List to Map

// 1.groupingBy :自定义 key 和 value
// 例子：key：id+memberId, value: memberId List
Map<String, List<String>> customGroupByMap = users.stream().collect(
           Collectors.groupingBy(e -> e.getId() + e.getMemberId(), Collectors.mapping(User::getMemberId, Collectors.toList())
           ));

// 2.groupingBy : normal
// 例子：key：memberId, value: User List
Map<String, List<User>> normalGroupByMap = users.stream()
           .filter(key -> key.getName() != null)
           .collect(Collectors.groupingBy(e -> e.getName(), Collectors.toList()));

// 3.to Map 注意:key必须不能重复，重复会爆错
Map<Integer, User> userIdMap = users.stream().collect(Collectors.toMap(User::getId, Function.identity()));
Map<Integer, User> otherWrite = users.stream().collect(Collectors.toMap(User::getId, e->e));

// 4.to Map key可以重复. 最后一个参数方法可自定义
Map<Integer, User> keyCanBeDuplicate = users.stream().collect(Collectors.toMap(User::getId, Function.identity(), (value1, value2) -> value2));

// 5.用toMap的形式 实现 groupingBy
Map<Integer, List<User>> toMapEqualsGroupBy = users.stream().collect(Collectors.toMap(User::getId, Collections::singletonList,
           (e1, e2) -> {
               List<User> userList = new ArrayList<>();
               userList.addAll(e1);
               userList.addAll(e2);
               return userList;
}));

reduce 金额相加

bargainHelps.stream().map(e -> e.getHelpBargainAmount()).reduce(BigDecimal.ZERO,BigDecimal::add);

根据属性去重 （对象的单个属性）

List<Book> unique = books.stream().collect(
                Collectors.collectingAndThen(Collectors.toCollection(
                        () -> new TreeSet<>(Comparator.comparing(o -> o.id))), ArrayList::new)

对象的多个属性

        List<ErrorSeatInfoDto> distinctErr = err.stream()
                .collect(
                        Collectors.collectingAndThen(
                                Collectors.toCollection(
                                        () -> new TreeSet<>(Comparator.comparing(o -> o.getAirline() + "_" + o.getCabinCode() + "_" + o.getChannelInfo().getSuppChannelName()))
                                ), ArrayList::new)
                );

对集合的每个元素进行操作，但不消费

cacheList.getFlightList().stream().peek(f -> f.setCabinList(null)).collect(Collectors.toList());

Map key进行排序

Map<Integer,List<CouponConfig>> sortPrizes 
 = perPriorityPrizes.entrySet().stream()
       .sorted(Map.Entry.comparingByKey())
       .collect(
    Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)
     );

排序：多个字段排序

ossPicinfo.sort((BannerOssPicInfoDto o1, BannerOssPicInfoDto o2) -> {
 if (o1.getPicOrder().equals(o2.getPicOrder())) {
     return o1.getAddTime().compareTo(o2.getAddTime());
 } else {
     return o1.getPicOrder().compareTo(o2.getPicOrder());
 }
});

排序：String字段排序 小心数字 是一位一位比较的

排序：对象的多个属性排序

List<User> users = new ArrayList<>();
users.add(new User(6, "2"));
users.add(new User(2,"2"));
users.add(new User(4,"4"));

users.sort(Comparator.comparing((User u) -> new BigDecimal(u.getName())).thenComparing(user -> user.getId()));

List to Map

// Collectors.toMap 
Map<Integer,PickupDayTimeCfgBO> map = pickupDayTimeCfgBOS.stream().collect(Collectors.toMap(PickupDayTimeCfgBO::getPickup, e -> e))