    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 */
    017package org.apache.commons.configuration2.tree;
    018
    019import java.util.Collections;
    020import java.util.HashSet;
    021import 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()} method exists. It
    044 * can be passed the name of a node, which should be considered a list node.
    045 * </p>
    046 *
    047 * @since 1.3
    048 */
    049public abstract class NodeCombiner
    050{
    051    /**
    052     * A default handler object for immutable nodes. This object can be used by
    053     * derived classes for dealing with nodes. However, it provides only limited
    054     * functionality; it supports only operations on child nodes, but no
    055     * references to parent nodes.
    056     */
    057    protected static final NodeHandler<ImmutableNode> HANDLER =
    058            createNodeHandler();
    059
    060    /** Stores a list with node names that are known to be list nodes. */
    061    private final Set<String> listNodes;
    062
    063    /**
    064     * Creates a new instance of {@code NodeCombiner}.
    065     */
    066    public NodeCombiner()
    067    {
    068        listNodes = new HashSet<>();
    069    }
    070
    071    /**
    072     * Adds the name of a node to the list of known list nodes. This means that
    073     * nodes with this name will never be combined.
    074     *
    075     * @param nodeName the name to be added
    076     */
    077    public void addListNode(final String nodeName)
    078    {
    079        listNodes.add(nodeName);
    080    }
    081
    082    /**
    083     * Returns a set with the names of nodes that are known to be list nodes.
    084     *
    085     * @return a set with the names of list nodes
    086     */
    087    public Set<String> getListNodes()
    088    {
    089        return Collections.unmodifiableSet(listNodes);
    090    }
    091
    092    /**
    093     * Checks if a node is a list node. This implementation tests if the given
    094     * node name is contained in the set of known list nodes. Derived classes
    095     * which use different criteria may overload this method.
    096     *
    097     * @param node the node to be tested
    098     * @return a flag whether this is a list node
    099     */
    100    public boolean isListNode(final ImmutableNode node)
    101    {
    102        return listNodes.contains(node.getNodeName());
    103    }
    104
    105    /**
    106     * Combines the hierarchies represented by the given root nodes. This method
    107     * must be defined in concrete sub classes with the implementation of a
    108     * specific combination algorithm.
    109     *
    110     * @param node1 the first root node
    111     * @param node2 the second root node
    112     * @return the root node of the resulting combined node structure
    113     */
    114    public abstract ImmutableNode combine(ImmutableNode node1,
    115            ImmutableNode node2);
    116
    117    /**
    118     * Creates a node handler object for immutable nodes which can be used by
    119     * sub classes to perform advanced operations on nodes.
    120     *
    121     * @return the node handler implementation
    122     */
    123    private static NodeHandler<ImmutableNode> createNodeHandler()
    124    {
    125        return new AbstractImmutableNodeHandler()
    126        {
    127            @Override
    128            public ImmutableNode getParent(final ImmutableNode node)
    129            {
    130                return null;
    131            }
    132
    133            @Override
    134            public ImmutableNode getRootNode()
    135            {
    136                return null;
    137            }
    138        };
    139    }
    140}