mem collection spliterators incorrectly claim IMMUTABLE characteristic

[arne-bdt]

Version

6.2.0-SNAPSHOT

What happened?

Description

The spliterators in org.apache.jena.mem.spliterator —
ArraySpliterator, ArraySubSpliterator, SparseArraySpliterator,
and SparseArraySubSpliterator — report Spliterator.IMMUTABLE from
their characteristics() method. This is inconsistent with their
actual behavior: each one snapshots set.size() at construction time
and throws ConcurrentModificationException from tryAdvance /
forEachRemaining when the underlying collection is structurally
modified during traversal.

The Javadoc for Spliterator.IMMUTABLE states that an immutable
spliterator's element source "cannot be structurally modified".
Reporting IMMUTABLE while detecting and reporting structural
modification is a contract violation and can mislead stream pipelines
or downstream callers that inspect characteristics.

Related issue: NONNULL not enforced

The same spliterators also report NONNULL, but the backing map and
set implementations (FastHashMap, FastHashSet, HashCommonMap,
HashCommonSet) accept null keys and values without complaint.
While none of the current internal callers pass null, the interface
contract was not documented and not enforced, so the NONNULL
characteristic was technically also unsubstantiated.

Proposed fix

• Drop IMMUTABLE from characteristics() in the four spliterators.
• Document on JenaMap, JenaMapIndexed, JenaSet,
  JenaSetHashOptimized, and JenaSetIndexed that null keys/values
  are not permitted.
• Add assert checks on the write paths of the map/set
  implementations to catch violations in tests.

Scope

Internal to jena-core memory collections. No public API change; the
affected interfaces are package-internal helpers used by the in-memory
graph stores.

Relevant output and stacktrace

Are you interested in making a pull request?

None