组件仅在悬停时绘制

Question

有人可以解释为什么我的组件只有在我将它们悬停在它们应该的位置时才会被绘制出来吗？

我设置了一个无边框，而不是可以在任何地方拖动，我正在尝试在右上角创建一个退出按钮但是在我将鼠标悬停在它上面之前它不会被绘制。 我在JFrame上绘制一个背景图像，然后绘制我的按钮并将整个事物设置为可见。

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. 必须在Event Dispatching Thread中执行与UI的所有交互
2. 您应该避免从顶级容器（如JFrame扩展，而是使用JPanel 。
3. 未能遵守paint链合同是防止任何儿童组件开始绘画
4. 覆盖执行自定义绘制的首选方法是paintComponent

您可能希望阅读

• 在AWT和Swing中绘画
• 事件派遣线程
• 执行自定义绘画

尝试这样的事情;

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);
        }
    }
}

您也可能希望阅读Java编程语言的代码约定，而不是通过忽略它们来建立任何朋友;）

Answer 2

如果我记得很清楚， ToolKit.getImage返回一个可能无法完全加载的Image 。 当您将鼠标悬停在它上面时，它可能已同时加载到后台。 而是这样做（类似于你的背景线）：

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

（为了更好地理解，您可能希望搜索MediaTracker ，我相信这是用来确保图像完全加载的。）