source: josm/src/org/openstreetmap/josm/actions/mapmode/AddLineSegmentAction.java@ 68

package org.openstreetmap.josm.actions.mapmode;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import org.openstreetmap.josm.Main;
import org.openstreetmap.josm.command.AddCommand;
import org.openstreetmap.josm.data.osm.LineSegment;
import org.openstreetmap.josm.data.osm.Node;
import org.openstreetmap.josm.data.osm.OsmPrimitive;
import org.openstreetmap.josm.gui.MapFrame;

/**
 * The user can add a new line segment between two nodes by pressing on the 
 * starting node and dragging to the ending node. 
 * 
 * No line segment can be created if there is already a line segment containing
 * both nodes.
 * 
 * @author imi
 */
public class AddLineSegmentAction extends MapMode implements MouseListener {

        /**
         * The first node the user pressed the button onto.
         */
        private Node first;
        /**
         * The second node used if the user releases the button.
         */
        private Node second;

        /**
         * Whether the hint is currently drawn on screen.
         */
        private boolean hintDrawn = false;
        
        /**
         * Create a new AddLineSegmentAction.
         * @param mapFrame The MapFrame this action belongs to.
         */
        public AddLineSegmentAction(MapFrame mapFrame) {
                super("Add Line Segment", "addlinesegment", "Add a line segment between two nodes.", "G", KeyEvent.VK_G, mapFrame);
        }

        @Override
        public void registerListener() {
                super.registerListener();
                mv.addMouseListener(this);
                mv.addMouseMotionListener(this);
        }

        @Override
        public void unregisterListener() {
                super.unregisterListener();
                mv.removeMouseListener(this);
                mv.removeMouseMotionListener(this);
                drawHint(false);
        }

        
        @Override
        public void actionPerformed(ActionEvent e) {
                super.actionPerformed(e);
                makeLineSegment();
        }

        /**
         * If user clicked on a node, from the dragging with that node. 
         */
        @Override
        public void mousePressed(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON1)
                        return;

                OsmPrimitive clicked = mv.getNearest(e.getPoint(), true);
                if (clicked == null || !(clicked instanceof Node))
                        return;

                drawHint(false);
                first = second = (Node)clicked;
        }

        /**
         * Draw a hint which nodes will get connected if the user release
         * the mouse button now.
         */
        @Override
        public void mouseDragged(MouseEvent e) {
                if ((e.getModifiersEx() & MouseEvent.BUTTON1_DOWN_MASK) == 0)
                        return;

                OsmPrimitive clicked = mv.getNearest(e.getPoint(), (e.getModifiersEx() & MouseEvent.ALT_DOWN_MASK) != 0);
                if (clicked == null || clicked == second || !(clicked instanceof Node))
                        return;

                drawHint(false);

                second = (Node)clicked;
                drawHint(true);
        }

        /**
         * If left button was released, try to create the line segment.
         */
        @Override
        public void mouseReleased(MouseEvent e) {
                if (e.getButton() == MouseEvent.BUTTON1) {
                        makeLineSegment();
                        first = null; // release line segment drawing
                }
        }

        /**
         * Create the line segment if first and second are different and there is
         * not already a line segment.
         */
        private void makeLineSegment() {
                if (first == null || second == null) {
                        first = null;
                        second = null;
                        return;
                }

                drawHint(false);
                
                Node start = first;
                Node end = second;
                first = second;
                second = null;
                
                if (start != end) {
                        // try to find a line segment
                        for (LineSegment ls : Main.main.ds.lineSegments)
                                if ((start == ls.from && end == ls.to) || (end == ls.from && start == ls.to))
                                        return; // already a line segment here - be happy, do nothing.

                        LineSegment ls = new LineSegment(start, end);
                        mv.editLayer().add(new AddCommand(Main.main.ds, ls));
                }

                mv.repaint();
        }

        /**
         * Draw or remove the hint line, depending on the parameter.
         */
        private void drawHint(boolean draw) {
                if (draw == hintDrawn)
                        return;
                if (first == null || second == null)
                        return;
                if (second == first)
                        return;

                Graphics g = mv.getGraphics();
                g.setColor(Color.BLACK);
                g.setXORMode(Color.WHITE);
                Point firstDrawn = mv.getScreenPoint(first.coor);
                Point secondDrawn = mv.getScreenPoint(second.coor);
                g.drawLine(firstDrawn.x, firstDrawn.y, secondDrawn.x, secondDrawn.y);
                hintDrawn = !hintDrawn;
        }
}