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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *     http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.configuration.tree;
18  
19  import java.util.Collections;
20  import java.util.HashSet;
21  import java.util.Set;
22  
23  /***
24   * <p>
25   * A base class for node combiner implementations.
26   * </p>
27   * <p>
28   * A <em>node combiner</em> is an object that knows how two hierarchical node
29   * structures can be combined into a single one. Of course, there are many
30   * possible ways of implementing such a combination, e.g. constructing a union,
31   * an intersection, or an "override" structure (were nodes in the first
32   * hierarchy take precedence over nodes in the second hierarchy). This abstract
33   * base class only provides some helper methods and defines the common interface
34   * for node combiners. Concrete sub classes will implement the diverse
35   * combination algorithms.
36   * </p>
37   * <p>
38   * For some concrete combiner implementations it is important to distinguish
39   * whether a node is a single node or whether it belongs to a list structure.
40   * Alone from the input structures, the combiner will not always be able to make
41   * this decision. So sometimes it may be necessary for the developer to
42   * configure the combiner and tell it, which nodes should be treated as list
43   * nodes. For this purpose the <code>addListNode()</code> method exists. It
44   * can be passed the name of a node, which should be considered a list node.
45   * </p>
46   *
47   * @author <a
48   * href="http://jakarta.apache.org/commons/configuration/team-list.html">Commons
49   * Configuration team</a>
50   * @version $Id: NodeCombiner.java 439648 2006-09-02 20:42:10Z oheger $
51   * @since 1.3
52   */
53  public abstract class NodeCombiner
54  {
55      /*** Stores a list with node names that are known to be list nodes. */
56      protected Set listNodes;
57  
58      /***
59       * Creates a new instance of <code>NodeCombiner</code>.
60       */
61      public NodeCombiner()
62      {
63          listNodes = new HashSet();
64      }
65  
66      /***
67       * Adds the name of a node to the list of known list nodes. This means that
68       * nodes with this name will never be combined.
69       *
70       * @param nodeName the name to be added
71       */
72      public void addListNode(String nodeName)
73      {
74          listNodes.add(nodeName);
75      }
76  
77      /***
78       * Returns a set with the names of nodes that are known to be list nodes.
79       *
80       * @return a set with the names of list nodes
81       */
82      public Set getListNodes()
83      {
84          return Collections.unmodifiableSet(listNodes);
85      }
86  
87      /***
88       * Checks if a node is a list node. This implementation tests if the given
89       * node name is contained in the set of known list nodes. Derived classes
90       * which use different criteria may overload this method.
91       *
92       * @param node the node to be tested
93       * @return a flag whether this is a list node
94       */
95      public boolean isListNode(ConfigurationNode node)
96      {
97          return listNodes.contains(node.getName());
98      }
99  
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 }