在Java Stream上调用collect方法出现意外结果

您正在处理的int/int不是可变的.因此，累加器和组合器都不会改变结果.因此，它保持0.

你可以通过使用一个可变对象(如Atomicupon)来完成操作，并使用一个finisher将其转换回NTFS来避免这个问题:

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 AtomicInteger 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.getAndAdd(result2.get());
        return result1;
    };

    //Create a Finisher to get the Integer value
    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
}