MongoCollection<Document> collection;
Document document = new Document();
collection.insertOne(document);
// document.get("_id") = 5a1786851ce328045c0c5276    <----- _id populated as expected after insert

public class Widget {
  private ObjectId id;

  public String getId() {
    return id;
  }

  public void setId(ObjectId id) {
    this.id = id;
  }

}

MongoCollection<Widget> collection;  // (PojoCodecProvider configured for automatic)
Widget widget = new Widget();
collection.insertOne(widget);
// widget.getId() == null    <-----   id value still null;

// The ID property is properly populated when using find
Widget widget = collection.find(...);
widget.getId() == 5a1786851ce328045c0c5276   <--- id value set as expected