code: @Nullable
      public Map<Sha256Hash, Integer> getAppearsInHashes() {
          return appearsInHashes != null ? ImmutableMap.copyOf(appearsInHashes) : null;
      }
docs: Returns a map of block [hashes] which contain the transaction mapped to relativity counters, or null if this
  transaction doesn't have that data because it's not stored in the wallet or because it has never appeared in a
  block.