bson/src/main/org/bson/BsonDocument.java
public BsonValue get(final Object key, final BsonValue defaultValue) {
        BsonValue value = get(key);
        return value != null ? value : defaultValue;
    }
public BsonValue get(final Object key, final BsonValue defaultValue) {
        BsonValue value = get(key);
        return value != null ? value : defaultValue;
    }
If the document does not contain the given key, return the given default value.  Otherwise, gets the value of the key.

@param key the key
@param defaultValue the default value
@return the value of the key as a BsonValue