source: josm/src/org/openstreetmap/josm/actions/mapmode/SelectionAction.java@ 94

package org.openstreetmap.josm.actions.mapmode;

import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.util.Collection;
import java.util.LinkedList;

import org.openstreetmap.josm.Main;
import org.openstreetmap.josm.data.osm.OsmPrimitive;
import org.openstreetmap.josm.gui.MapFrame;
import org.openstreetmap.josm.gui.SelectionManager;
import org.openstreetmap.josm.gui.SelectionManager.SelectionEnded;

/**
 * This MapMode enables the user to easy make a selection of different objects.
 * 
 * The selected objects are drawn in a different style.
 * 
 * Holding and dragging the left mouse button draws an selection rectangle. 
 * When releasing the left mouse button, all objects within the rectangle get 
 * selected. 
 * 
 * When releasing the left mouse button while the right mouse button pressed,
 * nothing happens (the selection rectangle will be cleared, however).
 *
 * When releasing the mouse button and one of the following keys was hold:
 *
 * If Alt key was hold, select all objects that are touched by the 
 * selection rectangle. If the Alt key was not hold, select only those objects 
 * completly within (e.g. for ways mean: only if all nodes of the way are 
 * within).  
 *
 * If Shift key was hold, the objects are added to the current selection. If
 * Shift key wasn't hold, the current selection get replaced.
 * 
 * If Ctrl key was hold, remove all objects under the current rectangle from
 * the active selection (if there were any). Nothing is added to the current
 * selection.
 *
 * Alt can be combined with Ctrl or Shift. Ctrl and Shift cannot be combined.
 * If both are pressed, nothing happens when releasing the mouse button.
 *
 * The user can also only click on the map. All total movements of 2 or less 
 * pixel are considered "only click". If that happens, the nearest Node will
 * be selected if there is any within 10 pixel range. If there is no Node within
 * 10 pixel, the nearest Segment (or Street, if user hold down the Alt-Key)
 * within 10 pixel range is selected. If there is no Segment within 10 pixel
 * and the user clicked in or 10 pixel away from an area, this area is selected. 
 * If there is even no area, nothing is selected. Shift and Ctrl key applies to 
 * this as usual. For more, @see MapView#getNearest(Point, boolean)
 *
 * @author imi
 */
public class SelectionAction extends MapMode implements SelectionEnded {

        /**
         * The SelectionManager that manages the selection rectangle.
         */
        private SelectionManager selectionManager;

        /**
         * Create a new SelectionAction in the given frame.
         * @param mapFrame The frame this action belongs to
         */
        public SelectionAction(MapFrame mapFrame) {
                super("Selection", "selection", "Select objects by dragging or clicking.", "S", KeyEvent.VK_S, mapFrame);
                this.selectionManager = new SelectionManager(this, false, mv);
        }

        @Override public void enterMode() {
                super.enterMode();
                selectionManager.register(mv);
        }

        @Override public void exitMode() {
                super.exitMode();
                selectionManager.unregister(mv);
        }


        /**
         * Check the state of the keys and buttons and set the selection accordingly.
         */
        public void selectionEnded(Rectangle r, boolean alt, boolean shift, boolean ctrl) {
                if (shift && ctrl)
                        return; // not allowed together

                Collection<OsmPrimitive> curSel;
                if (!ctrl && !shift)
                        curSel = new LinkedList<OsmPrimitive>(); // new selection will replace the old.
                else
                        curSel = Main.ds.getSelected(); 

                Collection<OsmPrimitive> selectionList = selectionManager.getObjectsInRectangle(r,alt);
                for (OsmPrimitive osm : selectionList)
                        if (ctrl)
                                curSel.remove(osm);
                        else
                                curSel.add(osm);
                Main.ds.setSelected(curSel);
                mv.repaint();
        }
}