code: public void wrap(final FileChannel fileChannel, final long offset, final long length)
      {
          unmap();
          this.fileChannel = fileChannel;
          map(offset, length);
      }
docs: Remap the buffer based on a new file, offset, and a length
  
  @param fileChannel the file to map
  @param offset      the offset of the file to start the mapping
  @param length      of the buffer from the given address