stanfordnlp/CoreNLP · error · RuntimeException

Cannot get max of attribute " + key + ", object of type: "…

Error message

Cannot get max of attribute " + key + ", object of type: " + obj.getClass()

What it means

Thrown by the CoreMapAttributeAggregator.MAX aggregate function when an attribute value under the given key is not Comparable, so a maximum cannot be computed across the CoreMaps; indicates a non-comparable object stored under that key.

Solutions

Use an attribute whose values are Comparable (numbers, strings, dates).
Normalize values to a Comparable type before aggregation.
Write a custom Aggregator using an explicit Comparator for non-Comparable types.

Example fix

// before
Object max = CoreMapAttributeAggregator.MaxAggregator.aggregate(maps, CoreAnnotations.TokensAnnotation.class); // List not Comparable
// after
Object max = CoreMapAttributeAggregator.MaxAggregator.aggregate(maps, CoreAnnotations.CharacterOffsetEndAnnotation.class); // Integer

Defensive patterns

Strategy: type-guard

Validate before calling

Object v = map.get(key);
if (v != null && !(v instanceof Comparable)) throw new IllegalStateException("Non-comparable value under key: " + key);

Type guard

static boolean isMaxable(Object o) { return o instanceof Comparable; }

Try / catch

try { max = aggregator.aggregate(maps, key); } catch (RuntimeException e) { if (e.getMessage().startsWith("Cannot get max")) { max = maxWithComparator(maps, key, myComparator); } else { throw e; } }

Prevention

Apply MAX only to scalar Comparable attributes (offsets, indexes, dates).
Normalize stored values to a Comparable type during annotation.
Add startup checks that aggregation keys hold scalar values.

When it happens

Trigger: Calling aggregate with the MAX aggregator on an attribute whose stored values do not implement Comparable (e.g. List, Tree, or arbitrary Object).

Common situations: Applying max aggregation to structural annotations rather than scalar numeric/date/string fields; misconfigured aggregation keys in pipeline or scoring code.

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

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/2cfd6932d4c04b70. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/pipeline/CoreMapAttributeAggregator.java:226

      return min;
    }
  };
  public static final CoreMapAttributeAggregator MAX = new CoreMapAttributeAggregator() {
    public Object aggregate(Class key, List<? extends CoreMap> in) {
      if (in == null) return null;
      Comparable max = null;
      for (CoreMap cm:in) {
        Object obj = cm.get(key);
        if (obj != null) {
          if (obj instanceof Comparable) {
            Comparable c = (Comparable) obj;
            if (max == null) {
              max = c;
            } else if (c.compareTo(max) > 0) {
              max = c;
            }
          } else {
            throw new RuntimeException("Cannot get max of attribute " + key + ", object of type: " + obj.getClass());
          }
        }
      }
      return max;
    }
  };

  public static final class MostFreqAggregator extends CoreMapAttributeAggregator {
    Set<Object> ignoreSet;
    public MostFreqAggregator()
    {
    }

    public MostFreqAggregator(Set<Object> set)
    {
      ignoreSet = set;
    }

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