src/main/java/org/apache/commons/configuration2/tree/ExpressionEngine.java

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 * 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
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
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 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
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 * See the License for the specific language governing permissions and
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package org.apache.commons.configuration2.tree;

import java.util.List;

/**
 * <p>
 * Definition of an interface for evaluating keys for hierarchical configurations.
 * </p>
 * <p>
 * An <em>expression engine</em> knows how to map a key for a configuration's property to a single or a set of
 * configuration nodes. Thus it defines the way how properties are addressed in this configuration. Methods of a
 * configuration that have to handle property keys (for example {@code getProperty()} or {@code addProperty()} do not interpret
 * the passed in keys on their own, but delegate this task to an associated expression engine. This expression engine
 * will then find out, which configuration nodes are addressed by the key.
 * </p>
 * <p>
 * Separating the task of evaluating property keys from the configuration object has the advantage that multiple
 * different expression languages (i.e. ways for querying or setting properties) can be supported. Just set a suitable
 * implementation of this interface as the configuration's expression engine, and you can use the syntax provided by
 * this implementation.
 * </p>
 * <p>
 * An {@code ExpressionEngine} can deal with nodes of different types. To achieve this, a {@link NodeHandler} for the
 * desired type must be passed to the methods.
 * </p>
 *
 * @since 1.3
 */
public interface ExpressionEngine {

    /**
     * Returns the key of an attribute. The passed in {@code parentKey} must reference the parent node of the attribute. A
     * concrete implementation must concatenate this parent key with the attribute name to a valid key for this attribute.
     *
     * @param parentKey the key to the node owning this attribute
     * @param attributeName the name of the attribute in question
     * @return the resulting key referencing this attribute
     */
    String attributeKey(String parentKey, String attributeName);

    /**
     * Determines a &quot;canonical&quot; key for the specified node in the expression language supported by this
     * implementation. This means that always a unique key if generated pointing to this specific node. For most concrete
     * implementations, this means that an index is added to the node name to ensure that there are no ambiguities with
     * child nodes having the same names.
     *
     * @param <T> the type of the node to be processed
     * @param node the node, for which the key must be constructed
     * @param parentKey the key of this node's parent (can be <strong>null</strong> for the root node)
     * @param handler the {@code NodeHandler} for accessing the node
     * @return the canonical key of this node
     */
    <T> String canonicalKey(T node, String parentKey, NodeHandler<T> handler);

    /**
     * Returns the key for the specified node in the expression language supported by an implementation. This method is
     * called whenever a property key for a node has to be constructed, for example by the
     * {@link org.apache.commons.configuration2.Configuration#getKeys() getKeys()} method.
     *
     * @param <T> the type of the node to be processed
     * @param node the node, for which the key must be constructed
     * @param parentKey the key of this node's parent (can be <strong>null</strong> for the root node)
     * @param handler the {@code NodeHandler} for accessing the node
     * @return this node's key
     */
    <T> String nodeKey(T node, String parentKey, NodeHandler<T> handler);

    /**
     * Returns information needed for an add operation. This method gets called when new properties are to be added to a
     * configuration. An implementation has to interpret the specified key, find the parent node for the new elements, and
     * provide all information about new nodes to be added.
     *
     * @param <T> the type of the node to be processed
     * @param root the root node
     * @param key the key for the new property
     * @param handler the {@code NodeHandler} for accessing the node
     * @return an object with all information needed for the add operation
     */
    <T> NodeAddData<T> prepareAdd(T root, String key, NodeHandler<T> handler);

    /**
     * Finds the nodes and/or attributes that are matched by the specified key. This is the main method for interpreting
     * property keys. An implementation must traverse the given root node and its children to find all results that are
     * matched by the given key. If the key is not correct in the syntax provided by that implementation, it is free to
     * throw a (runtime) exception indicating this error condition. The passed in {@code NodeHandler} can be used to gather
     * the required information from the node object.
     *
     * @param <T> the type of the node to be processed
     * @param root the root node of a hierarchy of nodes
     * @param key the key to be evaluated
     * @param handler the {@code NodeHandler} for accessing the node
     * @return a list with the results that are matched by the key (should never be <strong>null</strong>)
     */
    <T> List<QueryResult<T>> query(T root, String key, NodeHandler<T> handler);
}