Java.lang.nullpointerexception element cannot be mapped to a null key

问题描述：

java.lang.NullPointerException: element cannot be mapped to a null key

问题分析：

1、使用Collectors.groupingBy()进行分组时，分组值存在null值。

List<String> strList = new ArrayList<>(Arrays.asList("11", "12", "13", null, null));
Map<String, List<String>> map = strList.stream().collect(Collectors.groupingBy(x -> x));

解决办法：分组值为null时，默认值为空字符。

List<String> strList = new ArrayList<>(Arrays.asList("11", "12", "13", null, null));
Map<String, List<String>> map = strList.stream().collect(Collectors.groupingBy(x -> StrUtil.isEmpty(x) ? "" : x));