bson/src/main/org/bson/BsonDocument.java
public BsonObjectId getObjectId(final Object key, final BsonObjectId defaultValue) {
        if (!containsKey(key)) {
            return defaultValue;
        }
        return get(key).asObjectId();
    }
public BsonObjectId getObjectId(final Object key, final BsonObjectId defaultValue) {
        if (!containsKey(key)) {
            return defaultValue;
        }
        return get(key).asObjectId();
    }
If the document does not contain the given key, return the given default value.  Otherwise, gets the value of the key as a
BsonObjectId.

@param key the key
@param defaultValue the default value
@return the value of the key as a BsonObjectId
@throws org.bson.BsonInvalidOperationException if the document contains the key but the value is not of the expected type