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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 }