stanfordnlp/CoreNLP · error · java.lang.RuntimeException

Unexpected class in list while looking up word () in…

Error message

Unexpected class in list  while looking up word () in phrase 

What it means

While walking the trie during lookup, a List node must contain only Phrase and (at most one) Map elements. Encountering any other object type inside the list throws this RuntimeException, indicating the PhraseTable's internal structure was corrupted or tampered with.

Solutions

1. Rebuild the PhraseTable single-threaded (or under one lock) from the source phrase list.
2. Stop any code that touches the table's internal Map/List objects directly.
3. Use the immutable pattern: build once, then only call lookup/read methods from other threads.
4. Upgrade CoreNLP and report with a reproducer if corruption occurs without concurrent writes.

Example fix

// before
List<Object> list = (List<Object>) getRootTree().get(word);
list.add(myCustomObject); // poisons the table
// after
table.addPhrases(Arrays.asList(myPhraseStrings)); // only public API

// only public API may touch the table; assert no reflective access
table.addPhrases(phrases); // then treat as immutable

boolean isValidNode(Object o) { return o == null || o instanceof Phrase || o instanceof Map || o instanceof List; }

try { return table.lookup(wordList); } catch (RuntimeException e) { if (e.getMessage().contains("Unexpected class")) { table = rebuild(phrases); return table.lookup(wordList); } throw e; }

When it happens

Trigger: lookup(WordList) reaching a lookupList that holds an object that is neither Phrase nor Map - caused by concurrent addPhrase races or external mutation of the internal lists.

Common situations: Multi-threaded table construction; reflection/serialization experiments on the table; mixing objects from different PhraseTable instances.

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

• Unexpected class while looking up
• Invalid node pattern class: for variable
• Invalid node pattern variable class: for variable
• Invalid sequence pattern variable class:
• Unexpected class while adding word () in phrase

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/ce56a199e2d368a5. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/ling/tokensregex/PhraseTable.java:571

        for (Object obj:lookupList) {
          if (obj instanceof Phrase) {
            // check rest of the phrase matches
            Phrase phrase = (Phrase) obj;
            int matchedTokenEnd = checkWordListMatch(
              phrase, wordList, 0, wordList.size(), i, true);

            if (matchedTokenEnd >= 0) {
              return phrase;
            }
          } else if (obj instanceof Map) {
            if (nMaps == 1) {
              throw new RuntimeException("More than one map in list while looking up word "
                      + i + "(" + word + ") in phrase " + wordList.toString());
            }
            tree = (Map<String, Object>) obj;
            nMaps++;
          } else  {
            throw new RuntimeException("Unexpected class in list " + obj.getClass() + " while looking up word "
                    + i + "(" + word + ") in phrase " + wordList.toString());
          }
        }
        if (nMaps == 0) {
          return null;
        }
      } else {
        throw new RuntimeException("Unexpected class in list " + node.getClass() + " while looking up word "
                + i + "(" + word + ") in phrase " + wordList.toString());
      }
    }
    Phrase phrase = (Phrase) tree.get(PHRASE_END);
    if (phrase != null) {
      int matchedTokenEnd = checkWordListMatch(
        phrase, wordList, 0, wordList.size(), wordList.size(), true);
      return (matchedTokenEnd >= 0)? phrase:null;
    } else {
      return null;

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