stanfordnlp/CoreNLP · error · RuntimeException

Unexpected node class

Error message

Unexpected node class

What it means

TransducerGraph.processNode converts graph node objects into their string form, supporting Strings, Sets, and Block instances; any other node class triggers this RuntimeException. It is an internal type expectation violation in graph output/processing.

Solutions

1. Use String, Set, or TransducerGraph.Block as node objects
2. Convert custom node objects to Strings before adding them to the graph
3. Override/extend processNode in a subclass to handle your node class
4. Inspect node creation code to find where the unsupported class was introduced

Example fix

// before
Object node = new MyNode("x");
graph.addArc(node, target, input);
// after
Object node = "x"; // use a supported node type
graph.addArc(node, target, input);

// Only add supported node types to the graph
if (!(node instanceof String || node instanceof java.util.Set || node instanceof TransducerGraph.Block))
  throw new IllegalArgumentException("Unsupported node class: " + node.getClass());

boolean isSupportedNode(Object n) {
  return n instanceof String || n instanceof java.util.Set || n instanceof TransducerGraph.Block;
}

try {
  String s = graph.processNode(node);
} catch (RuntimeException e) {
  if ("Unexpected node class".equals(e.getMessage())) {
    String s = String.valueOf(node); // fallback rendering
  } else throw e;
}

When it happens

Trigger: Rendering/processing a TransducerGraph whose nodes were created with a custom object class rather than String, Set, or Block (e.g. user-defined node payloads passed to addArc or graph construction).

Common situations: Extending TransducerGraph with custom node types; loading graphs built by other code that used nonstandard node objects; refactoring that changed node representation.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Creating nondeterminism while inserting arc
• addFeature was called with a features object that is…
• Unknown value for span
• Attempting to remove features based on weight from a…
• String match result must be referred to by group id

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/5feceecf3f2d0cbc. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/fsm/TransducerGraph.java:477

  public static class SetToStringNodeProcessor implements NodeProcessor {
    private TreebankLanguagePack tlp;

    public SetToStringNodeProcessor(TreebankLanguagePack tlp) {
      this.tlp = tlp;
    }

    @Override
    public Object processNode(Object node) {
      Set s = null;
      if (node instanceof Set) {
        s = (Set) node;
      } else {
        if (node instanceof Block) {
          Block b = (Block) node;
          s = b.getMembers();
        } else {
          throw new RuntimeException("Unexpected node class");
        }
      }
      Object sampleNode = s.iterator().next();
      if (s.size() == 1) {
        if (sampleNode instanceof Block) {
          return processNode(sampleNode);
        } else {
          return sampleNode;
        }
      }
      // nope there's a set of things
      if (sampleNode instanceof String) {
        String str = (String) sampleNode;
        if (str.charAt(0) != '@') {
          // passive category...
          return tlp.basicCategory(str) + "-" + s.hashCode(); // TODO remove b/c there could be collisions
          //          return tlp.basicCategory(str) + "-" + System.identityHashCode(s);
        }

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