//差集（从旧对象中找出新对象没有的数据，即删除）
List delPaylist = oldList.stream()
.filter(item -> !newList.stream().map(e -> e.getId()).collect(Collectors.toList()).contains(item.getId()))
.collect(Collectors.toList());

//取出某个属性值封装集合

userList=list.stream().map(UserConversation::getConId).collect(Collectors.toList());

//int、double、long: 求和
double max = list.stream().mapToDouble(User::getHeight).sum();

//bigdecimal求和
shopOrderOtherFeeList.stream().map(ShopOrderOtherFee::getFeeAmount).reduce(BigDecimal.ZERO,BigDecimal::add);

//过滤
resultList.stream().filter(a ->a.getApproveSts() !=null&&
(a.getApproveSts().equals(“1”)||a.getApproveSts().equals(“3”))).collect(Collectors.toList());

//排序

// 正序 写法1
list.sort(Comparator.comparing(Integer::intValue));
System.out.println("=正序 写法 1");
list.forEach(System.out::println);

    // 正序 写法2(naturalOrder 自然排序就是正序)
    // Comparator.comparing 与Comparator.comparingInt，两者可以互相代替
    list.sort(Comparator.comparing(Integer::intValue, Comparator.naturalOrder()));
    System.out.println("=======正序 写法 2======");
    list.forEach(System.out::println);

    // 倒序  reverseOrder倒序
    // Comparator.comparing 与Comparator.comparingInt，两者不可以互相代替
    list.sort(Comparator.comparing(Integer::intValue, Comparator.reverseOrder()));
    System.out.println("=======倒序======");
    list.forEach(System.out::println);

通过指定字段分组Collectors.groupingBy().

Map<String, List> groupByStatus = userList.stream().collect(Collectors.groupingBy(User::getStatus));

//去重distinct().

List userIdList = userList.stream().distinct().collect(Collectors.toList());

//单字段多字段排序sort().

userList.sort(Comparator.comparing(User::getUserId));

userList.sort(Comparator.comparing(User::getUserId).thenComparing(User::getStatus));