stanfordnlp/CoreNLP · error · java.lang.RuntimeException

Unexpected class in list while looking up

Error message

Unexpected class in list  while looking up 

What it means

In the exhaustive multi-match lookup routine, each element popped from a List node must be a Phrase or a Map to push on the search stack. Any other class triggers this RuntimeException, signaling the trie's internal structure is not what the search expects.

Solutions

1. Populate the table under a single lock, then use it read-only from multiple threads.
2. Rebuild from the phrase list rather than repairing the corrupted instance.
3. Avoid keeping references to internal Map/List nodes returned by reflection.
4. Upgrade/report to Stanford NLP if single-threaded usage still throws.

Example fix

// before
thread1: table.addPhrase(p1); thread2: table.lookupByRegex(...);
// after
buildTable(); // complete all addPhrase calls first
matches = table.lookupByRegex(...); // now read-only

// finish all addPhrase calls before multi-match lookups
if (building) throw new IllegalStateException("table still being populated");

try { matches = table.lookupByRegex(...); } catch (RuntimeException e) { if (e.getMessage().contains("while looking up")) { table = rebuildTable(phrases); } else { throw e; } }

When it happens

Trigger: Calling the public lookup variants that enumerate all matches on a table whose lists contain foreign objects - after concurrent writes or external corruption.

Common situations: Unsynchronized population followed by multi-match lookups; direct manipulation of internal maps; version-mismatched deserialization.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• More than one map in list while looking up word () in…
• Unexpected class while adding word () in phrase
• More than one map in list while adding word () in phrase
• Unexpected class in list while adding word () in phrase
• Unexpected class in list while converting list to map

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/0319f779fd2876c9. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/ling/tokensregex/PhraseTable.java:826

          tree = (Map<String, Object>) node;
        } else if (node instanceof List) {
          // Search through list for matches to word (at this point, the table is small, so no Map)
          List lookupList = (List) node;
          for (Object obj:lookupList) {
            if (obj instanceof Phrase) {
              // check rest of the phrase matches
              Phrase phrase = (Phrase) obj;
              if (acceptablePhrases == null || acceptablePhrases.contains(phrase)) {
                int matchedTokenEnd = checkWordListMatch(
                  phrase, tokens, cur.tokenStart, cur.tokenEnd, i+1, matchEnd);
                if (matchedTokenEnd >= 0) {
                  matched.add(new PhraseMatch(phrase, cur.tokenStart, matchedTokenEnd));
                }
              }
            } else if (obj instanceof Map) {
              todoStack.push(new StackEntry((Map<String,Object>) obj, cur.tokenStart, i+1, cur.tokenEnd, -1));
            } else  {
              throw new RuntimeException("Unexpected class in list " + obj.getClass() + " while looking up " + word);
            }
          }
          break;
        } else {
          throw new RuntimeException("Unexpected class " + node.getClass() + " while looking up " + word);
        }
      }
      if (cur.continueAt >= 0) {
        int newStart = (cur.continueAt > cur.tokenStart)? cur.continueAt: cur.tokenStart+1;
        if (newStart < cur.tokenEnd) {
          todoStack.push(new StackEntry(cur.tree, newStart, newStart, cur.tokenEnd, newStart+1));
        }
      }
    }
    return matched;
  }

  public Iterator<Phrase> iterator() {

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