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 * https://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, for example 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 /**
47 * A default handler object for immutable nodes. This object can be used by derived classes for dealing with nodes.
48 * However, it provides only limited functionality; it supports only operations on child nodes, but no references to
49 * parent nodes.
50 */
51 protected static final NodeHandler<ImmutableNode> HANDLER = createNodeHandler();
52
53 /**
54 * Creates a node handler object for immutable nodes which can be used by sub classes to perform advanced operations on
55 * nodes.
56 *
57 * @return the node handler implementation
58 */
59 private static NodeHandler<ImmutableNode> createNodeHandler() {
60 return new AbstractImmutableNodeHandler() {
61 @Override
62 public ImmutableNode getParent(final ImmutableNode node) {
63 return null;
64 }
65
66 @Override
67 public ImmutableNode getRootNode() {
68 return null;
69 }
70 };
71 }
72
73 /** Stores a list with node names that are known to be list nodes. */
74 private final Set<String> listNodes;
75
76 /**
77 * Creates a new instance of {@code NodeCombiner}.
78 */
79 public NodeCombiner() {
80 listNodes = new HashSet<>();
81 }
82
83 /**
84 * Adds the name of a node to the list of known list nodes. This means that nodes with this name will never be combined.
85 *
86 * @param nodeName the name to be added
87 */
88 public void addListNode(final String nodeName) {
89 listNodes.add(nodeName);
90 }
91
92 /**
93 * Combines the hierarchies represented by the given root nodes. This method must be defined in concrete sub classes
94 * with the implementation of a specific combination algorithm.
95 *
96 * @param node1 the first root node
97 * @param node2 the second root node
98 * @return the root node of the resulting combined node structure
99 */
100 public abstract ImmutableNode combine(ImmutableNode node1, ImmutableNode node2);
101
102 /**
103 * Gets a set with the names of nodes that are known to be list nodes.
104 *
105 * @return a set with the names of list nodes
106 */
107 public Set<String> getListNodes() {
108 return Collections.unmodifiableSet(listNodes);
109 }
110
111 /**
112 * Checks if a node is a list node. This implementation tests if the given node name is contained in the set of known
113 * list nodes. Derived classes which use different criteria may overload this method.
114 *
115 * @param node the node to be tested
116 * @return a flag whether this is a list node
117 */
118 public boolean isListNode(final ImmutableNode node) {
119 return listNodes.contains(node.getNodeName());
120 }
121 }