下面定义的流API中有2个和3个arg版本的reduce

T reduce(T identity,
         BinaryOperator<T> accumulator)

字符串
和

<U> U reduce(U identity,
             BiFunction<U,? super T,U> accumulator,
             BinaryOperator<U> combiner)

型
我尝试用stream/parallel stream来计算一个句子中的单词数量，并使用累加器和合并器创建以下类

class WordCounter {
    private final int counter;
    private final boolean lastSpace;
    // constructor skipped
    public WordCounter accumulate(Character c) {
        if (Character.isWhitespace(c)) {
            return lastSpace ?
                    this :
                    new WordCounter(counter, true);
        } else {
            return lastSpace ?
                    new WordCounter(counter + 1, false) :
                    this;
        }
    }
    public WordCounter combine(WordCounter wordCounter) {
        return new WordCounter(counter + wordCounter.counter, wordCounter.lastSpace);
    }
}

型
现在我试着在main方法中用reduce的版本来测试它

public static void main(String[] args) {
        String SENTENCE = "please help";
        Stream<Character> stream = IntStream
                .range(0, SENTENCE.length())
                .mapToObj(SENTENCE::charAt);
        int count2 = stream.reduce(new WordCounter(0, true),
                WordCounter::accumulate
        ).getCounter();
}

型
当我尝试上面的方法时，编译器会抱怨，因为我正在使用来自static main的非静态WordCounter::accumulate
但是，在同一个静态main中，

int count1 = stream.reduce(new WordCounter(0, true),
                WordCounter::accumulate,
                WordCounter::combine
        ).getCounter();

型