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}