apache/iceberg · error · IllegalStateException

Not an instance of

Error message

Not an instance of 

What it means

GenericRecord.get(pos, javaClass) first fetches the raw value then casts it to the requested Java class; if the stored value is non-null but not an instance of the requested class, this IllegalStateException is thrown with both the class name and the value.

Solutions

1. Request the Java class matching the schema field's Iceberg type (Integer for int, Long for long, etc.)
2. Read the raw value with get(pos) and convert explicitly
3. Verify the record's schema matches what your accessor code assumes

Example fix

// before
Long ts = record.get(2, Long.class); // stored as Integer
// after
Integer ts = record.get(2, Integer.class);

Types.NestedField f = table.schema().columns().get(pos);
Class<?> expected = f.type().typeId() == Type.TypeID.LONG ? Long.class : f.type().typeId() == Type.TypeID.INTEGER ? Integer.class : /* ... */ Object.class;

<T> T getIfInstance(Record r, int pos, Class<T> c) { Object v = r.get(pos); return c.isInstance(v) ? c.cast(v) : null; }

try { Long v = record.get(pos, Long.class); } catch (IllegalStateException e) { if (e.getMessage().startsWith("Not an instance of")) { /* request correct class or convert */ } else throw e; }

When it happens

Trigger: Calling record.get(pos, Long.class) when the value is an Integer, or requesting String for an int field; class-based accessors (e.g. in GenericData readers) assuming a different Java type than stored.

Common situations: Java generic-get convenience usage where schema types changed; comparing int vs long conventions between Iceberg and engine data models; code shared across record types.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Cannot convert date
• Cannot convert dict encoded field
• Cannot convert time
• Cannot convert timestamp
• Cannot convert timestamptz

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/9ff7e1d57d3416cf. Report an issue: GitHub.

Appendix: source

Thrown at core/src/main/java/org/apache/iceberg/data/GenericRecord.java:138

  }

  @Override
  public int size() {
    return size;
  }

  @Override
  public Object get(int pos) {
    return values[pos];
  }

  @Override
  public <T> T get(int pos, Class<T> javaClass) {
    Object value = get(pos);
    if (value == null || javaClass.isInstance(value)) {
      return javaClass.cast(value);
    } else {
      throw new IllegalStateException("Not an instance of " + javaClass.getName() + ": " + value);
    }
  }

  @Override
  public <T> void set(int pos, T value) {
    values[pos] = value;
  }

  @Override
  public GenericRecord copy() {
    return new GenericRecord(this);
  }

  @Override
  public GenericRecord copy(Map<String, Object> overwriteValues) {
    return new GenericRecord(this, overwriteValues);
  }

View on GitHub (pinned to 86d9c8fc54)