将列表转换为映射 [英] Converting a List to Map

问题描述

我有一个我要转换为地图的字符串列表。我尝试了以下但我似乎无法弄清楚为什么它不起作用

I got a list of Strings that i want to convert to a map. I tried the below but i can't seem to figure out why its not working

List<String> dataList = new ArrayList<>( //code to create the list );

Map<String, Double> doubleMap = dataList.stream().collect(Collectors.toMap(o->o, Double::new));

我得到的是：

java.lang.NumberFormatException: For input string: "Test1"

它似乎试图将一个字符串放入值（这是一个Double）而不是创建一个空/ null double。

It seems to be trying to put a string into the value (which is a Double) instead of creating an empty/null double.

我基本上希望地图包含字符串，每条记录0.0。

I essentially want the map to contain the String, 0.0 for each record.

推荐答案

您正在尝试传递字符串到 public Double（String s）构造函数，如果 List 包含任何 String 无法解析为 double 。

You are trying to pass a String to the public Double(String s) constructor, which fails if your List contains any String that cannot be parsed as a double.

当您将 Double 构造函数的方法引用传递给 toMap ，相当于：

When you pass a method reference of a Double constructor to toMap, it's equivalent to :

Map<String, Double> doubleMap = dataList.stream().collect(Collectors.toMap(o->o, o->new Double(o)));

相反，请写：

Map<String, Double> doubleMap = dataList.stream().collect(Collectors.toMap(o->o, o->0.0));

这篇关于将列表转换为映射的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！