Collectors.toMap冲突处理

用后面的 value 覆盖前面的

Map<String, String> map = list.stream.collect(Collectors.toMap(Student::getName, Student::getAge, (value1, value2) -> value2));

将重复 key 的 value 进行拼接

Map<String,String> map = list.stream.collect(Collectors.toMap(Student::getName, Student::getAge, (value1, value2) -> value1 + "," + value2));

将重复 key 的 value 作为 List

Map<String, List<Product>> map = productList.stream().collect(Collectors.toMap(
     dto -> dto.getSkuId() + "_" + dto.getRegionId(), Lists::newArrayList, (value1, value2) -> {
         value1.addAll(value2);
         return value1;
     }
));