Java8 Stream 关于分组、聚合等常用的操作测试

在平常的业务处理中，总是很喜欢从数据库里查询数List<T>数据，然后自己通过Map遍历去分组，现在发现Stream的分组功能非常好用，在此记录和分享下。
/**
 * Stream的分组功能测试
 *
 * @author nanfengxiangbei
 * @date 2022-04-13
 */

public class GroupingByTest {

    /**
     * 对集合按照单个属性分组
     */
    @Test
    public void testGroupingBySingleProperty1() {
        List<String> items = Arrays.asList("apple", "apple", "banana", "apple", "orange", "banana", "papaya");

        // 1、分组，select
        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);


        // 2、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);


        // 3、分组、计数、排序
        Map<String, Long> finalMap = new LinkedHashMap<>();
        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);
    }

    /**
     * 对集合按照单个属性分组
     */
    @Test
    public void testGroupingBySingleProperty2() {
        // select age, List<people> from t_people group by age
        Stream<Person> people = Stream.of(new Person("Paul", 24, 20000),
                new Person("Mark", 30, 30000),
                new Person("Will", 28, 28000),
                new Person("William", 28, 28000));
        Map<Integer, List<Person>> peopleByAge = people.collect(
                Collectors.groupingBy(Person::getAge)
        );
        // {24=[Person{name='Paul', age=24, salary=20000}], 28=[Person{name='Will', age=28, salary=28000}, Person{name='William', age=28, salary=28000}], 30=[Person{name='Mark', age=30, salary=30000}]}
        System.out.println(peopleByAge);

        // select age, count(*) from t_people group by age
        Stream<Person> people2 = Stream.of(new Person("Paul", 24, 20000),
                new Person("Mark", 30, 30000),
                new Person("Will", 28, 28000),
                new Person("William", 28, 28000));
        Map<Integer, Long> peopleByAgeCount = people2.collect(
                Collectors.groupingBy(Person::getAge, Collectors.counting())
        );
        // {24=1, 28=2, 30=1}
        System.out.println(peopleByAgeCount);
        List<Person> list = Arrays.asList(new Person("Paul", 24, 20000),
                new Person("Mark", 30, 30000),
                new Person("Will", 28, 28000),
                new Person("Will", 28, 10000),
                new Person("William", 28, 28000));

        // select name, sum(salary) from t_people group by name
        Map<String, Long> peopleByName = list.stream().collect(
                Collectors.groupingBy(Person::getName, Collectors.summingLong(Person::getSalary))
        );
        // {24=1, 28=2, 30=1}
        System.out.println(peopleByName);

        // select name,List<Person> from t_people group by name
        Map<String, List<Person>> byName = list.stream().collect(
                Collectors.groupingBy(Person::getName)
        );
        System.out.println("根据名称分组：" + byName);

        // select * from t_people t where t.salary>8000
        List<Person> person = list.stream().filter(x -> x.getSalary() > 8000).collect(Collectors.toList());
        System.out.println("高于8000的员工信息：" + person);

        // select t.name from t_people t where t.salary>8000
        List<String> filterList = list.stream()
                .filter(person1 -> person1.getSalary() > 8000)
                .map(Person::getName)
                .collect(Collectors.toList());
        System.out.println("高于8000的员工姓名：" + filterList);

        // select name, max(salary) from t_people group by name
        Optional<Long> max = list.stream().map(Person::getSalary).max(Long::compare);
        System.out.println("max(salary)=" + max);
        Map<String, Person> max2 = list.stream().collect(
                Collectors.toMap(Person::getName, Function.identity(), (c1, c2) -> c1.getSalary() > c2.getSalary() ? c1 : c2)
        );
        System.out.println("max(salary)=" + max2);
    }

    /**
     * 对集合按照多个属性分组
     */
    @Test
    public void testGroupingByMultiProperty() {
        Stream<Person> people = Stream.of(new Person("Paul", 24, 20000),
                new Person("Mark", 30, 30000),
                new Person("Will", 28, 28000),
                new Person("William", 28, 28000));
        Map<String, List<Person>> collect = people.collect(
                Collectors.groupingBy(p -> p.age + "#" + p.salary)
        );
        // {28#28000=[Person{name='Will', age=28, salary=28000}, Person{name='William', age=28, salary=28000}], 30#30000=[Person{name='Mark', age=30, salary=30000}], 24#20000=[Person{name='Paul', age=24, salary=20000}]}
        System.out.println(collect);
    }

    /**
     * 嵌套GroupingBy
     */
    @Test
    public void testNestedGroupingBy() {
        Stream<Person> people = Stream.of(new Person("Paul", 24, 20000),
                new Person("Mark", 30, 30000),
                new Person("Will", 28, 28000),
                new Person("William", 28, 28000));

        Map<String, Map<Integer, List<Person>>> collect = people.collect(
                Collectors.groupingBy(Person::getName, Collectors.groupingBy(Person::getAge))
        );
        System.out.println(collect);
    }

    public static class Person {
        private String name;
        private int age;
        private long salary;

        Person(String name, int age, long salary) {
            this.name = name;
            this.age = age;
            this.salary = salary;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        public long getSalary() {
            return salary;
        }

        public void setSalary(long salary) {
            this.salary = salary;
        }

        @Override
        public String toString() {
            return String.format("Person{name='%s', age=%d, salary=%d}", name, age, salary);
        }
    }
}