public BsonString getString(final Object key, final BsonString defaultValue) {
        if (!containsKey(key)) {
            return defaultValue;
        }
        return get(key).asString();
    }
public BsonString getString ( final Object key , final BsonString defaultValue ) { if ( ! containsKey ( key ) ) { return defaultValue ; } return get ( key ) . asString ( ) ; }
If the document does not contain the given key, return the given default value.  Otherwise, gets the value of the key as a
BsonString.

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