public class Test1 {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9));
        C1 c1 = new C1();
        numbers.parallelStream().forEach(c1::method1);//操作元素
    }
    static class C1 {
        public void method1(Integer integer) {
            System.out.print(integer + "\t");
        }
    }
}

TreeMap<String, List<User>> treeMap = userList.stream()
                .sorted((o1, o2) -> o1.getAge() - o2.getAge())
                .collect(groupingBy(item -> item.getHeight, TreeMap::new, toList()));