java8 ->map遍历

 /**
     * map遍历
     * @Author: Mr.li
     * @Date: 2020/9/8
     */
    @Test
    public void demo() {
        List<Person> list = new ArrayList<>();
        for (int i = 0; i < 100; i++) {
            Person person = new Person();
            person.setId((int) (Math.random() * 10));
            person.setName("你不知" + i);
            list.add(person);
        }
        Map<Integer, List<Person>> map = list
                .stream()
                .collect(Collectors.groupingBy(Person::getId));

        System.out.println(map);
        map.forEach((k, v) -> System.out.println("key:value = " + k + ":" + v));
        map.forEach((k, v) -> {
            String idStr = v.stream().map(obj -> obj.getId().toString()).collect(Collectors.joining(","));
            String nameStr = v.stream().map(obj -> obj.getName()).collect(Collectors.joining(","));
            System.out.println(idStr);
            System.out.println(nameStr);
        });

        List<User> userList = new ArrayList<User>() {
            {
                add(new User("a", "1993/12/24", 26));
                add(new User("b", "1973/12/24", 46));
                add(new User("c", "1995/12/24", 24));
                add(new User("a", "1981/12/24", 38));
                add(new User("a", "1991/12/24", null));
            }
        };
        Map<String, Integer> nameMap = userList.stream()
                .filter(e -> e.getAge() != null)//过滤掉空的数据
                .collect(Collectors.toMap(User::getName,
                        User::getAge,
                        (value1, value2) -> value2));
        System.out.println(nameMap);
    }

 result: