apache/druid · error · IllegalStateException

Cannot convert [ ]

Error message

Cannot convert [%s]

What it means

ConciseBitmapFactory.makeImmutableBitmap only knows how to convert a WrappedConciseBitmap (its own mutable type). Passing any other MutableBitmap implementation throws IllegalStateException naming the offending class, guarding against mixing bitmap implementations within one factory.

Solutions

1. Create the mutable bitmap from the same factory: use conciseFactory.makeMutableBitmap() and pass that to makeImmutableBitmap()
2. Standardize on one bitmap factory across the system (druid.processing.bitmap.type config)
3. If conversion between types is needed, copy via iteration: create a new WrappedConciseBitmap and add all bits from the source
4. Check where the MutableBitmap originated — a factory mismatch elsewhere is the root cause

Example fix

// before
MutableBitmap mutable = roaringFactory.makeMutableBitmap();
ImmutableBitmap immutable = conciseFactory.makeImmutableBitmap(mutable); // ISE
// after
MutableBitmap mutable = conciseFactory.makeMutableBitmap();
mutable.or(mutableFromOtherFactory);
ImmutableBitmap immutable = conciseFactory.makeImmutableBitmap(mutable);

if (!(mutableBitmap instanceof WrappedConciseBitmap)) { throw new IllegalArgumentException("need a bitmap from ConciseBitmapFactory"); }

static boolean isConciseMutable(MutableBitmap b) { return b instanceof WrappedConciseBitmap; }

try { ImmutableBitmap im = factory.makeImmutableBitmap(mb); } catch (IllegalStateException e) { /* wrong factory: rebuild via makeMutableBitmap and copy bits */ }

When it happens

Trigger: Calling makeImmutableBitmap() with a mutable bitmap created by a different factory (e.g. WrappedRoaringBitmap or RoaringBitmapFactory's mutable type) instead of one from ConciseBitmapFactory.makeMutableBitmap().

Common situations: Configuring different bitmap factories for different components (e.g. index vs query) so bitmaps cross factory boundaries; refactoring code that switched to Roaring while a utility still uses Concise; passing generic MutableBitmap parameters built elsewhere.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• Can't get sketch from object of type
• Cannot deserialize type
• Cannot merge columns of type
• Cannot remove non Integer from integer BitSet
• Empty bitmapFullnessFactorizationStops:

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/91e01fe43260f8fd. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/collections/bitmap/ConciseBitmapFactory.java:96

  }

  @Override
  public MutableBitmap makeEmptyMutableBitmap()
  {
    return new WrappedConciseBitmap();
  }

  @Override
  public ImmutableBitmap makeEmptyImmutableBitmap()
  {
    return WRAPPED_IMMUTABLE_CONCISE_BITMAP;
  }

  @Override
  public ImmutableBitmap makeImmutableBitmap(MutableBitmap mutableBitmap)
  {
    if (!(mutableBitmap instanceof WrappedConciseBitmap)) {
      throw new ISE("Cannot convert [%s]", mutableBitmap.getClass());
    }
    return new WrappedImmutableConciseBitmap(
        ImmutableConciseSet.newImmutableFromMutable(
            ((WrappedConciseBitmap) mutableBitmap).getBitmap()
        )
    );
  }

  @Override
  public ImmutableBitmap mapImmutableBitmap(ByteBuffer b)
  {
    return new WrappedImmutableConciseBitmap(b.asIntBuffer());
  }

  @Override
  public ImmutableBitmap union(Iterable<ImmutableBitmap> b)
      throws ClassCastException
  {

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