public Set<String> getNamespacesWithQuotas() {
      Set<String> namespaces = new HashSet<>();
      for (TableName tn : tablesWithNamespaceQuotas) {
        namespaces.add(tn.getNamespaceAsString());
      }
      return namespaces;
    }
Returns an unmodifiable view of all tables in namespaces that have namespace quotas.
hbase-server/src/main/java/org/apache/hadoop/hbase/quotas/QuotaObserverChore.java