/*
 * 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.io;

import java.net.URL;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;

 A specialized implementation of a FileLocationStrategy which encapsulates an arbitrary number of FileLocationStrategy objects. 
 A collection with the wrapped FileLocationStrategy objects is passed at construction time. During a [locate() operation the wrapped strategies are called one after the other until one returns a non null URL. This URL is returned. If none of the wrapped strategies is able to resolve the passed in FileLocator, result is null. This is
similar to the chain of responsibility design pattern.

 This class, together with the provided concrete FileLocationStrategy implementations, offers a convenient way to customize the lookup for configuration files: Just add the desired concrete strategies to a CombinedLocationStrategy object. If necessary, custom strategies can be implemented if there are specific requirements. Note that the order in which strategies are added to a CombinedLocationStrategy matters: sub strategies are queried in the same order as they appear in the collection passed to the constructor. 
Since:
2.0/**
 * <p>
 * A specialized implementation of a {@code FileLocationStrategy} which
 * encapsulates an arbitrary number of {@code FileLocationStrategy} objects.
 * </p>
 * <p>
 * A collection with the wrapped {@code FileLocationStrategy} objects is passed
 * at construction time. During a [{@code locate()} operation the wrapped
 * strategies are called one after the other until one returns a non <b>null</b>
 * URL. This URL is returned. If none of the wrapped strategies is able to
 * resolve the passed in {@link FileLocator}, result is <b>null</b>. This is
 * similar to the <em>chain of responsibility</em> design pattern.
 * </p>
 * <p>
 * This class, together with the provided concrete {@code FileLocationStrategy}
 * implementations, offers a convenient way to customize the lookup for
 * configuration files: Just add the desired concrete strategies to a
 * {@code CombinedLocationStrategy} object. If necessary, custom strategies can
 * be implemented if there are specific requirements. Note that the order in
 * which strategies are added to a {@code CombinedLocationStrategy} matters: sub
 * strategies are queried in the same order as they appear in the collection
 * passed to the constructor.
 * </p>
 *
 * @since 2.0
 */
public class CombinedLocationStrategy implements FileLocationStrategy
{
    
A collection with all sub strategies managed by this object. /** A collection with all sub strategies managed by this object. */
    private final Collection<FileLocationStrategy> subStrategies;

    
Creates a new instance of CombinedLocationStrategy and initializes it with the provided sub strategies. The passed in collection must not be null or contain null elements.
Params:
subs – the collection with sub strategies
Throws:
IllegalArgumentException – if the collection is null or has
        null elements/**
     * Creates a new instance of {@code CombinedLocationStrategy} and
     * initializes it with the provided sub strategies. The passed in collection
     * must not be <b>null</b> or contain <b>null</b> elements.
     *
     * @param subs the collection with sub strategies
     * @throws IllegalArgumentException if the collection is <b>null</b> or has
     *         <b>null</b> elements
     */
    public CombinedLocationStrategy(
            final Collection<? extends FileLocationStrategy> subs)
    {
        if (subs == null)
        {
            throw new IllegalArgumentException(
                    "Collection with sub strategies must not be null!");
        }
        subStrategies =
                Collections
                        .unmodifiableCollection(new ArrayList<>(
                                subs));
        if (subStrategies.contains(null))
        {
            throw new IllegalArgumentException(
                    "Collection with sub strategies contains null entry!");
        }
    }

    
Returns a (unmodifiable) collection with the sub strategies managed by
this object.
Returns:
the sub FileLocationStrategy objects/**
     * Returns a (unmodifiable) collection with the sub strategies managed by
     * this object.
     *
     * @return the sub {@code FileLocationStrategy} objects
     */
    public Collection<FileLocationStrategy> getSubStrategies()
    {
        return subStrategies;
    }

    
{@inheritDoc} This implementation tries to locate the file by delegating
to the managed sub strategies.
/**
     * {@inheritDoc} This implementation tries to locate the file by delegating
     * to the managed sub strategies.
     */
    @Override
    public URL locate(final FileSystem fileSystem, final FileLocator locator)
    {
        for (final FileLocationStrategy sub : getSubStrategies())
        {
            final URL url = sub.locate(fileSystem, locator);
            if (url != null)
            {
                return url;
            }
        }

        return null;
    }
}