Remove path and file information from a filename returning only its extension component : Path « File « Java Tutorial

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2. File
3. Path

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

/*

 Derby - Class org.apache.derby.iapi.util.PropertyUtil

 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.

 */

public class Main {


  /**
   * Remove path and file information from a filename returning only its
   * extension  component
   *
   * @param uri The filename
   * @return The filename extension (with starting dot!) or null if filename extension is not found
   */
  public static String getExtension(String uri) {
      int dot = uri.lastIndexOf('.');
      if (dot > -1) {
          uri = uri.substring(dot);
          int slash = uri.lastIndexOf('/');
          if (slash > -1) {
              return null;
          } else {
              int sharp = uri.lastIndexOf('#');
              if (sharp > -1) {
                  // uri starts with dot already
                  return uri.substring(0, sharp);
              } else {
                  int mark = uri.lastIndexOf('?');
                  if (mark > -1) {
                      // uri starts with dot already
                      return uri.substring(0, mark);
                  } else {
                      return uri;
                  }
              }
          }
      } else {
          return null;
      }
  }


}