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.configuration2.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 structures can be combined into a single
29 * one. Of course, there are many possible ways of implementing such a combination, e.g. constructing a union, an
30 * intersection, or an "override" structure (were nodes in the first hierarchy take precedence over nodes in the second
31 * hierarchy). This abstract base class only provides some helper methods and defines the common interface for node
32 * combiners. Concrete sub classes will implement the diverse combination algorithms.
33 * </p>
34 * <p>
35 * For some concrete combiner implementations it is important to distinguish whether a node is a single node or whether
36 * it belongs to a list structure. Alone from the input structures, the combiner will not always be able to make this
37 * decision. So sometimes it may be necessary for the developer to configure the combiner and tell it, which nodes
38 * should be treated as list nodes. For this purpose the {@code addListNode()} method exists. It can be passed the name
39 * of a node, which should be considered a list node.
40 * </p>
41 *
42 * @since 1.3
43 */
44 public abstract class NodeCombiner {
45 /**
46 * A default handler object for immutable nodes. This object can be used by derived classes for dealing with nodes.
47 * However, it provides only limited functionality; it supports only operations on child nodes, but no references to
48 * parent nodes.
49 */
50 protected static final NodeHandler<ImmutableNode> HANDLER = createNodeHandler();
51
52 /** Stores a list with node names that are known to be list nodes. */
53 private final Set<String> listNodes;
54
55 /**
56 * Creates a new instance of {@code NodeCombiner}.
57 */
58 public NodeCombiner() {
59 listNodes = new HashSet<>();
60 }
61
62 /**
63 * Adds the name of a node to the list of known list nodes. This means that nodes with this name will never be combined.
64 *
65 * @param nodeName the name to be added
66 */
67 public void addListNode(final String nodeName) {
68 listNodes.add(nodeName);
69 }
70
71 /**
72 * Gets a set with the names of nodes that are known to be list nodes.
73 *
74 * @return a set with the names of list nodes
75 */
76 public Set<String> getListNodes() {
77 return Collections.unmodifiableSet(listNodes);
78 }
79
80 /**
81 * Checks if a node is a list node. This implementation tests if the given node name is contained in the set of known
82 * list nodes. Derived classes which use different criteria may overload this method.
83 *
84 * @param node the node to be tested
85 * @return a flag whether this is a list node
86 */
87 public boolean isListNode(final ImmutableNode node) {
88 return listNodes.contains(node.getNodeName());
89 }
90
91 /**
92 * Combines the hierarchies represented by the given root nodes. This method must be defined in concrete sub classes
93 * with the implementation of a specific combination algorithm.
94 *
95 * @param node1 the first root node
96 * @param node2 the second root node
97 * @return the root node of the resulting combined node structure
98 */
99 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 }