Lambda

//按性别获取用户名称 
Map<User.Sex, List<String>> map2 = users.stream()
	.collect(Collectors.groupingBy(User::getGender,
	Collectors.mapping(User::getName,Collectors.toList()))); 

//按性别统计用户数
Map<User.Sex, Integer> map = users.parallelStream()
	.collect(Collectors.groupingBy(User::getGender,
	Collectors.summingInt(p -> 1)));

//获取年龄大于12的用户列表
List<User> list = users.parallelStream()
	.filter(p -> p.age > 12).collect(Collectors.toList());

//获取所有男性用户的平均年龄 
Averager averageCollect = users.parallelStream()
	.filter(p -> p.getGender() == User.Sex.MALE)
	.map(User::getAge)
	.collect(Averager::new, Averager::accept, Averager::combine);

//按性别求年龄的总和
Map<User.Sex, Integer> map3 = users.stream().collect(
	Collectors.groupingBy(User::getGender,
	Collectors.reducing(0, User::getAge, Integer::sum)));

//按性别求年龄的平均值
Map<User.Sex, Double> map4 = users.stream().collect(
	Collectors.groupingBy(User::getGender,
	Collectors.averagingInt(User::getAge)));

//取得map
Map<String, String> newMap = resultDtos.stream()
	.collect(Collectors.toMap(FindNameListResultDto::getId,FindNameListResultDto::getName, (item1, item2) -> item1));

List<SpaceFloorInfo> floorInfos = getFloorByFloorIds(floorIds).getData();
Map<String, SpaceFloorInfo> floorMap = floorInfos.stream()
        .collect(Collectors.toMap(SpaceFloorInfo::getId, floor -> floor, (item1, item2) -> item1));

// 正常排序，1，2，3
list=list.stream().sorted(Comparator.comparing(VipCardVo::getVipCardType)).collect(Collectors.toList());

// 倒序 3，2，1
list=list.stream().sorted(Comparator.comparing(VipCardVo::getVipCardType).reversed()).collect(Collectors.toList());

// 正序
Collections.sort(people, Comparator.comparing(Person::getName));

// 反序
Collections.sort(people, Comparator.comparing(Person::getName).reversed());

// 多种条件的排序
people.sort(Comparator.comparing(Person::getName).thenComparing(Person::getAge));

// 某个属性去重
list.stream().collect((Collectors.collectingAndThen(
                Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(DboALineDiskCurrentDetailInfo::getEqmName))), ArrayList::new)));

long waitApproveCount = allOrder.stream().filter(order -> "111".equals(order.getOrderType())).count();

long stayMakeSum = r.getGoodsList().stream().mapToLong(GoodsModel::getNumber).sum();

Map<String, List<OrderModel>> orderMap = orders.stream().collect(Collectors.groupingBy(order -> order.getShopCode() + "_" + order.getOrderNo()));

集合判空

Optional.ofNullable(list).ifPresent(list1 -> list1.stream().forEach(System.out::println));
Optional.ofNullable(list).map(List::stream).orElseGet(Stream::empty).forEach(System.out::println);
     
String str = "有值";
String s = Optional.ofNullable(str).orElse("默认值");
Optional.ofNullable(str).ifPresent(strs -> System.out.println(strs));