将DataSnapshot转换为类对象

我是Firebase数据库的新手，并且正在关注Google的文档以从我的数据库中检索数据.
我有这个结构:

I'm newbie to Firebase Database and I'm following Google's Docs to retrive data from my database.
I have this structure:

root/
|
|--- 82JWZZZd***
|    |
|    |--- profile
|         |
|         |--- id: 1
|         |--- name: Arnold Schwarzenegger
|         |--- age: 70
|
|--- GB3FDPiw***
     |
     |--- profile
          |
          |--- id: 1
          |--- name: Sylvester Stallone 
          |--- age: 71

我想检索用户的个人资料信息，并且有一个Java类:

I want to retrive the profile information of a user and I have a Java class:

public class Profile{
    private int id;
    private String name;
    private int age;

    public Profile(){
        // Default constructor required for calls to DataSnapshot.getValue(User.class)
    }

    public Profile(int id, String name, int age){
        this.id = id;
        this.name = name;
        this.age = age;
    }
}

因此，正如我从Google文档中了解到的那样，我创建了一个侦听器:

So, as I learned from Google's Docs I create a Listener:

DatabaseReference reference = FirebaseDatabase.getInstance().getReference(getUid() + "/profile");
reference.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot.exists()){
            Toast.makeText(getContext(), "" + dataSnapshot.getValue(),
                    Toast.LENGTH_LONG).show();
            // Profile prf = dataSnapshot.getValue(Profile.class);
        }
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

烤面包给我看:
{id = 1，name = Arnold Schwarzenegger，年龄= 71}

但是，如果我取消注释该行

The Toast show me this:
{id=1, name=Arnold Schwarzenegger, age=71}

BUT if I uncomment the line

Profile prf = dataSnapshot.getValue(Profile.class);

我的应用程序崩溃.

my App CRASH.

我不了解Google文档的区别.有人可以帮我吗?我为此太苦了！

错误是:

I don't understand the difference with Google documentation. Can somebody help me? I'm struggling with this too much!

Error is:

com.google.firebase.database.DatabaseException: Class com.companyname.myApp.FirstFragment$Profile is missing a constructor with no arguments