stanfordnlp/CoreNLP · error · NoSuchElementException

TreeIterator exhausted

Error message

TreeIterator exhausted

What it means

Tree's iterator throws NoSuchElementException with this message when next() is called after the traversal stack is empty. This is the standard Java iterator contract; the message just identifies which iterator was over-advanced.

Solutions

1. Guard every next() with iterator.hasNext() (or use an enhanced for-loop over the tree)
2. Do not reuse an exhausted TreeIterator; obtain a fresh iterator() call
3. Make sure each iteration consumes exactly one next() call

Example fix

// before
while (true) { Tree t = it.next(); ... } // exhausts
// after
while (it.hasNext()) { Tree t = it.next(); ... }

if (!it.hasNext()) return null; Tree t = it.next();

Tree nextOrNull(Iterator<Tree> it) { return it.hasNext() ? it.next() : null; }

try { Tree t = it.next(); } catch (NoSuchElementException e) { /* iterator exhausted; stop loop */ }

When it happens

Trigger: Calling next() more times than there are nodes — typically a loop that calls next() without hasNext(), or reusing an exhausted iterator.

Common situations: Manual while-loops calling it.next() twice per iteration; nested iteration over the same iterator; storing next() results in two variables from a single call site.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

• DocumentIterator exhausted.
• No more elements from
• Can't set Tree labels
• ancestor: height cannot be negative
• Tree.valueOf() tree construction failed

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/a5d8d0e6da093573. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/trees/Tree.java:2313

  private static class TreeIterator implements Iterator<Tree> {

    private final List<Tree> treeStack;

    protected TreeIterator(Tree t) {
      treeStack = new ArrayList<>();
      treeStack.add(t);
    }

    @Override
    public boolean hasNext() {
      return (!treeStack.isEmpty());
    }

    @Override
    public Tree next() {
      int lastIndex = treeStack.size() - 1;
      if (lastIndex < 0) {
        throw new NoSuchElementException("TreeIterator exhausted");
      }
      Tree tr = treeStack.remove(lastIndex);
      Tree[] kids = tr.children();
      // so that we can efficiently use one List, we reverse them
      for (int i = kids.length - 1; i >= 0; i--) {
        treeStack.add(kids[i]);
      }
      return tr;
    }

    /**
     * Not supported
     */
    @Override
    public void remove() {
      throw new UnsupportedOperationException();
    }

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