Drawing lines between JPanels/JInternalFrames

[Gregg Bolinger]

I am slowly working on a proof of concept for a friend of mine and I need to create primative shaped nodes and be able to draw lines between them, drag the nodes around, and have the lines stay connected.

I know about JGraph however, I don't need as much functionality nor is their license going to fit with this project. So I need to implement the base functionality myself.

At first I was looking into custom JPanels but part of the functionality I need already exists in a JInternalFrame, but I don't need the frame decorated and I need to be able to drag using any part of the node. I was able to get a JPanel to drag where I wanted it, but the drawing of lines and specifically keeping them connected while dragging the JPanel is giving me fits. I don't have my code in front of me at the moment so I can't show what I've done.

What I am looking for is some suggestions to solving this. What have you done? Did you use custom JPanels or some other customized component, etc? How about the connection of lines? Thanks for any tips.

[Craig Wood]

I need to create primative shaped nodes
You could draw these on a JPanel.
part of the functionality I need already exists in a JInternalFrame
I wonder what this is.
How about the connection of lines?
This is usually easy enough. For the parent component which contains the node components you loop through the children and draw lines among them.
looking for ... some suggestions
For opaque components or (java.awt.geom) primitives you can draw to the center of the node before rendering the node. For non–opaque components like JLabel, JComponent or primitives you need to do a little more to get the lines to end at the edge of the node. I can demonstrate this if you are interested. Ditto if you want the lines to end at a corner or edge–center of the node depending on relative locations of any two nodes.
You can draw the lines among nodes with every trip through paintComponent or you can keep the lines in an array/list and reset them as needed for node moves.
Here's an idea for using opaque components with the connecting lines joined at the node centers.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.event.MouseInputAdapter;
 
public class LinkedComponents {
    LinkedPanel parentPanel;
 
    public LinkedComponents() {
        parentPanel = new LinkedPanel();
        parentPanel.setLayout(null);
        Rectangle r = new Rectangle(75, 75, 125, 40);
        for(int j = 0; j < 3; j++) {
            addPanel(parentPanel, r, "Panel " + (j+1));
            r.x += r.width/3 + 25;
            r.y += r.height + 45;
        }
        PanelMover panelMover = new PanelMover(parentPanel);
        parentPanel.addMouseListener(panelMover);
        parentPanel.addMouseMotionListener(panelMover);
    }
 
    private void addPanel(Container c, Rectangle bounds, String id) {
        JPanel panel = new JPanel();
        c.add(panel);
        panel.setBounds(bounds);
        panel.add(new JLabel(id, JLabel.CENTER));
        panel.setBorder(BorderFactory.createEtchedBorder());
    }
 
    private JPanel getContent() {
        return parentPanel;
    }
 
    public static void main(String[] args) {
        LinkedComponents test = new LinkedComponents();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test.getContent());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class LinkedPanel extends JPanel {
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        Component[] c = getComponents();
        for(int j = 0; j < c.length; j++) {
            Point2D.Double p1 = getCenter(c[j]);
            for(int k = j+1; k < c.length; k++) {
                Point2D.Double p2 = getCenter(c[k]);
                g2.draw(new Line2D.Double(p1, p2));
            }
        }
    }
 
    private Point2D.Double getCenter(Component c) {
        Point2D.Double p = new Point2D.Double();
        Rectangle r = c.getBounds();
        p.x = r.getCenterX();
        p.y = r.getCenterY();
        return p;
    }
}
 
class PanelMover extends MouseInputAdapter {
    LinkedPanel panel;
    Point offset = new Point();
    int selectedIndex;
    boolean dragging = false;
 
    PanelMover(LinkedPanel panel) {
        this.panel = panel;
    }
 
    public void mousePressed(MouseEvent e) {
        Point p = e.getPoint();
        // This way of identifying the target JPanel should work as
        // long as the target is not focusable and does not contain
        // any components that consume MouseEvents, eg, JTextField.
        Component[] c = panel.getComponents();
        for(int j = 0; j < c.length; j++) {
            Rectangle r = c[j].getBounds();
            if(r.contains(p)) {
                offset.x = p.x - r.x;
                offset.y = p.y - r.y;
                selectedIndex = j;
                dragging = true;
                break;
            }
        }
    }
 
    public void mouseReleased(MouseEvent e) {
        dragging = false;
    }
 
    public void mouseDragged(MouseEvent e) {
        if(dragging) {
            int x = e.getX() - offset.x;
            int y = e.getY() - offset.y;
            panel.getComponent(selectedIndex).setLocation(x, y);
            panel.repaint();
        }
    }
}

[Gregg Bolinger]

Thanks Craig. I am going to mess around with the code you have provided later this evening and then I am sure I'll come back with some questions.

[Jan Cumps]

I allways loved Sun's JDK demo app (comes with the JDK, including source) to learn about linking graphic objects.
demo\applets\GraphLayout\example1.html.

Go ahead, grab one of the objects and drag it around.

Regards, Jan
[ February 25, 2007: Message edited by: Jan Cumps ]

[Marcus Senna]

Hey, I am developing an application which requires the linking of components by lines, but i would like to do this at the click of a button.
I am also using opaque components, so i would like to know if you could help me with regard to positioning the ends of the lines at the edge of the components. Thanks in advance.

[Craig Wood]

i would like to do this at the click of a button
This would be easy to control with booleans in paintComponent which are set in the listeners of your buttons.
I am also using opaque components
If you are using opaque (filled background) components then the technique shown in LinkedComponents above should work okay.
help me with regard to positioning the ends of the lines at the edge of the components
This is necessary for non–opaque (transparent/translucent background) components and primitives.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.event.MouseInputAdapter;
 
public class TransparentLinks {
    ParentPanel parentPanel;
 
    public TransparentLinks() {
        parentPanel = new ParentPanel();
        parentPanel.setLayout(null);
        Rectangle r = new Rectangle(135, 75, 125, 40);
        for(int j = 0; j < 3; j++) {
            addLabel(parentPanel, r, "Label " + (j+1));
            r.x += ((j==0) ? -1 : 2)*r.width*3/4;
            r.y += r.height + 45;
        }
        XPMover labelMover = new XPMover(parentPanel);
        parentPanel.addMouseListener(labelMover);
        parentPanel.addMouseMotionListener(labelMover);
    }
 
    private void addLabel(Container c, Rectangle bounds, String id) {
        JLabel label = new JLabel(id, JLabel.CENTER);
        c.add(label);
        label.setBounds(bounds);
        label.setBorder(BorderFactory.createEtchedBorder());
    }
 
    private JPanel getContent() {
        return parentPanel;
    }
 
    public static void main(String[] args) {
        TransparentLinks test = new TransparentLinks();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test.getContent());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class ParentPanel extends JPanel {
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        Component[] c = getComponents();
        for(int j = 0; j < c.length; j++) {
            Rectangle r1 = c[j].getBounds();
            for(int k = j+1; k < c.length; k++) {
                Rectangle r2 = c[k].getBounds();
                double theta = getAngle(r1, r2);
                Point2D.Double p1 = getIntersectingPoint(theta, r1);
                theta = getAngle(r2, r1);
                Point2D.Double p2 = getIntersectingPoint(theta, r2);
                g2.draw(new Line2D.Double(p1, p2));
            }
        }
    }
 
    private double getAngle(Rectangle r1, Rectangle r2) {
        double cy1 = r1.getCenterY();
        double cx1 = r1.getCenterX();
        double cy2 = r2.getCenterY();
        double cx2 = r2.getCenterX();
        double dy = cy2 - cy1;
        double dx = cx2 - cx1;
        return Math.atan2(dy, dx);
    }
 
    private Point2D.Double getIntersectingPoint(double theta, Rectangle r) {
        double cx = r.getCenterX();
        double cy = r.getCenterY();
        double W = r.getWidth()/2;
        double H = r.getHeight()/2;
        double R = Point2D.distance(cx, cy, cx+W, cy+H);
        double x = cx + R * Math.cos(theta);
        double y = cy + R * Math.sin(theta);
        Point2D.Double p = new Point2D.Double();
        int outcode = r.outcode(x, y);
        switch(outcode) {
            case Rectangle.OUT_TOP:
                p.x = cx - H * ((x - cx)/(y - cy));
                p.y = cy - H;
                break;
            case Rectangle.OUT_LEFT:
                p.x = cx - W;
                p.y = cy - W * ((y - cy)/(x - cx));
                break;
            case Rectangle.OUT_BOTTOM:
                p.x = cx + H * ((x - cx)/(y - cy));
                p.y = cy + H;
                break;
            case Rectangle.OUT_RIGHT:
                p.x = cx + W;
                p.y = cy + W * ((y - cy)/(x - cx));
                break;
            default: System.out.println("non-cardinal outcode: " + outcode);
        }
        return p;
    }
}
 
class XPMover extends MouseInputAdapter {
    ParentPanel panel;
    Point offset = new Point();
    int selectedIndex;
    boolean dragging = false;
 
    XPMover(ParentPanel panel) {
        this.panel = panel;
    }
 
    public void mousePressed(MouseEvent e) {
        Point p = e.getPoint();
        // This way of identifying the target JPanel should work as
        // long as the target is not focusable and does not contain
        // any components that consume MouseEvents, eg, JTextField.
        Component[] c = panel.getComponents();
        for(int j = 0; j < c.length; j++) {
            Rectangle r = c[j].getBounds();
            if(r.contains(p)) {
                offset.x = p.x - r.x;
                offset.y = p.y - r.y;
                selectedIndex = j;
                dragging = true;
                break;
            }
        }
    }
 
    public void mouseReleased(MouseEvent e) {
        dragging = false;
    }
 
    public void mouseDragged(MouseEvent e) {
        if(dragging) {
            int x = e.getX() - offset.x;
            int y = e.getY() - offset.y;
            panel.getComponent(selectedIndex).setLocation(x, y);
            panel.repaint();
        }
    }
}

[Marcus Senna]

Thanks!!!..you have been a great help

[Gregg Bolinger]

I've been messing around with the code provided by Craig and doing some proof of concept work. It's pretty simple and seems to work nicely. The problem I am running into now is how to make the connecting lines their own component. I tried making them a JPanel on their own but was having issues keeping them connected to the other nodes. So what I have done in the meantime is I am keeping a list of Links. Each link has a target and parent object which are the nodes. in the paintComponent method, instead of drawing lines between every component I simple iterate over the list of links and draw lines from source to target.

While this approach works, it's not very robust. I'd like to more control over the links themselves. Any tips or suggestions?

[Craig Wood]

I'd like to more control over the links themselves.
Here's some exploratory code along these lines.
Looks like someone else has been thinking about this: see reply 3 on the thread How do I "connect" some buttons with the drawing?.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.event.MouseInputAdapter;
 
public class ComponentLinks {
    private JTabbedPane getContent() {
        JTabbedPane tabbedPane = new JTabbedPane();
        // Component with draggable labels.
        tabbedPane.addTab("draggables", getDragComponent());
        // Component with layout manager.
        tabbedPane.addTab("layout", getLayoutComponent());
        return tabbedPane;
    }
 
    private JPanel getDragComponent() {
        ComponentPanel cp = new ComponentPanel();
        cp.setLayout(null);
        int numComponents = 4;
        Rectangle r = new Rectangle(75, 75, 100, 35);
        for(int j = 0; j < numComponents; j++) {
            r.x = ((j+1)%2 == 0) ? 250 : 50;
            addLabel(cp, r, "Label " + (j+1));
            if(j%2 == 1)
                r.y += r.height + 75;
        }
        LineManager manager = new LineManager(cp.getComponents());
        cp.setLineManager(manager);
        ComponentMover labelMover = new ComponentMover(cp, manager);
        cp.addMouseListener(labelMover);
        cp.addMouseMotionListener(labelMover);
        return cp;
    }
 
     private void addLabel(Container c, Rectangle bounds, String id) {
        JLabel label = new JLabel(id, JLabel.CENTER);
        c.add(label);
        label.setBounds(bounds);
        label.setBorder(BorderFactory.createEtchedBorder());
    }
 
    private JPanel getLayoutComponent() {
        ComponentPanel cp = new ComponentPanel();
        cp.setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(5,5,5,5);
        gbc.weightx = 1.0;
        gbc.weighty = 1.0;
        int numComponents = 8;
        for(int j = 0; j < numComponents; j++) {
            gbc.gridwidth = ((j+1)%2 == 0) ? GridBagConstraints.REMAINDER
                                           : GridBagConstraints.RELATIVE;
            JLabel label = new JLabel("Label " + (j+1), JLabel.CENTER);
            label.setPreferredSize(new Dimension(80, 25));
            label.setBorder(BorderFactory.createEtchedBorder());
            cp.add(label, gbc);
        }
        final LineManager manager = new LineManager(cp.getComponents());
        cp.setLineManager(manager);
        cp.addComponentListener(new ComponentAdapter() {
            public void componentResized(ComponentEvent e) {
                Container src = (Container)e.getSource();
                Component[] c = src.getComponents();
                manager.resetLines(c);
                src.repaint();
            }
        });
        return cp;
    }
 
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new ComponentLinks().getContent());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class ComponentPanel extends JPanel {
    LineManager lineManager;
 
    public void setLineManager(LineManager lm) {
        lineManager = lm;
    }
 
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        lineManager.drawLines(g2);
    }
}
 
class LineManager {
    Line2D.Double[] lines;
    Rectangle[] rects;
 
    public LineManager(Component[] c) {
        int n = c.length;
        // The number of lines is the combination of
        // n components taken two at a time.
        int numLines = factorial(n)/(factorial(2)*factorial(n-2));
        lines = new Line2D.Double[numLines];
        rects = new Rectangle[c.length];
        for(int j = 0, count = 0; j < c.length; j++) {
            Rectangle r1 = c[j].getBounds();
            rects[j] = r1;
            for(int k = j+1; k < c.length; k++) {
                Rectangle r2 = c[k].getBounds();
                double theta = getAngle(r1, r2);
                Point2D.Double p1 = getIntersectingPoint(theta, r1);
                theta = getAngle(r2, r1);
                Point2D.Double p2 = getIntersectingPoint(theta, r2);
                lines[count++] = new Line2D.Double(p1, p2);
            }
        }
    }
 
    private double getAngle(Rectangle r1, Rectangle r2) {
        double cy1 = r1.getCenterY();
        double cx1 = r1.getCenterX();
        double cy2 = r2.getCenterY();
        double cx2 = r2.getCenterX();
        double dy = cy2 - cy1;
        double dx = cx2 - cx1;
        return Math.atan2(dy, dx);
    }
 
    private Point2D.Double getIntersectingPoint(double theta, Rectangle r) {
        double cx = r.getCenterX();
        double cy = r.getCenterY();
        double W = r.getWidth()/2;
        double H = r.getHeight()/2;
        double R = Point2D.distance(cx, cy, cx+W, cy+H);
        double x = cx + R * Math.cos(theta);
        double y = cy + R * Math.sin(theta);
        Point2D.Double p = new Point2D.Double();
        int outcode = r.outcode(x, y);
        switch(outcode) {
            case Rectangle.OUT_TOP:
                p.x = cx - H * ((x - cx)/(y - cy));
                p.y = cy - H;
                break;
            case Rectangle.OUT_LEFT:
                p.x = cx - W;
                p.y = cy - W * ((y - cy)/(x - cx));
                break;
            case Rectangle.OUT_BOTTOM:
                p.x = cx + H * ((x - cx)/(y - cy));
                p.y = cy + H;
                break;
            case Rectangle.OUT_RIGHT:
                p.x = cx + W;
                p.y = cy + W * ((y - cy)/(x - cx));
                break;
        }
        return p;
    }
 
    private static int factorial(int n) {
        return n == 0 ? 1 : n*factorial(n-1);
    }
 
    public void drawLines(Graphics2D g2) {
        for(int j = 0; j < lines.length; j++) {
            g2.draw(lines[j]);
            // Label the lines with their index in the lines array.
            //double x = lines[j].getX1() +
            //          (lines[j].getX2() - lines[j].getX1())/3 + 5;
            //double y = lines[j].getY1() +
            //          (lines[j].getY2() - lines[j].getY1())/3 + 5;
            //g2.drawString(String.valueOf(j), (float)x, (float)y);
        }
    }
 
    public void setLinesFor(Rectangle r, int index) {
        // Update the rect that is being dragged.
        rects[index].setRect(r);
        // Find the indices of the lines connected to
        // the dragged rect - these need to be reset.
        int[] indices = getLineIndices(index);
        // Reset the indicated lines.
        for(int j = 0, k = 0; j < rects.length; j++) {
            if(j == index) continue;
            Rectangle r2 = rects[j];
            double theta = getAngle(r, r2);
            Point2D.Double p1 = getIntersectingPoint(theta, r);
            theta = getAngle(r2, r);
            Point2D.Double p2 = getIntersectingPoint(theta, r2);
            lines[indices[k++]].setLine(p1, p2);
        }
    }
 
    /** Method called from dragComponent ComponentMover. */
    private int[] getLineIndices(int index) {
        // Find indices of lines that need to be changed.
        int[] indices = new int[rects.length-1];
        int lineIndex = 0;
        for(int j = 0, count = 0; j < index+1; j++) {
            for(int k = 0; k < rects.length; k++) {
                if((j < index & k == index) || (j == index && k > index))
                    indices[count++] = lineIndex;
                if(k > j)
                    lineIndex++;
            }
        }
        return indices;
    }
 
    /** Called from layoutComponent ComponentListener */
    public void resetLines(Component[] c) {
        for(int j = 0, count = 0; j < rects.length; j++) {
            Rectangle r1 = c[j].getBounds();
            rects[j] = r1;
            for(int k = j+1; k < rects.length; k++) {
                Rectangle r2 = c[k].getBounds();
                double theta = getAngle(r1, r2);
                Point2D.Double p1 = getIntersectingPoint(theta, r1);
                theta = getAngle(r2, r1);
                Point2D.Double p2 = getIntersectingPoint(theta, r2);
                lines[count++].setLine(p1, p2);
            }
        }
    }
}
 
class ComponentMover extends MouseInputAdapter {
    ComponentPanel panel;
    LineManager lineManager;
    Point offset = new Point();
    int selectedIndex;
    boolean dragging = false;
 
    public ComponentMover(ComponentPanel panel, LineManager lm) {
        this.panel = panel;
        lineManager = lm;
    }
 
    public void mousePressed(MouseEvent e) {
        Point p = e.getPoint();
        Component[] c = panel.getComponents();
        for(int j = 0; j < c.length; j++) {
            Rectangle r = c[j].getBounds();
            if(r.contains(p)) {
                offset.x = p.x - r.x;
                offset.y = p.y - r.y;
                selectedIndex = j;
                dragging = true;
                break;
            }
        }
    }
 
    public void mouseReleased(MouseEvent e) {
        dragging = false;
    }
 
    public void mouseDragged(MouseEvent e) {
        if(dragging) {
            int x = e.getX() - offset.x;
            int y = e.getY() - offset.y;
            // Update the component location.
            Component c = panel.getComponent(selectedIndex);
            c.setLocation(x, y);
            // Update lines that terminate at component.
            lineManager.setLinesFor(c.getBounds(), selectedIndex);
            panel.repaint();
        }
    }
}