apache/druid · error · UnsupportedOperationException

Cannot retainAll ona an IntegerSet

Error message

Cannot retainAll ona an IntegerSet

What it means

retainAll is intentionally unimplemented in IntegerSet; it is a stub that always throws UnsupportedOperationException. The message contains a typo ('ona an') but the semantics are simply that intersection-based removal is not supported by this class.

Source

Thrown at processing/src/main/java/org/apache/druid/collections/IntegerSet.java:145

  @Override
  public boolean addAll(Collection<? extends Integer> c)
  {
    boolean setChanged = false;
    for (Integer i : c) {
      if (!this.contains(i)) {
        setChanged = true;
        this.add(i);
      }
    }
    return setChanged;
  }

  @Override
  public boolean retainAll(Collection<?> c)
  {
    // Stub
    throw new UnsupportedOperationException("Cannot retainAll ona an IntegerSet");
  }

  @Override
  public boolean removeAll(Collection<?> c)
  {
    Iterator<?> it = c.iterator();
    boolean changed = false;
    while (it.hasNext()) {
      Integer val = (Integer) it.next();
      changed = remove(val) || changed;
    }
    return changed;
  }

  @Override
  public void clear()
  {
    mutableBitmap.clear();

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Implement intersection manually: iterate the set and remove elements not in the other collection via iterator.remove() with contains() checks — note removeAll IS supported
2. Copy to a HashSet<Integer>, perform retainAll there, then clear() and addAll() back into the IntegerSet
3. Build a new IntegerSet containing only elements that pass contains() on the other collection
4. If retainAll is core to your logic, use HashSet<Integer> instead of IntegerSet

Example fix

// before
integerSet.retainAll(keep); // UnsupportedOperationException
// after
integerSet.removeIf(v -> !keep.contains(v)); // removeIf uses remove(), which is supported

try { set.retainAll(keep); } catch (UnsupportedOperationException e) { set.removeIf(v -> !keep.contains(v)); }

When it happens

Trigger: Any call to retainAll(collection) on an IntegerSet, e.g. keeping only the elements present in another collection.

Common situations: Code written generically against the Set interface (or refactored to use IntegerSet where a HashSet was before) that relies on retainAll for set intersection.

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

• Reverse lookup not allowed.
• Serialization not supported here
• Map column doesn't support getRow()
• Map column doesn't support lookupName()
• Map column doesn't support idLookup()

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/67a64b8dd0c1e272. Report an issue: GitHub.