1: list转map 当map的value值为空时会报错空指针异常(InvocationTargetException)有以下两种处理方式//解决方案一使用Optional类处理nullHashMapString, String cityProvinceMap cityProvinceList.stream().collect(Collectors.toMap(s - Optional.ofNullable(s.getCityName()).orElse(null), s - Optional.ofNullable(s.getProvince()).orElse(unknown), (a, b) - b, HashMap::new));//解决方案二直接使用collect()方法进行规约操作HashMapString, String cityProvinceMap2 cityProvinceList.stream().collect(HashMap::new, (map, item) - map.put(item.getCityName(), item.getProvince()), HashMap::putAll);