对list中某个对象的属性求和
 
BigDecimal money = userList.stream().map(User::getInvoiceMoney).reduce(BigDecimal::add).orElse(BigDecimal.ZERO);
 
分组
 
Map<String,List<User>> listMap = userList.stream().collect(Collectors.groupingBy(User::getBusinessType));
 
 map的遍历
 
for (Map.Entry<String,List<User>> entry : listMap.entrySet()) {
    String businessType = entry.getKey();
    List<User> userList= entry.getValue();
}
 
filter 过滤
 
userList.stream().filter(s -> s.getIsDefault() || s.getFirstAccount()).findFirst().orElse(null);