Jdk1.8 集合处理

@Test
    public void test() {
        List<User> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            User user = new User();
            user.setId(i);
            user.setUsername(i + "aaa");
            list.add(user);
        }
        User user = new User();
        user.setId(1);
        user.setUsername(1+ "aaa");
        list.add(user);

        List<Integer> collect = list.stream().map(User::getId).collect(Collectors.toList());
        System.out.println(collect);

        // (a, b) -> b) 若map存在key，则用b的值覆盖a的值
        Map<Integer, User> map = list.stream().collect(Collectors.toMap(User::getId, Function.identity(), (a, b) -> b));
        System.out.println(JSONUtils.toJSONString(map));

        Map<Integer, List<User>> integerListMap = list.stream().collect(Collectors.groupingBy(User::getId));
        System.out.println(JSONUtils.toJSONString(integerListMap));

    }@Test
    public void test() {