Java8 stream 中利用 groupingBy 进行多字段分组求和

Java8的groupBy实现集合的分组，类似Mysql的group by分组功能，注意得到的是一个map

对集合按照单个属性分组

case1：

List<String> items =
        Arrays.asList("apple", "apple", "banana",
                "apple", "orange", "banana", "papaya");
// 分组
Map<String, List<String>> result1 = items.stream().collect(
        Collectors.groupingBy(
                Function.identity()
        )
);
//{papaya=[papaya], orange=[orange], banana=[banana, banana], apple=[apple, apple, apple]}
System.out.println(result1);
// 分组计数
Map<String, Long> result2 = items.stream().collect(
        Collectors.groupingBy(
                Function.identity(), Collectors.counting()
        )
);
// {papaya=1, orange=1, banana=2, apple=3}
System.out.println(result2);
Map<String, Long> finalMap = new LinkedHashMap<>();
//分组, 计数和排序
result2.entrySet().stream()
        .sorted(Map.Entry.<String, Long>comparingByValue().reversed())
        .forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
// {apple=3, banana=2, papaya=1, orange=1}
System.out.println(finalMap);

case2:

User user1 = new User("zhangsan", "beijing", 10);
User user2 = new User("zhangsan", "beijing", 20);
User user3 = new User("lisi", "shanghai", 30);
List<User> list = new ArrayList<User>();
list.add(user1);
list.add(user2);
list.add(user3);
Map<String, List<User>> collect = list.stream()
        .collect(
                Collectors.groupingBy(
                        User::getName/*, Collectors.counting()*/
                )
        );
//{lisi=[User{age=30, name='lisi', address='shanghai'}], 
// zhangsan=[User{age=10, name='zhangsan', address='beijing'}, User{age=20, name='zhangsan', address='beijing'}]}
System.out.println(collect);