stream流根据一个字段分组同时提取另一字段

//测试类部分
class Test{
    private String phone;

    private String message;

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

// 模拟数据
List<Test> arr = new ArrayList<>();
for (int i = 0; i < 10; i++) {
    Test s = new Test();
    if (i<5){
        s.setPhone("1111");
    }else {
        s.setPhone("2222");
    }
    s.setMessage("aaaa");
    arr.add(s);
}
// stream流
Map<String, List<String>> collect = arr.stream().collect(Collectors.groupingBy(Test::getPhone, Collectors.mapping(Test::getMessage, Collectors.toList())));

输出结果
{2222=[aaaa, aaaa, aaaa, aaaa, aaaa], 1111=[aaaa, aaaa, aaaa, aaaa, aaaa]}