是否有可能只迭代一次流并执行2个或更多操作？ [英] Is it possible to iterate a stream only once and perform 2 or more operations?

问题描述

给定代码

 列表< Integer> numbers = Arrays.asList（2，4，3）; $（$，$，$，$，$，$，$，$，$）$ sum $ total $ $（1，（x，y） - > x * y）; 
 int multiplyTotal = numbers.stream（）。 
  

是否可以在迭代流时执行两个操作？

另外，请注意，每个reduce都有不同的标识：-3和1.

解决方案

创建一个自定义的类，并使用可变的缩减：

  public static void main（String [] args）{
列表与LT;整数> numbers = Arrays.asList（2，4，3）; 
 
 result result = numbers.stream（）。collect（Result :: new，Result :: consume，Result :: combine）; 
 System.out.println（result =+ result）; 
} 
 
 private static class结果{
 private int sum = 0; 
 private int product = 1; 
 
 public void consume（int i）{
 sum + = i + 3; 
 product * = i; 
} 
 
 public void combine（Result r）{
 // READ 
 sum + = r.sum + 3; 
产品* = r.product; 
} 
 
 @Override 
 public String toString（）{
 returnResult {+ 
sum =+ sum + 
 ，product =+ product + 
'}'; 
} 
} 
  

但是我怀疑你这样做会获得很多收益只需迭代两次就可以了。

编辑：

请注意，上面的combine（）在使用并行流时调用将产生与顺序流不一致的结果。 Holger在评论中提到，这个问题是由于操作不尊重缩减的身份前提条件引起的：应用任何具有身份的值V的减少应该产生V，但是它产生V + 3以0作为身份。

确保不要使用并行流来计算结果。

Given the code

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

int sumTotal = numbers.stream().reduce(-3, (x, y) -> x + y + 3);
int multiplyTotal = numbers.stream().reduce(1, (x, y) -> x * y);

Is it possible to perform both operations while iterating the stream only once?

Also, notice each reduce has a different identity: -3 and 1.

解决方案

You can create a custom class, and use a mutable reduction:

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

    Result result = numbers.stream().collect(Result::new, Result::consume, Result::combine);
    System.out.println("result = " + result);
}

private static class Result {
    private int sum = 0;
    private int product = 1;

    public void consume(int i) {
        sum += i + 3;
        product *= i;
    }

    public void combine(Result r) {
        // READ note below
        sum += r.sum + 3;
        product *= r.product;
    }

    @Override
    public String toString() {
        return "Result{" +
            "sum=" + sum +
            ", product=" + product +
            '}';
    }
}

But I doubt you will gain much by doing that over simply iterating twice as you're doing.

EDIT:

Note that the combine() method above, invoked when using a parallel stream, will produce results that are inconsistent with a sequential stream. The problem is caused, as Holger mentions in the comments, by the fact that the operation doesn't respect the identity preconditions of a reduction: applying the reduction on any value V with the identity is supposed to produce V, but it produces V + 3 with 0 as identity.

Make sure not to use a parallel stream to compute that result.

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