code: @Override
      public Set<Span> getChildren(String tenant, String id) {
          if (id == null) {
              throw new NullPointerException("Id should not be null!");
          }
  
          Set<Span> children = childrenCache.get(id);
  
          return children == null ? null : Collections.unmodifiableSet(children);
      }
docs: Note that method assumes that id was changed with {@link SpanUniqueIdGenerator#toUnique(Span)}.