code: public MappedByteBuffer map(long offset, int length) throws IOException {
          offset += contentStart;
          if (mode == AccessMode.READ_ONLY) {
              return channel.map(FileChannel.MapMode.READ_ONLY, offset, length);
          } else {
              return channel.map(FileChannel.MapMode.READ_WRITE, offset, length);
          }
      }
docs: Create a {@link MappedByteBuffer} mapping the given region. The size of the HEADER will be respected
  automatically. <br>
  <br>
  WARNING: This method is for professional use only.
  
  @param offset
  @param length
  @return a {@link MappedByteBuffer} pointing to the part of the file specified by offset and length.
  @throws IOException