流和减少合同

Question

完成this example

public class FilterUsingReduce {

    public static <I> List<I> filter(Stream<I> stream, Predicate<I> predicate) {
        List<I> initial = new ArrayList<>();
        return stream.reduce(initial,
                             (List<I> acc, I x) -> {
                                if (predicate.test(x)) {
                                    // We are copying data from acc to new list instance. It is very inefficient,
                                    // but contract of Stream.reduce method requires that accumulator function does
                                    // not mutate its arguments.
                                    // Stream.collect method could be used to implement more efficient mutable reduction,
                                    // but this exercise asks to use reduce method explicitly.
                                    List<I> newAcc = new ArrayList<>(acc);
                                    newAcc.add(x);
                                    return newAcc;
                                } else {
                                    return acc;
                                }
                             },
                             FilterUsingReduce::combineLists);
    }

    private static <I> List<I> combineLists(List<I> left, List<I> right) {
        // We are copying left to new list to avoid mutating it. 
        List<I> newLeft = new ArrayList<>(left);
        newLeft.addAll(right);
        return newLeft;
    }

}

作者表示，减少合同可以防止争议。实际上，在javadoc中没有找到任何关于它的内容。他是什么意思？