@edu.umd.cs.findbugs.annotations.SuppressWarnings(value = "NP_NONNULL_PARAM_VIOLATION",
      justification = "it is valid to pass NULL to CompletableFuture#completedFuture")
  private CompletableFuture<Void> closeConn() {
    CompletableFuture<AsyncConnection> f = future.get();
    if (f == null) {
      return CompletableFuture.completedFuture(null);
    }
    CompletableFuture<Void> closeFuture = new CompletableFuture<>();
    addListener(f, (conn, error) -> {
      if (error == null) {
        IOUtils.closeQuietly(conn, e -> LOG.warn("failed to close conn", e));
      }
      closeFuture.complete(null);
    });
    return closeFuture;
  }
For the example we're just using one qualifier.
hbase-examples/src/main/java/org/apache/hadoop/hbase/client/example/AsyncClientExample.java