Lines disappear when scrolling

[Sharon whipple]

Hi all,
I am drawing lines on JComponent/JPanel wrapped in Jscrollpane (along with two JTables) the lines connect the rows of the left and right tables.

When I scroll down, the lines erased by the top part of the container,
So i tried to use the repaint method and now the lines disperses completely.

please advise


Here is the canvas:

   
public class LineConnectorsPanel extends JPanel{ //JComponent {
 ArrayList<Line> lines =null;
 
 public void paintComponent(Graphics g) {
super.paintComponent(g);  
           
for (Line line : lines) {
Point p1 = line.getP1();
Point p2 = line.getP2();
p1.setX(p1.getX() + (int)getComponentGraphics(g).getClipBounds().getX());
p1.setY(p1.getY() + (int)getComponentGraphics(g).getClipBounds().getY());
 
p2.setX(p2.getX() + (int)getComponentGraphics(g).getClipBounds().getX());
p2.setY(p2.getY() + (int)getComponentGraphics(g).getClipBounds().getY());
 
Graphics2D g2 = (Graphics2D) g;
        g2.setStroke(new BasicStroke(4));
g2.setColor(p1.getColor());
g2.drawLine(0,p1.getY(),getWidth(),p2.getY());
System.out.println("drawing Line from : " + p1 + " to "  + p2);
   }
   ///   ..... removed getters and setters
 }

And this is the ScrollBar listener:

 
getTablesScroll().getVerticalScrollBar().addAdjustmentListener( new AdjustmentListener() {
   public void adjustmentValueChanged(AdjustmentEvent e) {
getLineConnectorsPanel1().repaint();
}
}

[Craig Wood]

There are so many ways to get into trouble with drawing that it's difficult to
know where to start with what you've presented.
Some comments:
Looks like you are using custom Point and Line classes. I wonder what's going on
in the Line class: could you be changing the line locations with each of the p1.setX(p1.getX() +...–type calls?

I experimented a bit with the getComponentGraphics method and it seems to
alter the graphics context, at least the color, during the drawing of the lines.
Maybe you are trying to draw only the parts of the lines that need re-drawing.
Swing will do this for you automatically (double–buffering).

The scrollPane will automatically repaint the graphics located within its view
extent.
Here's an idea:

import java.awt.*;
import javax.swing.*;
 
public class Marching extends JPanel {
    private JScrollPane getContent() {
        JPanel panel = new JPanel(new GridLayout(1,0));
        panel.add(new JTable(32,2));
        panel.add(this);
        panel.add(new JTable(32,2));
        return new JScrollPane(panel);
    }
 
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int w = getWidth();
        JTable table = (JTable)getParent().getComponent(0);
        int rowHeight = table.getRowHeight();
        g2.setPaint(Color.red);
        for(int j = 0; j < table.getRowCount(); j++) {
            int y = j*rowHeight + rowHeight/2 -1;
            g2.drawLine(0, y, w-1, y);
        }
    }
 
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new Marching().getContent());
        f.setSize(500,300);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}