src/main/java/org/apache/commons/configuration2/ImmutableHierarchicalConfiguration.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
 *
 * 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;

import java.util.List;

import org.apache.commons.configuration2.tree.ExpressionEngine;

/**
 * <p>
 * An interface for immutable hierarchical configurations.
 * </p>
 * <p>
 * There are some sources of configuration data that cannot be stored very well in a flat configuration object (like
 * {@link BaseConfiguration}) because then their structure is lost. A prominent example are XML documents.
 * </p>
 * <p>
 * This interface extends the basic {@link ImmutableConfiguration} interface by structured access to configuration
 * properties. An {@link ExpressionEngine} is used to evaluate complex property keys and to map them to nodes of a
 * tree-like structure.
 * </p>
 *
 * @since 2.0
 */
public interface ImmutableHierarchicalConfiguration extends ImmutableConfiguration {

    /**
     * Gets the expression engine used by this configuration. This method will never return <strong>null</strong>; if no specific
     * expression engine was set, the default expression engine will be returned.
     *
     * @return the current expression engine
     */
    ExpressionEngine getExpressionEngine();

    /**
     * Gets the maximum defined index for the given key. This is useful if there are multiple values for this key. They
     * can then be addressed separately by specifying indices from 0 to the return value of this method.
     *
     * @param key the key to be checked
     * @return the maximum defined index for this key
     */
    int getMaxIndex(String key);

    /**
     * Gets the name of the root element of this configuration. This information may be of use in some cases, for example for
     * sub configurations created using the {@code immutableConfigurationsAt()} method. The exact meaning of the string
     * returned by this method is specific to a concrete implementation. For instance, an XML configuration might return the
     * name of the document element.
     *
     * @return the name of the root element of this configuration
     */
    String getRootElementName();

    /**
     * Returns a list of immutable configurations for all direct child elements of the node selected by the given key. With
     * this method it is possible to inspect the content of a hierarchical structure; all children of a given node can be
     * queried without having to know their exact names. If the passed in key does not point to a single node, an empty list
     * is returned. This is also the result if the node referred to by the key does not have child elements.
     *
     * @param key the key for selecting the desired parent node
     * @return a collection with immutable configurations for all child nodes of the selected parent node
     */
    List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(String key);

    /**
     * Returns an immutable hierarchical configuration for the node specified by the given key. This is a short form for
     * {@code immutableConfigurationAt(key,
     * <strong>false</strong>)}.
     *
     * @param key the key that selects the sub tree
     * @return a hierarchical configuration that contains this sub tree
     */
    ImmutableHierarchicalConfiguration immutableConfigurationAt(String key);

    /**
     * <p>
     * Returns an immutable hierarchical configuration object that wraps the configuration node specified by the given key.
     * This method provides an easy means of accessing sub trees of a hierarchical configuration. In the returned
     * configuration the sub tree can directly be accessed, it becomes the root node of this configuration. Because of this
     * the passed in key must select exactly one configuration node; otherwise an {@code IllegalArgumentException} will be
     * thrown.
     * </p>
     * <p>
     * The difference between this method and the {@link #immutableSubset(String)} method is that {@code immutableSubset()}
     * supports arbitrary subsets of configuration nodes while {@code immutableConfigurationAt()} only returns a single sub
     * tree. Please refer to the documentation of the {@code SubnodeConfiguration} class to obtain further information about
     * subnode configurations and when they should be used.
     * </p>
     *
     * @param key the key that selects the sub tree
     * @param supportUpdates a flag whether the returned subnode configuration should be able to handle updates of its
     *        parent
     * @return a hierarchical configuration that contains this sub tree
     */
    ImmutableHierarchicalConfiguration immutableConfigurationAt(String key, boolean supportUpdates);

    /**
     * Returns a list of immutable configurations for all configuration nodes selected by the given key. This method will
     * evaluate the passed in key (using the current {@code ExpressionEngine}) and then create an immutable subnode
     * configuration for each returned node (like {@link #immutableConfigurationAt(String)}}). This is especially useful
     * when dealing with list-like structures. As an example consider the configuration that contains data about database
     * tables and their fields. If you need access to all fields of a certain table, you can simply do
     *
     * <pre>
     * List&lt;ImmutableHierarchicalConfiguration&gt; fields =
     *   config.immutableConfigurationsAt("tables.table(0).fields.field");
     * for(Iterator&lt;ImmutableHierarchicalConfiguration&gt; it = fields.iterator();
     *   it.hasNext();)
     * {
     *     ImmutableHierarchicalConfiguration sub = it.next();
     *     // now the children and attributes of the field node can be
     *     // directly accessed
     *     String fieldName = sub.getString("name");
     *     String fieldType = sub.getString("type");
     *     ...
     * </pre>
     *
     * @param key the key for selecting the desired nodes
     * @return a list with immutable hierarchical configuration objects; each configuration represents one of the nodes
     *         selected by the passed in key
     */
    List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(String key);
}