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()} 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://commons.apache.org/configuration/team-list.html">Commons 49 * Configuration team</a> 50 * @version $Id: NodeCombiner.java 1206476 2011-11-26 16:19:53Z 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<String> listNodes; 57 58 /** 59 * Creates a new instance of {@code NodeCombiner}. 60 */ 61 public NodeCombiner() 62 { 63 listNodes = new HashSet<String>(); 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<String> 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 overridden to create special view nodes. 115 * This base implementation returns a new instance of 116 * {@link ViewNode}. 117 * 118 * @return the new view node 119 */ 120 protected ViewNode createViewNode() 121 { 122 return new ViewNode(); 123 } 124 }