为什么Stream.Reduce采用BinaryOperator而不是BiFunction？[副本]

Question

对于我的具体情况，我想在约简中使用函数合成；例如：

BiFunction<ImmutableSet<Integer>, ImmutableSet<Integer>, Sets.SetView<Integer>> f = Sets::intersection;
Function<Sets.SetView<Integer>, ImmutableSet<Integer>> g = Sets.SetView::immutableCopy;
BiFunction<ImmutableSet<Integer>, ImmutableSet<Integer>, ImmutableSet<Integer>> biFunction = f.andThen(g);
ImmutableSet<Integer> intersection = Stream.of(ImmutableSet.of(1, 2, 3), ImmutableSet.of(1, 2), ImmutableSet.of(4))
    .reduce(biFunction)
    .orElse(ImmutableSet.of());

这有一个编译错误：

argument mismatch BiFunction cannot be converted to BinaryOperator

相反，我需要做的是：

ImmutableSet<Integer> intersection = Stream.of(ImmutableSet.of(1, 2, 3), ImmutableSet.of(1, 2), ImmutableSet.of(4))
    .reduce((a, b) -> Sets.intersection(a, b).immutableCopy())
    .orElse(ImmutableSet.of());

但是，这就失去了composition提供的无点样式。

为什么流API是这样设计的？BinaryOperator是BiFunction，所以用超类型声明Reduce方法的参数不是更有意义吗？

Answer 1

reduce操作必须采用相同类型的参数并返回相同类型。如果没有，就会出现类型不匹配。这正是binaryoperator的含义:binaryoperator 扩展了bifunction

您可以创建bifunction而不是使用lambda。然后创建BinaryOperator:

import java.util.function.BinaryOperator;
import java.util.function.BiFunction;
import java.util.function.Function;

import java.util.stream.Stream;

import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Sets;

public class StackOverflowTest {
  public static void main(String[] args) {

    BiFunction<ImmutableSet<Integer>, ImmutableSet<Integer>, Sets.SetView<Integer>> f = Sets::intersection;
    Function<Sets.SetView<Integer>, ImmutableSet<Integer>> g = Sets.SetView::immutableCopy;

    BiFunction<ImmutableSet<Integer>, ImmutableSet<Integer>, ImmutableSet<Integer>> biFunction = f.andThen(g);

    BinaryOperator<ImmutableSet<Integer>> biOperator = biFunction::apply;

    ImmutableSet<Integer> intersection =
       Stream.of(ImmutableSet.of(1, 2, 3),
                 ImmutableSet.of(1, 2),
                 ImmutableSet.of(1, 4)) // added a 1
             .reduce(biOperator)
             .orElse(ImmutableSet.of());

    System.out.println(intersection);

/*
prints:
[1]
*/
  }
}