@CanIgnoreReturnValue
  public DirectoryEntry requireDoesNotExist(Path pathForException)
      throws FileAlreadyExistsException {
    if (exists()) {
      throw new FileAlreadyExistsException(pathForException.toString());
    }
    return this;
  }
Checks that this entry does not exist, throwing an exception if it does.

@return this
@throws FileAlreadyExistsException if this entry does not exist
jimfs/src/main/java/com/google/common/jimfs/DirectoryEntry.java