public String getPrefix() {
    return _prefix.get();
  }
@Test
  public void testPrefix() {
    // The prefix should be an empty string if no directory is provided.
    // Otherwise, listFiles will return no matches.
    GcsUri gcsUri = new GcsUri(URI.create("gs://bucket_name/"));
    assertEquals("", gcsUri.getPrefix());
    gcsUri = new GcsUri(URI.create("gs://bucket_name"));
    assertEquals("", gcsUri.getPrefix());
    // The prefix should end with the delimiter if there are directories, which
    // ensures searches do not return false positive matches.
    gcsUri = new GcsUri(URI.create("gs://bucket_name/dir/"));
    assertEquals("dir/", gcsUri.getPrefix());
    gcsUri = new GcsUri(URI.create("gs://bucket_name/dir"));
    assertEquals("dir/", gcsUri.getPrefix());
  }