java8 实现分组求和，平均等数据统计

系统管理员 2023-07-01 11:56 23阅读 0赞

### java8 实现分组聚合，平均等数据统计 ###

*   *  写在前面
     *  一、分组实现
     *   *  1.1、简单分组
         *  1.2、集合分组
         *  1.3、分组求和
         *  1.4、分组求平均

## 写在前面 ##

## 一、分组实现 ##

### 1.1、简单分组 ###

Map<String, Long> departmentNames = departmentDbs.stream()
                    .collect(Collectors.toMap(
                    			Department::getDepartmentName, 
                    			Department::getDepartmentId)
                    		);

### 1.2、集合分组 ###

/** * 分组函数使用 * 这个示例返回，分组后的集合对象 */
        @Test
        public void t12_2() { 
            List<Person> roster = Arrays.asList(
                    new Person("as", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zx", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zc", LocalDate.now(), Person.Sex.MALE,"Test@",12),
                    new Person("zv", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zb", LocalDate.now(), Person.Sex.MALE,"Test@",12),
                    new Person("zn", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zm", LocalDate.now(), Person.Sex.MALE,"Test@",12)
            );
    
            Map<Person.Sex, List<Person>> collect = roster.stream()
                    .collect(
                            Collectors.groupingBy(Person::getGender)
                    );
            collect.forEach((k,v) -> System.out.println(k + " : " + v));
        }

### 1.3、分组求和 ###

/** * 分组函数并求和 * MALE : 36 * FEMALE : 48 */
        @Test
        public void t14() { 
            List<Person> roster = Arrays.asList(
                    new Person("as", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zx", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zc", LocalDate.now(), Person.Sex.MALE,"Test@",12),
                    new Person("zv", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zb", LocalDate.now(), Person.Sex.MALE,"Test@",12),
                    new Person("zn", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zm", LocalDate.now(), Person.Sex.MALE,"Test@",12)
            );
    
            Map<Person.Sex, Integer> totalAgeByGender =
                    roster.stream()
                            .collect(
                                    Collectors.groupingBy(
                                            Person::getGender,
                                            Collectors.reducing(
                                                    0,
                                                    Person::getAge,
                                                    Integer::sum))
                            );
            totalAgeByGender.forEach((k,v) -> System.out.println(k + " : " + v));
        }

### 1.4、分组求平均 ###

/** * 分组求平均 * MALE : 12.0 * FEMALE : 12.0 */
        @Test
        public void t15() { 
            List<Person> roster = Arrays.asList(
                    new Person("as", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zx", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zc", LocalDate.now(), Person.Sex.MALE,"Test@",12),
                    new Person("zv", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zb", LocalDate.now(), Person.Sex.MALE,"Test@",12),
                    new Person("zn", LocalDate.now(), Person.Sex.FEMALE,"Test@",12),
                    new Person("zm", LocalDate.now(), Person.Sex.MALE,"Test@",12)
            );
    
            Map<Person.Sex, Double> averageAgeByGender = roster.stream()
                    .collect(
                            Collectors.groupingBy(
                                    Person::getGender,
                                    Collectors.averagingInt(Person::getAge)));
            averageAgeByGender.forEach((k,v) -> System.out.println(k + " : " + v));
        }