parquet-avro/src/main/java/org/apache/parquet/avro/AvroSchemaConverter.java
public static Schema getNonNull(Schema schema) {
    if (schema.getType().equals(Schema.Type.UNION)) {
      List<Schema> schemas = schema.getTypes();
      if (schemas.size() == 2) {
        if (schemas.get(0).getType().equals(Schema.Type.NULL)) {
          return schemas.get(1);
        } else if (schemas.get(1).getType().equals(Schema.Type.NULL)) {
          return schemas.get(0);
        } else {
          return schema;
        }
      } else {
        return schema;
      }
    } else {
      return schema;
    }
  }
public static Schema getNonNull(Schema schema) {
    if (schema.getType().equals(Schema.Type.UNION)) {
      List<Schema> schemas = schema.getTypes();
      if (schemas.size() == 2) {
        if (schemas.get(0).getType().equals(Schema.Type.NULL)) {
          return schemas.get(1);
        } else if (schemas.get(1).getType().equals(Schema.Type.NULL)) {
          return schemas.get(0);
        } else {
          return schema;
        }
      } else {
        return schema;
      }
    } else {
      return schema;
    }
  }
Given a schema, check to see if it is a union of a null type and a regular schema,
and then return the non-null sub-schema. Otherwise, return the given schema.

@param schema The schema to check
@return The non-null portion of a union schema, or the given schema