source: josm/src/org/openstreetmap/josm/data/osm/OsmPrimitive.java@ 90

package org.openstreetmap.josm.data.osm;

import java.text.SimpleDateFormat;
import java.util.Collection;
import java.util.Collections;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

import org.openstreetmap.josm.data.osm.visitor.Visitor;


/**
 * An OSM primitive can be associated with a key/value pair. It can be created, deleted
 * and updated within the OSM-Server.
 *
 * Although OsmPrimitive is designed as a base class, it is not to be meant to subclass
 * it by any other than from the package org.openstreetmap.josm.data.osm (thus the
 * visibility of the constructor). The available primitives are a fixed set that are given
 * by the server environment and not an extendible data stuff. 
 *
 * @author imi
 */
abstract public class OsmPrimitive {

        /**
         * The key/value list for this primitive.
         */
        public Map<String, String> keys;

        /**
         * Unique identifier in OSM. This is used to reidentify objects in the server.
         * An id of 0 means an unknown id. The object has not been uploaded yet to 
         * know what id it will get.
         */
        public long id = 0;

        /**
         * <code>true</code>, if the object has been modified since it was loaded from
         * the server. In this case, on next upload, this object will be updated.
         * Deleted objects are deleted from the server. If the objects are added (id=0),
         * the modified is ignored and the object is added to the server. 
         */
        public boolean modified = false;

        /**
         * <code>true</code>, if the object has been deleted.
         */
        public boolean deleted = false;

        /**
         * If set to true, this object is currently selected.
         */
        public volatile boolean selected = false;

        /**
         * Time of last modification to this object. This is not set by JOSM but
         * read from the server and delivered back to the server unmodified. It is
         * used to check against edit conflicts.
         */
        public Date timestamp = null;

        /**
         * Implementation of the visitor scheme. Subclases have to call the correct
         * visitor function.
         * @param visitor The visitor from which the visit() function must be called.
         */
        abstract public void visit(Visitor visitor);

        public final void delete(boolean deleted) {
                this.deleted = deleted;
                selected = false;
                modified = true;
        }

        /**
         * Equal, if the id (and class) is equal. If both ids are 0, use the super classes
         * equal instead.
         * 
         * An primitive is equal to its incomplete counter part.
         */
        @Override public final boolean equals(Object obj) {
                if (obj == null || getClass() != obj.getClass() || id == 0 || ((OsmPrimitive)obj).id == 0)
                        return super.equals(obj);
                return id == ((OsmPrimitive)obj).id;
        }

        /**
         * Return the id as hashcode or supers hashcode if 0.
         * 
         * An primitive has the same hashcode as its incomplete counter part.
         */
        @Override public final int hashCode() {
                return id == 0 ? super.hashCode() : (int)id;
        }

        /**
         * Set the given value to the given key
         * @param key The key, for which the value is to be set.
         * @param value The value for the key.
         */
        public final void put(String key, String value) {
                if (value == null)
                        remove(key);
                else {
                        if (keys == null)
                                keys = new HashMap<String, String>();
                        keys.put(key, value);
                }
        }
        /**
         * Remove the given key from the list.
         */
        public final void remove(String key) {
                if (keys != null) {
                        keys.remove(key);
                        if (keys.isEmpty())
                                keys = null;
                }
        }

        public final String get(String key) {
                return keys == null ? null : keys.get(key);
        }

        public final Collection<Entry<String, String>> entrySet() {
                if (keys == null)
                        return Collections.emptyList();
                return keys.entrySet();
        }

        public final Collection<String> keySet() {
                if (keys == null)
                        return Collections.emptyList();
                return keys.keySet();
        }

        /**
         * Get and write all attributes from the parameter. Does not fire any listener, so
         * use this only in the data initializing phase
         */
        public void cloneFrom(OsmPrimitive osm) {
                keys = osm.keys == null ? null : new HashMap<String, String>(osm.keys);
                id = osm.id;
                modified = osm.modified;
                deleted = osm.deleted;
                selected = osm.selected;
                timestamp = osm.timestamp;
        }

        /**
         * Perform an equality compare for all non-volatile fields not only for the id 
         * but for the whole object (for conflict resolving etc)
         */
        public boolean realEqual(OsmPrimitive osm) {
                return
                id == osm.id && 
                modified == osm.modified && 
                deleted == osm.deleted &&
                (timestamp == null ? osm.timestamp==null : timestamp.equals(osm.timestamp)) &&
                (keys == null ? osm.keys==null : keys.equals(osm.keys));
        }
        
        public String getTimeStr() {
                return timestamp == null ? null : new SimpleDateFormat("y-M-d H:m:s").format(timestamp);
        }
}