apache/beam · error · RuntimeException

Cannot infer schema from unparameterized collection.

Error message

Cannot infer schema from unparameterized collection.

What it means

fieldFromType infers an FieldType.iterable(elementType) for Collection-typed fields, but this requires the collection's element type parameter. If the Collection is declared raw (no generic parameter), the element type cannot be inferred and this RuntimeException is thrown.

Solutions

1. Parameterize the collection field, e.g. List<String> or List<MyRowPojo>.
2. Supply the field type explicitly in the schema rather than relying on inference.
3. Use a TypeDescriptor that preserves the element type when calling inference APIs.
4. For heterogeneous data, model as List<Row> with a defined row schema.

Example fix

// before
private List items; // raw collection
// after
private List<String> items;

for (java.lang.reflect.Field f : pojoClass.getDeclaredFields()) {
  if (Collection.class.isAssignableFrom(f.getType())
      && !(f.getGenericType() instanceof java.lang.reflect.ParameterizedType)) {
    throw new IllegalArgumentException("Raw Collection field: " + f.getName());
  }
}

boolean hasParameterizedCollection(java.lang.reflect.Field f) {
  return f.getGenericType() instanceof java.lang.reflect.ParameterizedType
      && Collection.class.isAssignableFrom(f.getType());
}

try {
  Schema s = Schema.of(pojoClass);
} catch (RuntimeException e) {
  if (e.getMessage() != null && e.getMessage().contains("unparameterized collection")) {
    throw new IllegalStateException("Parameterize Collection fields, e.g. List<String>", e);
  }
  throw e;
}

When it happens

Trigger: Schema inference encounters a field declared as a raw Collection/List (e.g. `private List items;` without `<T>`), reached from fieldType/fieldFromType.

Common situations: Pre-generics or sloppily typed POJOs, fields typed as Collection/List/Iterable without a type argument, or descriptors built from raw Classes losing parameterization.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Cannot infer schema from unparameterized map.
• Arrow schema conversion does not support Beam type
• Cannot call getFromRowFunction when there is no schema
• Cannot call getSchema when there is no schema
• Cannot call getToRowFunction when there is no schema

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/ce14d41ba3b8e26a. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/utils/StaticSchemaInference.java:216

      return FieldType.BYTES;
    } else if (type.isSubtypeOf(TypeDescriptor.of(Iterable.class))) {
      TypeDescriptor<Iterable<?>> iterable = type.getSupertype(Iterable.class);
      if (iterable.getType() instanceof ParameterizedType) {
        ParameterizedType ptype = (ParameterizedType) iterable.getType();
        java.lang.reflect.Type[] params = ptype.getActualTypeArguments();
        checkArgument(params.length == 1);
        // TODO: should this be AbstractCollection?
        if (type.isSubtypeOf(TypeDescriptor.of(Collection.class))) {
          return FieldType.array(
              fieldFromType(
                  TypeDescriptor.of(params[0]), fieldValueTypeSupplier, alreadyVisitedSchemas));
        } else {
          return FieldType.iterable(
              fieldFromType(
                  TypeDescriptor.of(params[0]), fieldValueTypeSupplier, alreadyVisitedSchemas));
        }
      } else {
        throw new RuntimeException("Cannot infer schema from unparameterized collection.");
      }
    } else {
      return FieldType.row(schemaFromClass(type, fieldValueTypeSupplier, alreadyVisitedSchemas));
    }
  }
}

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