为什么在此jpanel上显示这种jmenu

Question

这说明了我的问题

im尝试复制诸如程序之类的油漆，但是每当我单击屏幕时，菜单栏都会在面板区域显示。除非我单击屏幕(绘制(，否则它不会做。菜单栏是在面板之前添加的，任何帮助将不胜感激。

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

一个问题：您需要在PaintComponent方法中调用super.paintComponent(g);。否则，jpanel不能为自己的自我做家务绘画。

如果您希望图纸持续存在，那么要么创建一个线条列表，然后在for循环中绘制的线条绘制，或者更好地将绘画绘制到bufferedimage上，然后在paintComponent方法中显示。

。

，例如，

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

您未能兑现油漆链的要求...

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);
}

除了不需要public之外，paintComponent的工作之一是准备Graphics上下文进行绘画。因此，您应该做的第一件事是在进行任何自定义绘画之前调用super.paintComponent。

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是一个共享的上下文，它是在需要绘制的其他组件之间共享的，因此非常重要的是，您必须在绘画之前对其进行适当准备。

在方法存在之前，您对上下文进行的任何重大修改(例如转换或渲染提示(也很重要。

我建议您看一下秋千上的自定义绘画和绘画，以更好地了解绘画系统的实际运作方式