重绘时未调用paintComponent（）

Question

I have been working on a program that requires the painting of lines on an image when triggered by a mouse drag event, however within my code when the call to paintComponent is made through repaint() the paintComponent method doesn't execute. I have read the tutorials on swing graphics and also had some input from other programmers on this but have so far not been able to find a solution that works for me.

I have posted a small SSCCE to help pinpoint the area of code from within my program that isn't acting as expected in an attempt to simplify this for myself and anyone looking over my code.

Thanks in advance for anyone that takes the time to look this over for me.

Below are the two separate classes I am using.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;

public class TestGraphics {
private JLayeredPane contentPane;

public void newImage() {
    try {
        JFileChooser fileChooser = new JFileChooser(".");
        int status = fileChooser.showOpenDialog(null);

        if (status == JFileChooser.APPROVE_OPTION) {
            File selectedFile = fileChooser.getSelectedFile();
            System.out.println("The selected file is from the: " + selectedFile.getParent() + " Drive");
            System.out.println("Name of file: " + selectedFile.getName());
            System.out.println("Opening file");

            BufferedImage buffImage = ImageIO.read(new File(selectedFile.getAbsolutePath()));
            ImageIcon image = new ImageIcon(buffImage);
            JLabel label = new JLabel(image);
            label.setSize(label.getPreferredSize());

            label.setLocation(0, 0);

            contentPane = new JLayeredPane();
            contentPane.setBackground(Color.WHITE);
            contentPane.setOpaque(true);
            //getTabbedPane().setComponentAt(tabNum, contentPane);
            contentPane.add(label);
            contentPane.setPreferredSize(new Dimension(label.getWidth(), label.getHeight()));

            Segmentation segmentation = new Segmentation();
            segmentation.addListeners(label); //call to addListeners method

            JFrame frame = new JFrame();
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.getContentPane().add(contentPane);
            frame.setResizable(false);
            frame.pack();
            frame.setLocationByPlatform(true);
            frame.setVisible(true);

        } else if(status == JFileChooser.CANCEL_OPTION) {
            JOptionPane.showMessageDialog(null, "Canceled");
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            TestGraphics tg = new TestGraphics();
            tg.newImage();
        }
    });
}

}

and the other.

import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseEvent;
import java.awt.geom.Line2D;
import java.util.ArrayList;
import javax.swing.JLabel;
import java.awt.event.MouseAdapter;

public class Segmentation extends JLabel {

private static final long serialVersionUID = -1481861667880271052L;  // unique id   
private static final Color LINES_COLOR = Color.red;
public static final Color CURRENT_LINE_COLOR = new Color(255, 200, 200);
ArrayList<Line2D> lineList = new ArrayList<Line2D>();
Line2D currentLine = null;
MyMouseAdapter mouse = new MyMouseAdapter();

public void addListeners(Component component) { 
    MyMouseAdapter myMouseAdapter = new MyMouseAdapter();
    component.addMouseListener(myMouseAdapter);
    component.addMouseMotionListener(myMouseAdapter);
}

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

    System.out.println("repainting");

    g2.setColor(LINES_COLOR);
    for (Line2D line : lineList) {
        g2.draw(line);
    }
    if (currentLine != null) {
        g2.setColor(CURRENT_LINE_COLOR);
        g2.draw(currentLine);
    }
}

private class MyMouseAdapter extends MouseAdapter {
    Point p1 = null;

    @Override
    public void mousePressed(MouseEvent e) {
        p1 = e.getPoint();
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        if (currentLine != null) {
            currentLine = new Line2D.Double(p1, e.getPoint());
            lineList.add(currentLine);
            currentLine = null;
            p1 = null;
            System.out.println("about to repaint");
            repaint();
        }
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        if (p1 != null) {
            currentLine = new Line2D.Double(p1, e.getPoint());
            repaint();
        }
    }
}

}

Answer 1

You never add your Segmentation instance to the component hierarchy. So your paintComponent will never be called.

You should have somewhere something like this:

Segmentation segmentation = new Segmentation();
// ...
component.add(segmentation); // assuming that component is part of a visible component hierarchy

EDIT:

You don't register the mouse listeners on the appropriate component. The following code seems to work quite alright:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Line2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.util.ArrayList;

import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;

public class TestGraphics {
    private JLayeredPane contentPane;

    public void newImage() {
        try {
            JFileChooser fileChooser = new JFileChooser(".");
            int status = fileChooser.showOpenDialog(null);

            if (status == JFileChooser.APPROVE_OPTION) {
                File selectedFile = fileChooser.getSelectedFile();
                System.out.println("The selected file is from the: " + selectedFile.getParent() + " Drive");
                System.out.println("Name of file: " + selectedFile.getName());
                System.out.println("Opening file");

                BufferedImage buffImage = ImageIO.read(new File(selectedFile.getAbsolutePath()));
                ImageIcon image = new ImageIcon(buffImage);

                contentPane = new JLayeredPane();
                contentPane.setBackground(Color.WHITE);
                contentPane.setOpaque(true);
                // getTabbedPane().setComponentAt(tabNum, contentPane);
                Dimension d = new Dimension(image.getIconWidth(), image.getIconHeight());
                Segmentation segmentation = new Segmentation();
                segmentation.setIcon(image);
                segmentation.setSize(d);
                contentPane.setPreferredSize(d);
                contentPane.add(segmentation);

                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setContentPane(contentPane);
                frame.setResizable(false);
                frame.pack();
                frame.setLocationByPlatform(true);
                frame.setVisible(true);

            } else if (status == JFileChooser.CANCEL_OPTION) {
                JOptionPane.showMessageDialog(null, "Canceled");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static class Segmentation extends JLabel {

        private static final long serialVersionUID = -1481861667880271052L; // unique id
        private static final Color LINES_COLOR = Color.red;
        public static final Color CURRENT_LINE_COLOR = new Color(255, 200, 200);
        ArrayList<Line2D> lineList = new ArrayList<Line2D>();
        Line2D currentLine = null;
        MyMouseAdapter mouse = new MyMouseAdapter();

        public Segmentation() {
            addMouseListener(mouse);
            addMouseMotionListener(mouse);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

            System.out.println("repainting");

            g2.setColor(LINES_COLOR);
            for (Line2D line : lineList) {
                g2.draw(line);
            }
            if (currentLine != null) {
                g2.setColor(CURRENT_LINE_COLOR);
                g2.draw(currentLine);
            }
        }

        private class MyMouseAdapter extends MouseAdapter {
            Point p1 = null;

            @Override
            public void mousePressed(MouseEvent e) {
                p1 = e.getPoint();
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                if (currentLine != null) {
                    currentLine = new Line2D.Double(p1, e.getPoint());
                    lineList.add(currentLine);
                    currentLine = null;
                    p1 = null;
                    System.out.println("about to repaint");
                    repaint();
                }
            }

            @Override
            public void mouseDragged(MouseEvent e) {
                if (p1 != null) {
                    currentLine = new Line2D.Double(p1, e.getPoint());
                    repaint();
                }
            }
        }

    }

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

}