java - 如何使用 Collectors.averagingDouble 计算双数组的平均值？

Question

我不明白为什么下面的代码无法编译:

import java.util.Arrays;
import java.util.stream.Collectors;

public class AppMain {

    public static void main(String args[]) {

        double[] x = {5.4, 5.56, 1.0};
        double avg = Arrays.stream(x).collect(Collectors.averagingDouble(n -> n));
    }
}

错误信息完全不清楚。

The method collect(Supplier<R>, ObjDoubleConsumer<R>, BiConsumer<R,R>) in the type DoubleStream is not applicable for the arguments (Collector<Object,?,Double>)
    Type mismatch: cannot convert from Collector<Object,capture#1-of ?,Double> to Supplier<R>
    Type mismatch: cannot convert from Object to double

Answer 1

Arrays.stream(x)对于 double数组返回 DoubleStream . DoubleStream界面有不同collect方法比 Stream接口(interface)，它不接受 Collector .

您可以简单地使用 average() DoubleStream的方法| :

double avg = Arrays.stream(x).average().getAsDouble();

如果你坚持使用avergingDouble你需要一个Stream<Double> ，您可以通过以下方式获得:

double[] x = {5.4, 5.56, 1.0};
double avg = Arrays.stream(x).boxed().collect(Collectors.averagingDouble(n -> n));

或通过:

Double[] x = {5.4, 5.56, 1.0};
double avg = Arrays.stream(x).collect(Collectors.averagingDouble(n -> n));