如何收集DoubleStream到列表

Question

我有以下代码:

Stream.of("1,2,3,4".split(",")).mapToDouble(Double::valueOf).collect(Collectors.toList());

我想回来List<Double>.

此代码无法编译.

我看到错误:

Error:(57, 69) java: method collect in interface java.util.stream.DoubleStream cannot be applied to given types;
  required: java.util.function.Supplier<R>,java.util.function.ObjDoubleConsumer<R>,java.util.function.BiConsumer<R,R>
  found: java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.List<java.lang.Object>>
  reason: cannot infer type-variable(s) R
    (actual and formal argument lists differ in length)

如何解决这个问题？

Answer 1

你可以用boxed().这将a DoubleStream(原始双精度流,返回者mapToDouble)映射到a Stream<Double>.

Stream.of("1,2,3,4".split(",")).mapToDouble(Double::parseDouble).boxed().collect(Collectors.toList());

请注意,我更改Double::valueOf为Double::parseDouble:这可以防止Double返回的by Double.valueOf取消装入基元double.

但是你为什么mapToDouble要开始使用？你可以map像这样使用:

Stream.of("1,2,3,4".split(",")).map(Double::valueOf).collect(Collectors.toList());