How connect dragable labels with line

Question

When I load the program labels will appear within the window. You can ckick
and drag these labels. The code for this
is very simple. The code works perfectly. However I want to extend my code to do
the following :
(1) I want to add an extra label .. it does not matter where on the screen
[ JLabel label = new JLabel() ]
(2) When I press this button I want a line to appear between the two
moveable labels ( ie the two labels are joined together by the line)
(3)Now here is the hard part.. Whenever I move either of the two labels
the line stretches so that the two labels are still connected.
start--->
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;
import javax.swing.border.*;
public class Connection
public static void main(String[] args)
JFrame f = new JFrame("Connection");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.getContentPane().add(new ConnectionPanel());
f.setSize(400,300);
f.setLocation(200,200);
f.setVisible(true);
class ConnectionPanel extends JPanel
JLabel label1, label2, label3, label4;
JLabel[] labels;
JLabel selectedLabel;
int offsetX, offsetY;
boolean dragging;
public ConnectionPanel()
setOpaque(false);
setLayout(null);
addLabels();
addMouseListener(new LabelSelector());
addMouseMotionListener(new LabelMover());
public void paintComponent(Graphics g)
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2.setPaint(Color.white);
g2.fillRect(0, 0, getWidth(), getHeight());
g2.setPaint(Color.blue);
Point
p1 = getCenter(label1),
p2 = getCenter(label2),
p3 = getCenter(label3),
p4 = getCenter(label4);
g2.draw(new Line2D.Double(p1.x, p1.y, p2.x, p2.y));
g2.draw(new Line2D.Double(p1.x, p1.y, p3.x, p3.y));
g2.draw(new Line2D.Double(p1.x, p1.y, p4.x, p4.y));
g2.draw(new Line2D.Double(p2.x, p2.y, p3.x, p3.y));
g2.draw(new Line2D.Double(p2.x, p2.y, p4.x, p4.y));
g2.draw(new Line2D.Double(p3.x, p3.y, p4.x, p4.y));
super.paintComponent(g);
private Point getCenter(JComponent c)
Rectangle r = c.getBounds();
return new Point((int)r.getCenterX(), (int)r.getCenterY());
private class LabelSelector extends MouseAdapter
public void mousePressed(MouseEvent e)
Point p = e.getPoint();
for(int i = 0; i < labels.length; i++)
Rectangle r = labels.getBounds();
if(r.contains(p))
selectedLabel = labels[i];
offsetX = p.x - r.x;
offsetY = p.y - r.y;
dragging = true;
break;
public void mouseReleased(MouseEvent e)
dragging = false;
private class LabelMover extends MouseMotionAdapter
public void mouseDragged(MouseEvent e)
if(dragging)
int x = e.getX() - offsetX;
int y = e.getY() - offsetY;
Rectangle r = selectedLabel.getBounds();
selectedLabel.setBounds(x, y, r.width, r.height);
repaint();
private void addLabels()
int w = 125;
int h = 25;
Border border = BorderFactory.createEtchedBorder();
label1 = new JLabel("Label 1", JLabel.CENTER);
label2 = new JLabel("Label 2", JLabel.CENTER);
label3 = new JLabel("Label 3", JLabel.CENTER);
label4 = new JLabel("Label 4", JLabel.CENTER);
labels = new JLabel[4];
labels[0] = label1; labels[1] = label2; labels[2] = label3; labels
[3] = label4;
for(int i = 0; i < labels.length; i++)
labels[i].setBorder(border);
labels[i].setOpaque(true);
labels[i].setBackground(Color.white);
add(labels[i]);
label1.setBounds(40, 40, w, h);
label2.setBounds(215, 40, w, h);
label3.setBounds(40, 200, w, h);
label4.setBounds(215, 200, w, h);
end-->
In this code: How to add labels when to me it is necessary
(From menu: add new label)?

Answer 1

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Line2D;
import javax.swing.*;
import javax.swing.event.MouseInputAdapter;
public class Connection
    ConnectionPanel connectionPanel;
    LabelWrangler wrangler;
    public Connection()
        connectionPanel = new ConnectionPanel();
        wrangler = new LabelWrangler(connectionPanel);
        connectionPanel.addMouseListener(wrangler);
        connectionPanel.addMouseMotionListener(wrangler);
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(getUIPanel(), "North");
        f.getContentPane().add(connectionPanel);
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    private JPanel getUIPanel()
        JButton add = new JButton("add label");
        add.addActionListener(new ActionListener()
            public void actionPerformed(ActionEvent e)
                wrangler.willAcceptLabel = true;
        JCheckBox lineBox = new JCheckBox("show lines");
        lineBox.addActionListener(new ActionListener()
            public void actionPerformed(ActionEvent e)
                connectionPanel.showConnectionLines();
        JPanel panel = new JPanel();
        panel.add(add);
        panel.add(lineBox);
        return panel;
    public static void main(String[] args)
        new Connection();
class ConnectionPanel extends JPanel
    boolean showConnections;
    int labelCount;
    final int
        N  = 2,
        NW = 3,
        W  = 1,
        SW = 9,
        S  = 8,
        SE = 12,
        E  = 4,
        NE = 6;
    public ConnectionPanel()
        showConnections = false;
        labelCount = 0;
        setLayout(null);
    protected void paintComponent(Graphics g)
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        if(showConnections)
            Component[] c = getComponents();
            Rectangle r1 = new Rectangle();
            Rectangle r2 = new Rectangle();
            for(int j = 0; j < c.length - 1; j++)
                c[j].getBounds(r1);
                for(int k = j + 1; k < c.length; k++)
                    c[k].getBounds(r2);
                    Point p1 = getClosestPoint(r1, r2);
                    Point p2 = getClosestPoint(r2, r1);
                    g2.draw(new Line2D.Double(p1.x, p1.y, p2.x, p2.y));
    private Point getClosestPoint(Rectangle r1, Rectangle r2)
        double cx = r2.getCenterX();
        double cy = r2.getCenterY();
        Point p = new Point();
        int outcode = r1.outcode(cx, cy);
        switch(outcode)
            case N:
                p.x = r1.x + r1.width/2 - 1;
                p.y = r1.y;
                break;
            case NW:
                p.x = r1.x;
                p.y = r1.y;
                break;
            case W:
                p.x = r1.x;
                p.y = r1.y + r1.height/2 - 1;
                break;
            case SW:
                p.x = r1.x;
                p.y = r1.y + r1.height - 1;
                break;
            case S:
                p.x = r1.x + r1.width/2 - 1;
                p.y = r1.y + r1.height - 1;
                break;
            case SE:
                p.x = r1.x + r1.width - 1;
                p.y = r1.y + r1.height - 1;
                break;
            case E:
                p.x = r1.x + r1.width - 1;
                p.y = r1.y + r1.height/2 - 1;
                break;
            case NE:
                p.x = r1.x + r1.width - 1;
                p.y = r1.y;
                break;
            default /* INSIDE == 0 */:
                System.out.println("INSIDE");
        return p;
    public void addLabel(Point p)
        JLabel label = new JLabel("label " + ++labelCount);
        label.setBorder(BorderFactory.createEtchedBorder());
        add(label);
        Dimension d = label.getPreferredSize();
        label.setBounds(p.x - d.width/2, p.y - d.height/2, d.width, d.height);
        repaint();
    public void showConnectionLines()
        showConnections = !showConnections;
        repaint();
class LabelWrangler extends MouseInputAdapter
    ConnectionPanel connectionPanel;
    Component selectedLabel;
    Point offset;
    boolean dragging, willAcceptLabel;
    public LabelWrangler(ConnectionPanel cp)
        connectionPanel = cp;
        offset = new Point();
        willAcceptLabel = false;
        dragging = false;
    public void mousePressed(MouseEvent e)
        Point p = e.getPoint();
        // has a label been selected for dragging?
        Component[] c = connectionPanel.getComponents();
        Rectangle r = new Rectangle();
        for(int j = 0; j < c.length; j++)
            c[j].getBounds(r);
            if(r.contains(p))
                selectedLabel = c[j];
                offset.x = p.x - r.x;
                offset.y = p.y - r.y;
                dragging = true;
                break;
        // if no selection, are we adding a new label?
        if(!dragging && willAcceptLabel)
            connectionPanel.addLabel(p);
            willAcceptLabel = false;
    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;
            selectedLabel.setLocation(x, y);
            connectionPanel.repaint();
}