public boolean isLeftSide() {
        return leftSide;
    }
Create a new {@link TimestampedKeyAndJoinSide} instance for the right join side if the provide {@code key} is not {@code null}.

@param key      the key
@param <K>      the type of the key
@return a new {@link TimestampedKeyAndJoinSide} instance for the right join side if the provide {@code key} is not {@code null}
streams/src/main/java/org/apache/kafka/streams/state/internals/TimestampedKeyAndJoinSide.java