org.apache.commons.configuration2.tree.NodeCombiner Maven / Gradle / Ivy
Show all versions of commons-configuration2 Show documentation
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.configuration2.tree;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
/**
*
* A base class for node combiner implementations.
*
*
* A node combiner is an object that knows how two hierarchical node
* structures can be combined into a single one. Of course, there are many
* possible ways of implementing such a combination, e.g. constructing a union,
* an intersection, or an "override" structure (were nodes in the first
* hierarchy take precedence over nodes in the second hierarchy). This abstract
* base class only provides some helper methods and defines the common interface
* for node combiners. Concrete sub classes will implement the diverse
* combination algorithms.
*
*
* For some concrete combiner implementations it is important to distinguish
* whether a node is a single node or whether it belongs to a list structure.
* Alone from the input structures, the combiner will not always be able to make
* this decision. So sometimes it may be necessary for the developer to
* configure the combiner and tell it, which nodes should be treated as list
* nodes. For this purpose the {@code addListNode()} method exists. It
* can be passed the name of a node, which should be considered a list node.
*
*
* @since 1.3
*/
public abstract class NodeCombiner
{
/**
* A default handler object for immutable nodes. This object can be used by
* derived classes for dealing with nodes. However, it provides only limited
* functionality; it supports only operations on child nodes, but no
* references to parent nodes.
*/
protected static final NodeHandler HANDLER =
createNodeHandler();
/** Stores a list with node names that are known to be list nodes. */
private final Set listNodes;
/**
* Creates a new instance of {@code NodeCombiner}.
*/
public NodeCombiner()
{
listNodes = new HashSet<>();
}
/**
* Adds the name of a node to the list of known list nodes. This means that
* nodes with this name will never be combined.
*
* @param nodeName the name to be added
*/
public void addListNode(final String nodeName)
{
listNodes.add(nodeName);
}
/**
* Returns a set with the names of nodes that are known to be list nodes.
*
* @return a set with the names of list nodes
*/
public Set getListNodes()
{
return Collections.unmodifiableSet(listNodes);
}
/**
* Checks if a node is a list node. This implementation tests if the given
* node name is contained in the set of known list nodes. Derived classes
* which use different criteria may overload this method.
*
* @param node the node to be tested
* @return a flag whether this is a list node
*/
public boolean isListNode(final ImmutableNode node)
{
return listNodes.contains(node.getNodeName());
}
/**
* Combines the hierarchies represented by the given root nodes. This method
* must be defined in concrete sub classes with the implementation of a
* specific combination algorithm.
*
* @param node1 the first root node
* @param node2 the second root node
* @return the root node of the resulting combined node structure
*/
public abstract ImmutableNode combine(ImmutableNode node1,
ImmutableNode node2);
/**
* Creates a node handler object for immutable nodes which can be used by
* sub classes to perform advanced operations on nodes.
*
* @return the node handler implementation
*/
private static NodeHandler createNodeHandler()
{
return new AbstractImmutableNodeHandler()
{
@Override
public ImmutableNode getParent(final ImmutableNode node)
{
return null;
}
@Override
public ImmutableNode getRootNode()
{
return null;
}
};
}
}