source: josm/src/org/openstreetmap/josm/data/projection/Projection.java@ 39

package org.openstreetmap.josm.data.projection;

import java.util.LinkedList;
import java.util.List;

import javax.swing.JComponent;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

import org.openstreetmap.josm.data.Bounds;
import org.openstreetmap.josm.data.GeoPoint;

/**
 * Classes subclass this are able to convert lat/lon values to 
 * planear screen coordinates.
 * 
 * @author imi
 */
abstract public class Projection implements Cloneable {

        /**
         * The event list with all state chaned listener
         */
        List<ChangeListener> listener = new LinkedList<ChangeListener>();
        
        /**
         * Convert from lat/lon to northing/easting. 
         * 
         * @param p             The geo point to convert. x/y members of the point are filled.
         */
        abstract public void latlon2xy(GeoPoint p);
        
        /**
         * Convert from norting/easting to lat/lon.
         * 
         * @param p             The geo point to convert. lat/lon members of the point are filled.
         */
        abstract public void xy2latlon(GeoPoint p);

        
        // description functions
        
        /**
         * Describe the projection converter in one or two words.
         */
        @Override
        abstract public String toString();
        
        // miscellous functions
        
        /**
         * If the projection supports any configuration, this function return
         * the configuration panel. If no configuration needed, 
         * return <code>null</code>.
         * 
         * The items on the configuration panel should not update the configuration
         * directly, but remember changed settings so a call to commitConfigurationPanel
         * can set them.
         * 
         * This function also rolls back all changes to the configuration panel interna
         * components.
         */
        abstract public JComponent getConfigurationPanel();
        /**
         * Commits any changes from components created by addToConfigurationPanel.
         * The projection should now obtain the new settings. If any setting has
         * changed, the implementation have to call to fireStateChanged to inform
         * the listeners.
         */
        abstract public void commitConfigurationPanel();

        /**
         * Initialize itself with the given bounding rectangle (regarding lat/lon).
         * 
         * This function should initialize own parameters needed to do the
         * projection at best effort.
         * 
         * Init must not fire an state changed event, since it is usually called
         * during the initialization of the mapFrame.
         *
         * This implementation does nothing. It is provided only for subclasses
         * to initialize their data members.
         */
        public void init(Bounds b) {}
        
        /**
         * Add an event listener to the state changed event queue. If passed 
         * <code>null</code>, nothing happens.
         */
        public final void addChangeListener(ChangeListener l) {
                if (l != null)
                        listener.add(l);
        }
        /**
         * Remove an event listener from the event queue. If passed 
         * <code>null</code>, nothing happens.
         */
        public final void removeChangeListener(ChangeListener l) {
                listener.remove(l);
        }
        /**
         * Fire an ChangeEvent to every listener on the queue.
         */
        public final void fireStateChanged() {
                ChangeEvent e = null;
                for(ChangeListener l : listener) {
                        if (e == null)
                                e = new ChangeEvent(this);
                        l.stateChanged(e);
                }
        }
}