lambda多个字段分组

按用户id和渠道分组。示例

import org.apache.commons.lang3.tuple.Pair;

public static void main(String[] args) {
        List<FrequentlyUsedShopDTO> list = new ArrayList<>();
        list.add(new FrequentlyUsedShopDTO().setChannel("W").setUserId(1).setFrequency(1));
        list.add(new FrequentlyUsedShopDTO().setChannel("W").setUserId(1).setFrequency(11));
        list.add(new FrequentlyUsedShopDTO().setChannel("W").setUserId(2).setFrequency(2));
        list.add(new FrequentlyUsedShopDTO().setChannel("y").setUserId(1).setFrequency(3));
        list.add(new FrequentlyUsedShopDTO().setChannel("y").setUserId(1).setFrequency(31));
        list.add(new FrequentlyUsedShopDTO().setChannel("y").setUserId(2).setFrequency(4));
        list.add(new FrequentlyUsedShopDTO().setChannel("y").setUserId(2).setFrequency(41));
        list.add(new FrequentlyUsedShopDTO().setChannel("W").setUserId(3).setFrequency(5));
       // 方式1
//        Map<Pair<Integer, String>, List<FrequentlyUsedShopDTO>> collect = list.stream()
//        .collect(Collectors.groupingBy(new Function<FrequentlyUsedShopDTO, Pair<Integer, String>>() {
//            @Override
//            public Pair<Integer, String> apply(FrequentlyUsedShopDTO frequentlyUsedShopDTO) {
//                return Pair.of(frequentlyUsedShopDTO.getUserId(), frequentlyUsedShopDTO.getChannel());
//            }
//        }));
        // 方式2
        Map<Pair<Integer, String>, List<FrequentlyUsedShopDTO>> collect = list.stream().
                collect(Collectors.groupingBy(user -> Pair.of(user.getUserId(), user.getChannel()),
                        Collectors.mapping((FrequentlyUsedShopDTO m) -> m, toList())));
        collect.forEach((key, value) -> {
            System.out.println(key.getKey() + " " + key.getValue() + " " + key.getLeft() + " " + key.getRight());
            System.out.println(value);
        });

打印结果：