在 Java Stream 上调用收集方法出现意外结果

Question

我正在尝试理解这个Java Stream方法

 <R> R collect(Supplier<R> supplier,
                  BiConsumer<R, ? super T> accumulator,
                  BiConsumer<R, R> combiner);

下面的代码应该给出结果 55 put 而不是 0 被打印出来。 有人可以向我解释一下出了什么问题以及如何更改它以便打印 55 吗？

  List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

        // Create a supplier that returns a new mutable result container, in this case an Integer accumulator.
                Supplier<Integer> supplier = () -> 0;

        // Create an accumulator that adds the square of the current element to the result container.
                BiConsumer<Integer, Integer> accumulator = (result, element) -> result += element * element;

        // Create a combiner that adds two result containers together.
                BiConsumer<Integer, Integer> combiner = (result1, result2) -> result1 += result2;

        // Collect the results of the reduction operation into a single Integer value.
                Integer sumOfSquares = numbers.stream().collect(supplier, accumulator, combiner);

                System.out.println(sumOfSquares); // 55

Answer 1

您正在处理不可变的整数/整数。因此，累加器和组合器都不会改变结果。因此保持为 0。

您可以通过使用像 AtomicInteger 这样的可变对象来执行操作并使用完成器将其转换回 Integer 来避免此问题：

public static void main(String[] args) {
    List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

    // Create a supplier that returns a new mutable result container, in this case an Integer accumulator.
    Supplier<AtomicInteger> supplier = () -> new AtomicInteger(0);

    // Create an accumulator that adds the square of the current element to the result container.
    BiConsumer<AtomicInteger, Integer> accumulator = (result, element) -> result.getAndAdd(element * element);

    // Create a combiner that adds two result containers together.
    BinaryOperator<AtomicInteger> combiner = (result1, result2) -> {
        result1.set(result1.getAndAdd(result2.get()));
        return result1;
    };

    Function<AtomicInteger, Integer> finisher = AtomicInteger::get;

    // Collect the results of the reduction operation into a single Integer value.
    Integer sumOfSquares = numbers.stream().collect(Collector.of(supplier, accumulator, combiner, finisher));

    System.out.println(sumOfSquares); // 55
}