零基础学Java优学网Stream API课：告别繁琐代码，轻松掌握集合处理技巧

Collections.sort(list, new Comparator() {

public int compare(String s1, String s2) {
    return s1.length() - s2.length();
}

});

List topUsers = orders.stream()

.filter(order -> order.getCreateTime().isAfter(monthAgo))
.collect(Collectors.groupingBy(Order::getUserId, 
         Collectors.summingDouble(Order::getTotalAmount)))
.entrySet().stream()
.sorted(Map.Entry.<Long, Double>comparingByValue().reversed())
.limit(10)
.map(entry -> userService.findById(entry.getKey()))
.collect(Collectors.toList());