java8 分组并进行求和组合运算

List<String> items =
        Arrays.asList("apple", "apple", "banana",
                "apple", "orange", "banana", "papaya");

Map<String, Long> result =
        items.stream().collect(
                Collectors.groupingBy(
                        Function.identity(), Collectors.counting()
                )
        );

System.out.println(result);

Map<Long, AdSlotData> adPVNumMap = adSlotDataList.stream()
                .collect(Collectors.toMap(AdSlotData::getSlotId, adSlotData -> adSlotData, (k1, k2) -> k1));