What causes "Cannot remove from key set" in stanfordnlp/CoreNLP?

Collect keys to remove into a separate list, then call counter.remove(key) after iteration.

stanfordnlp/CoreNLP · error · UnsupportedOperationException

Cannot remove from key set

Error message

Cannot remove from key set

What it means

Counters.java wraps a counter's key set in an Iterator whose remove() is deliberately unsupported. The library exposes the key set for iteration only; mutating the backing counter through the iterator would corrupt the internal total maintained by the wrapper. Call UnsupportedOperationException to signal that structural removal must go through the Counter API instead.

Solutions

Collect keys to remove into a separate list, then call counter.remove(key) after iteration.
Use Counters.retainKeys(counter, predicate) or similar filtering helper instead of iterator.remove().
Build a new Counter containing only the keys you want to keep.

Example fix

// before
for (Iterator<String> it = counter.keySet().iterator(); it.hasNext();) {
  String k = it.next();
  if (counter.getCount(k) < 0.5) it.remove(); // throws
}
// after
List<String> toRemove = new ArrayList<>();
for (String k : counter.keySet()) {
  if (counter.getCount(k) < 0.5) toRemove.add(k);
}
toRemove.forEach(counter::remove);

Defensive patterns

Strategy: validation

Validate before calling

if (counter == null || counter.keySet() == null) throw new IllegalArgumentException("counter must not be null");
// plan removals via keys, never via iterator.remove()

Try / catch

// should never rely on catching; structure code to avoid iterator.remove()
try {
  keySetIterator.remove();
} catch (UnsupportedOperationException e) {
  pendingRemovals.add(currentKey); // defer removal
}

Prevention

Never call remove() on iterators of Counter collection views; treat them as read-only.
Collect keys during iteration and remove after the loop with counter.remove(key).
Prefer Counters.retainKeys/filtering helpers over manual iteration.

When it happens

Trigger: Calling java.util.Iterator.remove() while iterating counter.keySet() (or any view backed by this iterator), e.g. `for (E k : counter.keySet()) { it.remove(); }` or removing via the for-each collection's iterator.

Common situations: Developers pruning keys below a threshold or deleting zero-count entries while iterating a Counter, a pattern that works on plain HashMap keySets but not on this view.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/9e922cd8367cd7e3. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/stats/Counters.java:2767

      }

      public Set<E> keySet() {
        return new AbstractSet<E>() {
          @Override
          public Iterator<E> iterator() {
            return new Iterator<E>() {
              Iterator<E> it = map.keySet().iterator();

              public boolean hasNext() {
                return it.hasNext();
              }

              public E next() {
                return it.next();
              }

              public void remove() {
                throw new UnsupportedOperationException("Cannot remove from key set");
              }
            };
          }

          @Override
          public int size() {
            return map.size();
          }
        };
      }

      public double remove(E key) {
        final Number removed = map.remove(key);
        if (removed != null) {
          final double rv = removed.doubleValue();
          total -= rv;
          return rv;
        }

View on GitHub (pinned to 1b7edd19c4)