Merges the two paths to create a valid version of the second path : Path « File « Java Tutorial

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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.
 *
 */

import java.io.File;

/**
 * 
 * FileUtils is a collection of routines for common file system operations.
 * 
 * @author Dan Jemiolo (danj)
 * 
 */

public final class FileUtils {

  /**
   * 
   * Merges the two paths to create a valid version of the second path. This
   * method should be used when you encounter a relative path in a document and
   * must resolve it based on the path of the current document. An example would
   * be: <br>
   * <br>
   * <b>original path</b> - files/customers/Orders.xml <br>
   * <br>
   * <b>relative path</b> - ../Accounts.xml <br>
   * <br>
   * <b>result</b> - files/customers/Accounts.xml <br>
   * <br>
   * The only time this method cannot be used is if the original path is for a
   * file that is in the root (has no directory as part of its path) and the
   * relative path starts with "..".
   * 
   * @param originalPath
   *          The path of the file that references another file.
   * 
   * @param relativePath
   *          The path of the other file, which is relative to the original.
   * 
   * @return A proper path for the other file, one that can be used to open and
   *         verify the file.
   * 
   */
  public static String createRelativePath(String originalPath, String relativePath) {
    if (originalPath == null)
      throw new NullPointerException("NullOriginalPath");

    if (relativePath == null)
      throw new NullPointerException("NullRelativePath");

    //
    // remove ./ if present
    //
    if (relativePath.startsWith("./"))
      relativePath = relativePath.substring(2);

    //
    // remove any .. reference by taking off the last section/ of
    // the original path
    //
    if (relativePath.startsWith("../")) {
      int slash = originalPath.lastIndexOf('/');
      originalPath = originalPath.substring(0, slash);
      relativePath = relativePath.substring(3);
    }

    int slash = originalPath.lastIndexOf('/');

    if (slash < 0)
      return relativePath;

    String dir = originalPath.substring(0, slash + 1);
    return dir + relativePath;
  }

}