为什么在此JPanel上显示此JMenu？

Question

This illustrates my problem

Im trying to copy a paint like program but whenever I click on the screen the menu bar is displayed in the panel area. It doesn't do it unless I click on the screen (to draw). The menu bar is added before the panel, any help would be much appreciated.

public class Main {
public static Frame frame;
public static Panel panel;
public static MenuBar menubar;
public static void main(String[] args) {
    frame = new Frame();
    panel = new Panel();
    menubar = new MenuBar();
    frame.setJMenuBar(menubar);
    frame.addMouseMotionListener(panel);
    frame.add(panel);
    frame.setVisible(true);     
}

}

public class Frame extends JFrame {
public Frame() {
    setTitle("Paint");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setExtendedState(JFrame.MAXIMIZED_BOTH);
}

}

public class Panel extends JPanel implements MouseMotionListener {

public boolean isMouseDown = false;
public int x1,y1, x2, y2;
public Color colour = Color.BLACK;
public int size = 3;

public void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    g2.setColor(colour);
    g2.setStroke(new BasicStroke(size));
    g2.drawLine(x1 - 10, y1 - 80, x2 - 10, y2 - 80);
}

public void mouseDragged(MouseEvent arg0) {
    x1 = x2;
    y1 = y2;
    x2 = arg0.getX();
    y2 = arg0.getY();
    repaint();
}

public void mouseMoved(MouseEvent arg0) {
    x1 = x2;
    y1 = y2;
    x2 = arg0.getX();
    y2 = arg0.getY();
}

public void mouseClicked(MouseEvent arg0) {}

public void mouseEntered(MouseEvent arg0) {}

public void mouseExited(MouseEvent arg0) {}

}

public class MenuBar extends JMenuBar {

JMenu file = new JMenu("File");
JMenu brush = new JMenu("Pen");
JMenu colour = new JMenu("Colour");
Font font = new Font("Times New Romans", Font.PLAIN, 18);

public MenuBar() {  
   //JMenuBar Code here, left black as very long
}

}

Answer 1

One problems: You need to call super.paintComponent(g); within your paintComponent method. Else the JPanel cannot do its house-keeping painting of its own self.

If you want the drawings to persist, then either create an ArrayList of lines that are then drawn within paintComponent in a for loop, or better, paint onto a BufferedImage which is then displayed within the paintComponent method.

eg,

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Stroke;
import java.awt.event.ActionEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import javax.swing.*;

public class MyMain {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }

    private static void createAndShowGui() {
        MyPainting mainPanel = new MyPainting();
        MenuCreator menuCreator = new MenuCreator(mainPanel);

        JFrame frame = new JFrame("MyPainting");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.add(mainPanel);
        frame.setJMenuBar(menuCreator.getMenubar());
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }
}

@SuppressWarnings("serial")
class MyPainting extends JPanel {
    private static final int IMG_W = 600;
    private static final int IMG_H = 450;
    private static final int STROKE_W = 3;
    private static final Stroke STROKE = new BasicStroke(STROKE_W);
    private BufferedImage img = new BufferedImage(IMG_W, IMG_H, BufferedImage.TYPE_INT_ARGB);
    private Color drawColor = Color.BLACK;

    public MyPainting() {
        setPreferredSize(new Dimension(IMG_W, IMG_H));
        MyMouse myMouse = new MyMouse();
        addMouseListener(myMouse);
        addMouseMotionListener(myMouse);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (img != null) {
            g.drawImage(img, 0, 0, this);
        }
    }

    private class MyMouse extends MouseAdapter {
        private Graphics2D g2 = null;
        private Point p0;

        @Override
        public void mousePressed(MouseEvent e) {
            if (img == null) {
                return;
            }
            g2 = img.createGraphics();
            g2.setStroke(STROKE);
            g2.setColor(drawColor);
            p0 = e.getPoint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (p0 == null) {
                return;
            }
            drawLine(e);

        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (p0 == null) {
                return;
            }
            drawLine(e);
            g2.dispose();
            p0 = null;
        }

        private void drawLine(MouseEvent e) {
            Point p1 = e.getPoint();
            g2.drawLine(p0.x, p0.y, p1.x, p1.y);
            repaint();
            p0 = p1;
        }

    }

    public void setDrawColor(Color drawColor) {
        this.drawColor = drawColor;
    }

    public void clear() {
        img = new BufferedImage(IMG_W, IMG_H, BufferedImage.TYPE_INT_ARGB);
        repaint();
    }
}

class MenuCreator {
    private JMenuBar menubar = new JMenuBar();
    private MyPainting myPainting;

    public MenuCreator(MyPainting myPainting) {
        JMenuItem clearDrawing = new JMenuItem(new AbstractAction("Clear Drawing") {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                if (myPainting != null) {
                    myPainting.clear();
                }
            }
        });
        JMenu fileMenu = new JMenu("File");
        fileMenu.add(clearDrawing);

        JMenu colourMenu = new JMenu("Colour");
        for (MyColor myColor : MyColor.values()) {
            colourMenu.add(new JMenuItem(new ColorAction(myColor)));
        }

        menubar.add(fileMenu);
        menubar.add(new JMenu("Pen"));
        menubar.add(colourMenu);
        this.myPainting = myPainting;
    }

    public JMenuBar getMenubar() {
        return menubar;
    }

    private class ColorAction extends AbstractAction {
        private MyColor myColor;

        public ColorAction(MyColor myColor) {
            super(myColor.getText());
            this.myColor = myColor;
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            if (myPainting != null) {
                myPainting.setDrawColor(myColor.getColor());
            }
        }
    }
}

enum MyColor {
    BLACK("Black", Color.BLACK), RED("Red", Color.RED), ORANGE("Orange", Color.ORANGE), BLUE("Blue", Color.BLUE);

    private String text;
    private Color color;
    private MyColor(String text, Color color) {
        this.text = text;
        this.color = color;
    }

    public String getText() {
        return text;
    }

    public Color getColor() {
        return color;
    }

}

Answer 2

You failed to honour the requirements of the paint chain...

public void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    g2.setColor(colour);
    g2.setStroke(new BasicStroke(size));
    g2.drawLine(x1 - 10, y1 - 80, x2 - 10, y2 - 80);
}

Apart from not needing to be public , one of the jobs of paintComponent is to prepare the Graphics context for painting. So, one of the first things you should do is call super.paintComponent before you do any custom painting.

protected void paintComponent(Graphics g) {
    super. paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    g2.setColor(colour);
    g2.setStroke(new BasicStroke(size));
    g2.drawLine(x1 - 10, y1 - 80, x2 - 10, y2 - 80);
}

Graphics is a shared context, it is shared amongst the other components which need to be painted, so it is very important that you ensure that it's properly prepared before you paint to it.

It's also important that any significant modifications you make to the context (like transformations or rendering hints) are reversed before the method exists

I would recommend taking a look at Performing Custom Painting and Painting in Swing to gain a better understanding into how the painting system actually works