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 structures can be combined into a single
029 * one. Of course, there are many possible ways of implementing such a combination, e.g. constructing a union, an
030 * intersection, or an "override" structure (were nodes in the first hierarchy take precedence over nodes in the second
031 * hierarchy). This abstract base class only provides some helper methods and defines the common interface for node
032 * combiners. Concrete sub classes will implement the diverse combination algorithms.
033 * </p>
034 * <p>
035 * For some concrete combiner implementations it is important to distinguish whether a node is a single node or whether
036 * it belongs to a list structure. Alone from the input structures, the combiner will not always be able to make this
037 * decision. So sometimes it may be necessary for the developer to configure the combiner and tell it, which nodes
038 * should be treated as list nodes. For this purpose the {@code addListNode()} method exists. It can be passed the name
039 * of a node, which should be considered a list node.
040 * </p>
041 *
042 * @since 1.3
043 */
044public abstract class NodeCombiner {
045    /**
046     * A default handler object for immutable nodes. This object can be used by derived classes for dealing with nodes.
047     * However, it provides only limited functionality; it supports only operations on child nodes, but no references to
048     * parent nodes.
049     */
050    protected static final NodeHandler<ImmutableNode> HANDLER = createNodeHandler();
051
052    /** Stores a list with node names that are known to be list nodes. */
053    private final Set<String> listNodes;
054
055    /**
056     * Creates a new instance of {@code NodeCombiner}.
057     */
058    public NodeCombiner() {
059        listNodes = new HashSet<>();
060    }
061
062    /**
063     * Adds the name of a node to the list of known list nodes. This means that nodes with this name will never be combined.
064     *
065     * @param nodeName the name to be added
066     */
067    public void addListNode(final String nodeName) {
068        listNodes.add(nodeName);
069    }
070
071    /**
072     * Gets a set with the names of nodes that are known to be list nodes.
073     *
074     * @return a set with the names of list nodes
075     */
076    public Set<String> getListNodes() {
077        return Collections.unmodifiableSet(listNodes);
078    }
079
080    /**
081     * Checks if a node is a list node. This implementation tests if the given node name is contained in the set of known
082     * list nodes. Derived classes which use different criteria may overload this method.
083     *
084     * @param node the node to be tested
085     * @return a flag whether this is a list node
086     */
087    public boolean isListNode(final ImmutableNode node) {
088        return listNodes.contains(node.getNodeName());
089    }
090
091    /**
092     * Combines the hierarchies represented by the given root nodes. This method must be defined in concrete sub classes
093     * with the implementation of a specific combination algorithm.
094     *
095     * @param node1 the first root node
096     * @param node2 the second root node
097     * @return the root node of the resulting combined node structure
098     */
099    public abstract ImmutableNode combine(ImmutableNode node1, ImmutableNode node2);
100
101    /**
102     * Creates a node handler object for immutable nodes which can be used by sub classes to perform advanced operations on
103     * nodes.
104     *
105     * @return the node handler implementation
106     */
107    private static NodeHandler<ImmutableNode> createNodeHandler() {
108        return new AbstractImmutableNodeHandler() {
109            @Override
110            public ImmutableNode getParent(final ImmutableNode node) {
111                return null;
112            }
113
114            @Override
115            public ImmutableNode getRootNode() {
116                return null;
117            }
118        };
119    }
120}