apache/beam · warning · NoSuchElementException

NoSuchElementException

Error message

NoSuchElementException

What it means

The iterator returned by BeamFnDataInboundObserver's Elements queue enforces the Iterator contract: calling next() past the end of the current batch array throws NoSuchElementException. Callers must check hasNext() before next().

Source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/fn/data/BeamFnDataInboundObserver.java:145

  // Copies the elements of list to an array and removes references to elements that
  // have been iterated past.
  private static class DiscardingIterator<T> implements Iterator<T> {
    private int index = 0;
    private final @Nullable Object[] array;

    DiscardingIterator(List<T> list) {
      this.array = list.toArray();
    }

    @Override
    public boolean hasNext() {
      return index < array.length;
    }

    @Override
    public T next() {
      if (index >= array.length) {
        throw new NoSuchElementException();
      }
      @SuppressWarnings("unchecked")
      T result = (T) array[index];
      array[index] = null;
      ++index;
      return result;
    }
  }

  private static <T> Iterator<T> createDiscardingIterator(List<T> list) {
    if (list.isEmpty()) {
      return Collections.emptyIterator(); // Optimize empty lists, which are common for timers.
    }
    return new DiscardingIterator<>(list);
  }

  /**
   * Uses the callers thread to process all elements received until we receive the end of the stream

View on GitHub (pinned to 12126d8942)

Solutions

1. Always guard next() with hasNext().
2. Prefer enhanced-for or stream iteration over manual index handling.
3. Catch NoSuchElementException to detect iteration overrun defensively in generic code.

Example fix

// before
while (true) { Data d = it.next(); ... }
// after
while (it.hasNext()) { Data d = it.next(); ... }

while (it.hasNext()) { T v = it.next(); }

try { v = it.next(); } catch (NoSuchElementException e) { /* overrun */ }

When it happens

Trigger: Calling next() on the data/timer element iterator without checking hasNext(), or calling it more times than there are elements in the current Elements message.

Common situations: Hand-written iteration loops over Elements.Data/Elements.Timers lists; off-by-one when mixing hasNext/next calls.

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

• Instruction id was poisoned
• Unregistering consumer which was not registered.
• PoisonedException
• UnsupportedOperationException
• Unable to find inbound data receiver for instruction %s and

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/1cd095aaa3df998e. Report an issue: GitHub.