            }
          });

        this.addMouseMotionListener(new MouseMotionAdapter() {
            @Override public void mouseDragged(MouseEvent e) {
              Point p = getLocation();
              setLocation(p.x + e.getX() - Offset.x, p.y + e.getY() - Offset.y);
            }
          });
    }

    @Override public void paint(Graphics g) {
        g.drawImage(Background, 0,0,this.getWidth(),this.getHeight(), null);
    }
}

Answer 1

1. All interactions with the UI must be executed from within the Event Dispatching Thread
2. You should avoid extending from top level containers, like JFrame , instead, use JPanel instead.
3. Failing to honor the paint chain contract is preventing any child components from begin painted
4. The preferred method to override for performing custom painting is paintComponent

You might like to have a read through

• Painting in AWT and Swing
• The Event Dispatch Thread
• Performing Custom Painting

Try something like this instead;

public class BadPaint01 {

    public static void main(String[] args) {
        new BadPaint01();
    }

    public BadPaint01() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame();
                Image Icon = Toolkit.getDefaultToolkit().getImage("Images/lol.jpg");
                frame.setIconImage(Icon);
                frame.setUndecorated(true);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new GUI());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static class GUI extends JPanel {

        private Image Background = null;
        private static Point Offset = new Point();

        public GUI() {
            AddListeners();
            SetCustomTheme();
            LoadBackground();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(300, 300);
        }

        private void Layout() {
            GroupLayout Info = new GroupLayout(this);
            setLayout(Info);
            JButton Button = new JButton();

            Info.setHorizontalGroup(
                    Info.createSequentialGroup()
                    .addComponent(Button));

            Info.setVerticalGroup(
                    Info.createParallelGroup()
                    .addComponent(Button));
        }

        private void SetCustomTheme() {
            try {
                UIManager.setLookAndFeel("javax.swing.plaf.nimbus.NimbusLookAndFeel");
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
            }
        }

        private void LoadBackground() {
            try {
                Background = ImageIO.read(getClass().getResource("Images/meh.png"));
            } catch (Exception Ex) {
            }
        }

        private void AddListeners() {
            this.addMouseListener(new MouseAdapter() {
                @Override
                public void mousePressed(MouseEvent e) {
                    Offset.x = e.getX();
                    Offset.y = e.getY();
                }
            });

            this.addMouseMotionListener(new MouseMotionAdapter() {
                @Override
                public void mouseDragged(MouseEvent e) {
                    Point p = getLocation();
                    setLocation(p.x + e.getX() - Offset.x, p.y + e.getY() - Offset.y);
                }
            });
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g); //To change body of generated methods, choose Tools | Templates.
            g.drawImage(Background, 0, 0, this.getWidth(), this.getHeight(), null);
        }
    }
}

You also might like to have read through Code Conventions for the Java Programming Language , you're not going to make any friends by ignoring them ;)

Answer 2

If I remember well, ToolKit.getImage returns an Image which may not be completely loaded. When you hover over it, it probably has been loaded in the background meanwhile. Instead do this (similar to your line for Background):

ImageIcon Icon = new ImageIcon(ImageIO.read(getClass().getResource("Images/lol.png")));
setIconImage(Icon);

(For better understanding you may want to search for MediaTracker , which I believe was used to make sure, that the image is loaded completely.)