为什么 java 流 "reduce()" 累积相同的对象

Question

        ComparisonResults comparisonResults = requestsList
                .parallelStream()
                .map(item -> getResponse(item))
                .map(item -> compareToBl(item))
                .reduce(new ComparisonResults(), (result1, result2) ->
                {
                    result1.addSingleResult(result2);
                    return result1;
                });

当

    private ComparisonResults compareToBl(CompleteRoutingResponseShort completeRoutingResponseShortFresh) {
...

        ComparisonResults comparisonResults = new ComparisonResults();
...
            return comparisonResults;
}

然而当我调试时：

 .reduce(new ComparisonResults(), (result1, result2) ->
                    {
                        result1.addSingleResult(result2);
                        return result1;
                    });

我看到 result1 和 result2 总是同一个对象（想法）

result1 等于 result2

Answer 1

addSingleResult 应该 return 一个修改为 this 副本的新对象，因此您应该将代码更改为：

.reduce(new ComparisonResults(), (result1, result2) ->
                {
                    return result1.addSingleResult(result2);
                });

否则，您总是return使用同一个实例（没有修改）。

来自 Java documentation：

The reduce operation always returns a new value. However, the accumulator function also returns a new value every time it processes an element of a stream. Suppose that you want to reduce the elements of a stream to a more complex object, such as a collection. This might hinder the performance of your application. If your reduce operation involves adding elements to a collection, then every time your accumulator function processes an element, it creates a new collection that includes the element, which is inefficient. It would be more efficient for you to update an existing collection instead. You can do this with the Stream.collect method, which the next section describes.

为什么 java 流 "reduce()" 累积相同的对象

why java stream "reduce()" accumulates the same object

java

concurrency

reduce

multithreading

java-stream