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The Apache Commons IO library contains utility classes, stream implementations, file filters, file comparators, endian transformation classes, and much more.
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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
 * 
 *      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.io.comparator;

import java.io.File;
import java.io.Serializable;
import java.util.Comparator;

import org.apache.commons.io.FilenameUtils;
import org.apache.commons.io.IOCase;

/**
 * Compare the file name extensions for order
 * (see {@link FilenameUtils#getExtension(String)}).
 * 
 * This comparator can be used to sort lists or arrays of files
 * by their file extension either in a case-sensitive, case-insensitive or
 * system dependent case sensitive way. A number of singleton instances
 * are provided for the various case sensitivity options (using {@link IOCase})
 * and the reverse of those options.
 * 

 * Example of a case-sensitive file extension sort using the
 * {@link #EXTENSION_COMPARATOR} singleton instance:
 * 
 *       List<File> list = ...
 *       ((AbstractFileComparator) ExtensionFileComparator.EXTENSION_COMPARATOR).sort(list);
 * 
 * 
 * Example of a reverse case-insensitive file extension sort using the
 * {@link #EXTENSION_INSENSITIVE_REVERSE} singleton instance:
 * 
 *       File[] array = ...
 *       ((AbstractFileComparator) ExtensionFileComparator.EXTENSION_INSENSITIVE_REVERSE).sort(array);
 * 
 * 
 *
 * @version $Id: ExtensionFileComparator.java 1642757 2014-12-01 21:09:30Z sebb $
 * @since 1.4
 */
public class ExtensionFileComparator extends AbstractFileComparator implements Serializable {

    private static final long serialVersionUID = 1928235200184222815L;

    /** Case-sensitive extension comparator instance (see {@link IOCase#SENSITIVE}) */
    public static final Comparator EXTENSION_COMPARATOR = new ExtensionFileComparator();

    /** Reverse case-sensitive extension comparator instance (see {@link IOCase#SENSITIVE}) */
    public static final Comparator EXTENSION_REVERSE = new ReverseComparator(EXTENSION_COMPARATOR);

    /** Case-insensitive extension comparator instance (see {@link IOCase#INSENSITIVE}) */
    public static final Comparator EXTENSION_INSENSITIVE_COMPARATOR
                                                = new ExtensionFileComparator(IOCase.INSENSITIVE);

    /** Reverse case-insensitive extension comparator instance (see {@link IOCase#INSENSITIVE}) */
    public static final Comparator EXTENSION_INSENSITIVE_REVERSE
                                                = new ReverseComparator(EXTENSION_INSENSITIVE_COMPARATOR);

    /** System sensitive extension comparator instance (see {@link IOCase#SYSTEM}) */
    public static final Comparator EXTENSION_SYSTEM_COMPARATOR = new ExtensionFileComparator(IOCase.SYSTEM);

    /** Reverse system sensitive path comparator instance (see {@link IOCase#SYSTEM}) */
    public static final Comparator EXTENSION_SYSTEM_REVERSE = new ReverseComparator(EXTENSION_SYSTEM_COMPARATOR);

    /** Whether the comparison is case sensitive. */
    private final IOCase caseSensitivity;

    /**
     * Construct a case sensitive file extension comparator instance.
     */
    public ExtensionFileComparator() {
        this.caseSensitivity = IOCase.SENSITIVE;
    }

    /**
     * Construct a file extension comparator instance with the specified case-sensitivity.
     *
     * @param caseSensitivity how to handle case sensitivity, null means case-sensitive
     */
    public ExtensionFileComparator(final IOCase caseSensitivity) {
        this.caseSensitivity = caseSensitivity == null ? IOCase.SENSITIVE : caseSensitivity;
    }

    /**
     * Compare the extensions of two files the specified case sensitivity.
     * 
     * @param file1 The first file to compare
     * @param file2 The second file to compare
     * @return a negative value if the first file's extension
     * is less than the second, zero if the extensions are the
     * same and a positive value if the first files extension
     * is greater than the second file.
     * 
     */
    public int compare(final File file1, final File file2) {
        final String suffix1 = FilenameUtils.getExtension(file1.getName());
        final String suffix2 = FilenameUtils.getExtension(file2.getName());
        return caseSensitivity.checkCompareTo(suffix1, suffix2);
    }

    /**
     * String representation of this file comparator.
     *
     * @return String representation of this file comparator
     */
    @Override
    public String toString() {
        return super.toString() + "[caseSensitivity=" + caseSensitivity + "]";
    }
}