laserninja opened a new issue, #18117:
URL: https://github.com/apache/iceberg/issues/18117
### Apache Iceberg version
main (development), commit `c4784768b`
### Query engine
Other: Java API
### Please describe the bug
The public single-value serializer writes an optional null map value, but
its parser cannot read that output.
```java
Types.MapType type = Types.MapType.ofOptional(
1, 2, Types.StringType.get(), Types.IntegerType.get());
Map<String, Integer> value = new HashMap<>();
value.put("a", null);
String json = SingleValueParser.toJson(type, value);
SingleValueParser.fromJson(type, json);
```
The JSON is `{"keys":["a"],"values":[null]}`. Parsing throws
`NullPointerException: null value in entry: a=null`.
Expected: recover the same map. Iceberg permits optional map values.
`mapFromJson` currently uses `ImmutableMap.Builder`, which rejects all null
values.
Proposed fix: allow optional null values while retaining immutable results
and rejecting null/duplicate keys and null required values. This report
concerns the single-value parser, not support for non-null map column defaults,
which are separately disallowed by the field builder.
### Willingness to contribute
- [x] I can contribute a fix for this bug independently
This report and its reproduction were prepared with AI assistance and
verified locally.
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