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

package org.openstreetmap.josm.actions.mapmode;

import java.awt.Cursor;
import java.awt.Point;
import java.awt.event.KeyEvent;
import java.awt.event.MouseEvent;
import java.util.Collection;

import javax.swing.JOptionPane;

import org.openstreetmap.josm.Main;
import org.openstreetmap.josm.command.Command;
import org.openstreetmap.josm.command.MoveCommand;
import org.openstreetmap.josm.data.GeoPoint;
import org.openstreetmap.josm.data.osm.Node;
import org.openstreetmap.josm.data.osm.OsmPrimitive;
import org.openstreetmap.josm.gui.MapFrame;

/**
 * Move is an action that can move all kind of OsmPrimitives (except Keys for now).
 * 
 * If any object is selected, all selected objects are moved. If no object is 
 * selected, the nearest object will be selected and moved. In this case, the
 * object will be unselected as soon as movement stopped.
 * 
 * @author imi
 */
public class MoveAction extends MapMode {

        /**
         * The old cursor before the user pressed the mouse button.
         */
        private Cursor oldCursor;
        /**
         * The position of the mouse before the user moves a node.
         */
        private Point mousePos;
        /**
         * Non-<code>null</code>, if no object was selected before movement 
         * (and so the object get unselected after mouse release).
         */
        private OsmPrimitive singleOsmPrimitive;

        /**
         * Create a new MoveAction
         * @param mapFrame The MapFrame, this action belongs to.
         */
        public MoveAction(MapFrame mapFrame) {
                super("Move", "move", "Move selected objects around.", "M", KeyEvent.VK_M, 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);
        }

        
        /**
         * If the left mouse button is pressed, move all currently selected
         * objects.
         */
        @Override
        public void mouseDragged(MouseEvent e) {
                if ((e.getModifiersEx() & MouseEvent.BUTTON1_DOWN_MASK) == 0)
                        return;
                
                if (mousePos == null) {
                        mousePos = e.getPoint();
                        singleOsmPrimitive = null;
                }

                GeoPoint mouseGeo = mv.getPoint(e.getX(), e.getY(), false);
                GeoPoint mouseStartGeo = mv.getPoint(mousePos.x, mousePos.y, false);
                double dx = mouseGeo.x - mouseStartGeo.x;
                double dy = mouseGeo.y - mouseStartGeo.y;
                if (dx == 0 && dy == 0)
                        return;

                Collection<OsmPrimitive> selection = Main.main.ds.getSelected();
                Collection<Node> affectedNodes = MoveCommand.getAffectedNodes(selection);
                
                // check if any coordinate would be outside the world
                for (OsmPrimitive osm : affectedNodes) {
                        if (osm instanceof Node && ((Node)osm).coor.isOutSideWorld()) {
                                JOptionPane.showMessageDialog(Main.main, "Cannot move objects outside of the world.");
                                return;
                        }
                }
                
                Command c = mv.editLayer().lastCommand();
                if (c instanceof MoveCommand && affectedNodes.equals(((MoveCommand)c).objects))
                        ((MoveCommand)c).moveAgain(dx,dy);
                else
                        mv.editLayer().add(new MoveCommand(selection, dx, dy));
                
                mv.repaint();
                mousePos = e.getPoint();
        }

        /**
         * Look, whether any object is selected. If not, select the nearest node.
         * If there are no nodes in the dataset, do nothing.
         * 
         * If the user did not press the left mouse button, do nothing.
         * 
         * Also remember the starting position of the movement and change the mouse 
         * cursor to movement.
         */
        @Override
        public void mousePressed(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON1)
                        return;

                if (Main.main.ds.getSelected().size() == 0) {
                        OsmPrimitive osm = mv.getNearest(e.getPoint(), (e.getModifiersEx() & MouseEvent.ALT_DOWN_MASK) != 0);
                        if (osm != null)
                                osm.setSelected(true);
                        singleOsmPrimitive = osm;
                        mv.repaint();
                } else
                        singleOsmPrimitive = null;
                
                mousePos = e.getPoint();
                oldCursor = mv.getCursor();
                mv.setCursor(Cursor.getPredefinedCursor(Cursor.MOVE_CURSOR));
        }
        
        /**
         * Restore the old mouse cursor.
         */
        @Override
        public void mouseReleased(MouseEvent e) {
                mv.setCursor(oldCursor);
                if (singleOsmPrimitive != null) {
                        singleOsmPrimitive.setSelected(false);
                        mv.repaint();
                }
        }
}