source: josm/src/org/openstreetmap/josm/data/osm/Key.java@ 8

package org.openstreetmap.josm.data.osm;

import java.util.Collection;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;

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


/**
 * A key can be associated together with a value to any osm primitive.
 *
 * @author imi
 */
public class Key extends OsmPrimitive {

        /**
         * The key's name
         */
        public final String name;

        /**
         * All keys are stored here.
         */
        private static Map<String, Key> allKeys = new HashMap<String, Key>();
        
        /**
         * Generate a key with the given name. You cannot call this directly but
         * have to use the static constructor. This makes sure, you get every key
         * only once.
         */
        private Key(String name) {
                this.name = name;
        }

        /**
         * Get an instance of the key with the given name.
         * @param name  The name of the key to get.
         * @return An shared instance of the key with the given name. 
         */
        public static Key get(String name) {
                Key key = allKeys.get(name);
                if (key == null) {
                        key = new Key(name);
                        allKeys.put(name, key);
                }
                return key;
        }
        
        /**
         * Return an empty list, since keys cannot have nodes. 
         */
        @Override
        public Collection<Node> getAllNodes() {
                return new LinkedList<Node>();
        }

        /**
         * Keys are equal, when their name is equal, regardless of their other keys.
         */
        @Override
        public boolean equals(Object obj) {
                if (!(obj instanceof Key))
                        return false;
                return name.equals(((Key)obj).name);
        }

        @Override
        public int hashCode() {
                return name.hashCode();
        }

        @Override
        public void visit(Visitor visitor) {
                visitor.visit(this);
        }
}