Stream常用方法

1. List to Map

   // 1.groupingBy :自定义 key 和 value
   // 例子：key：id+memberId, value: memberId List
   Map> 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> 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 userIdMap = users.stream().collect(Collectors.toMap(User::getId, Function.identity()));
   Map 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 keyCanBeDuplicate = users.stream().collect(Collectors.toMap(User::getId, Function.identity(), (value1, value2) -> value2));
   
   // 5.用toMap的形式 实现 groupingBy
   Map> toMapEqualsGroupBy = users.stream().collect(Collectors.toMap(User::getId, Collections::singletonList,
              (e1, e2) -> {
                  List userList = new ArrayList<>();
                  userList.addAll(e1);
                  userList.addAll(e2);
                  return userList;
   }));
   

2. reduce 金额相加

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

3. 根据属性去重 （对象的单个属性）

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

4. 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());
   

5. Map key进行排序

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

6. 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());
    }
   });
   

7. 排序：String字段排序 小心数字 是一位一位比较的

8. 排序：对象的多个属性排序

   List 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()));
   

9. List to Map

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