stanfordnlp/CoreNLP · error · RuntimeException

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

Error message

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

What it means

The MIN aggregator requires each value for the attribute key to implement Comparable; it casts to Comparable and compares. If the stored object is not Comparable it throws a RuntimeException stating the type cannot be min'd.

Solutions

Aggregate over an attribute whose values are Comparable (Integer, Double, String, etc.).
Convert values to a Comparable type before aggregating.
Provide a custom aggregator with an explicit Comparator for the type.

Example fix

// before
Object min = CoreMapAttributeAggregator.MinAggregator.aggregate(maps, CoreAnnotations.SentencesAnnotation.class); // List is not Comparable
// after
Object min = CoreMapAttributeAggregator.MinAggregator.aggregate(maps, CoreAnnotations.IndexAnnotation.class); // Integer is Comparable

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 isComparable(Object o) { return o instanceof Comparable; }

Try / catch

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

Prevention

Reserve min/max aggregation for Comparable-typed attributes.
Convert values to Integer/Double/String before storing them on the CoreMap.
Write a custom aggregator with an explicit Comparator for complex types.

When it happens

Trigger: Calling aggregate with the MIN aggregator on an attribute whose values are not Comparable objects (e.g. a raw Object, List, or Tree stored under the key).

Common situations: Selecting a coref/map key for min aggregation where values are complex types; using MIN on textual annotations whose runtime type does not implement Comparable.

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/fc8ee749f695f2f3. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/pipeline/CoreMapAttributeAggregator.java:204

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

View on GitHub (pinned to 1b7edd19c4)