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 * @version $Id: NodeCombiner.java 1790899 2017-04-10 21:56:46Z ggregory $
048 * @since 1.3
049 */
050public abstract class NodeCombiner
051{
052    /**
053     * A default handler object for immutable nodes. This object can be used by
054     * derived classes for dealing with nodes. However, it provides only limited
055     * functionality; it supports only operations on child nodes, but no
056     * references to parent nodes.
057     */
058    protected static final NodeHandler<ImmutableNode> HANDLER =
059            createNodeHandler();
060
061    /** Stores a list with node names that are known to be list nodes. */
062    private final Set<String> listNodes;
063
064    /**
065     * Creates a new instance of {@code NodeCombiner}.
066     */
067    public NodeCombiner()
068    {
069        listNodes = new HashSet<>();
070    }
071
072    /**
073     * Adds the name of a node to the list of known list nodes. This means that
074     * nodes with this name will never be combined.
075     *
076     * @param nodeName the name to be added
077     */
078    public void addListNode(String nodeName)
079    {
080        listNodes.add(nodeName);
081    }
082
083    /**
084     * Returns a set with the names of nodes that are known to be list nodes.
085     *
086     * @return a set with the names of list nodes
087     */
088    public Set<String> getListNodes()
089    {
090        return Collections.unmodifiableSet(listNodes);
091    }
092
093    /**
094     * Checks if a node is a list node. This implementation tests if the given
095     * node name is contained in the set of known list nodes. Derived classes
096     * which use different criteria may overload this method.
097     *
098     * @param node the node to be tested
099     * @return a flag whether this is a list node
100     */
101    public boolean isListNode(ImmutableNode node)
102    {
103        return listNodes.contains(node.getNodeName());
104    }
105
106    /**
107     * Combines the hierarchies represented by the given root nodes. This method
108     * must be defined in concrete sub classes with the implementation of a
109     * specific combination algorithm.
110     *
111     * @param node1 the first root node
112     * @param node2 the second root node
113     * @return the root node of the resulting combined node structure
114     */
115    public abstract ImmutableNode combine(ImmutableNode node1,
116            ImmutableNode node2);
117
118    /**
119     * Creates a node handler object for immutable nodes which can be used by
120     * sub classes to perform advanced operations on nodes.
121     *
122     * @return the node handler implementation
123     */
124    private static NodeHandler<ImmutableNode> createNodeHandler()
125    {
126        return new AbstractImmutableNodeHandler()
127        {
128            @Override
129            public ImmutableNode getParent(ImmutableNode node)
130            {
131                return null;
132            }
133
134            @Override
135            public ImmutableNode getRootNode()
136            {
137                return null;
138            }
139        };
140    }
141}