    001    /*
    002     * Licensed to the Apache Software Foundation (ASF) under one or more
    003     * contributor license agreements.  See the NOTICE file distributed with
    004     * this work for additional information regarding copyright ownership.
    005     * The ASF licenses this file to You under the Apache License, Version 2.0
    006     * (the "License"); you may not use this file except in compliance with
    007     * the License.  You may obtain a copy of the License at
    008     *
    009     *     http://www.apache.org/licenses/LICENSE-2.0
    010     *
    011     * Unless required by applicable law or agreed to in writing, software
    012     * distributed under the License is distributed on an "AS IS" BASIS,
    013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014     * See the License for the specific language governing permissions and
    015     * limitations under the License.
    016     */
    017    package org.apache.commons.configuration.tree;
    018
    019    import java.util.Collections;
    020    import java.util.HashSet;
    021    import java.util.Set;
    022
    023    /**
    024     * <p>
    025     * A base class for node combiner implementations.
    026     * </p>
    027     * <p>
    028     * A <em>node combiner</em> is an object that knows how two hierarchical node
    029     * structures can be combined into a single one. Of course, there are many
    030     * possible ways of implementing such a combination, e.g. constructing a union,
    031     * an intersection, or an "override" structure (were nodes in the first
    032     * hierarchy take precedence over nodes in the second hierarchy). This abstract
    033     * base class only provides some helper methods and defines the common interface
    034     * for node combiners. Concrete sub classes will implement the diverse
    035     * combination algorithms.
    036     * </p>
    037     * <p>
    038     * For some concrete combiner implementations it is important to distinguish
    039     * whether a node is a single node or whether it belongs to a list structure.
    040     * Alone from the input structures, the combiner will not always be able to make
    041     * this decision. So sometimes it may be necessary for the developer to
    042     * configure the combiner and tell it, which nodes should be treated as list
    043     * nodes. For this purpose the <code>addListNode()</code> method exists. It
    044     * can be passed the name of a node, which should be considered a list node.
    045     * </p>
    046     *
    047     * @author <a
    048     * href="http://commons.apache.org/configuration/team-list.html">Commons
    049     * Configuration team</a>
    050     * @version $Id: NodeCombiner.java 764168 2009-04-11 06:23:13Z rgoers $
    051     * @since 1.3
    052     */
    053    public abstract class NodeCombiner
    054    {
    055        /** Stores a list with node names that are known to be list nodes. */
    056        protected Set listNodes;
    057
    058        /**
    059         * Creates a new instance of <code>NodeCombiner</code>.
    060         */
    061        public NodeCombiner()
    062        {
    063            listNodes = new HashSet();
    064        }
    065
    066        /**
    067         * Adds the name of a node to the list of known list nodes. This means that
    068         * nodes with this name will never be combined.
    069         *
    070         * @param nodeName the name to be added
    071         */
    072        public void addListNode(String nodeName)
    073        {
    074            listNodes.add(nodeName);
    075        }
    076
    077        /**
    078         * Returns a set with the names of nodes that are known to be list nodes.
    079         *
    080         * @return a set with the names of list nodes
    081         */
    082        public Set getListNodes()
    083        {
    084            return Collections.unmodifiableSet(listNodes);
    085        }
    086
    087        /**
    088         * Checks if a node is a list node. This implementation tests if the given
    089         * node name is contained in the set of known list nodes. Derived classes
    090         * which use different criteria may overload this method.
    091         *
    092         * @param node the node to be tested
    093         * @return a flag whether this is a list node
    094         */
    095        public boolean isListNode(ConfigurationNode node)
    096        {
    097            return listNodes.contains(node.getName());
    098        }
    099
    100        /**
    101         * Combines the hierarchies represented by the given root nodes. This method
    102         * must be defined in concrete sub classes with the implementation of a
    103         * specific combination algorithm.
    104         *
    105         * @param node1 the first root node
    106         * @param node2 the second root node
    107         * @return the resulting combined node structure
    108         */
    109        public abstract ConfigurationNode combine(ConfigurationNode node1,
    110                ConfigurationNode node2);
    111
    112        /**
    113         * Creates a new view node. This method will be called whenever a new view
    114         * node is to be created. It can be overriden to create special view nodes.
    115         * This base implementation returns a new instance of
    116         * <code>{@link ViewNode}</code>.
    117         *
    118         * @return the new view node
    119         */
    120        protected ViewNode createViewNode()
    121        {
    122            return new ViewNode();
    123        }
    124    }