DatabaseException in firebase android

Question

I am a newbie in Android application development. I am trying to retrieve data from Firebase but i am not able to type cast string to user defined object.Firstly it was giving me ClasscastException that it cannot convert string to user-defined object.

here is my code:

MainActivity.java

public class MainActivity3 extends AppCompatActivity implements View.OnClickListener {

    FirebaseDatabase db = FirebaseDatabase.getInstance();
    DatabaseReference databasereference;
    ListView l1;
    List<User> names;
    ArrayAdapter<User>arrayAdapter;
    Button b;
    User u = new User();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);

        l1 = findViewById(R.id.listvw);
        b = findViewById(R.id.list_button);

        b.setOnClickListener(this);
 }

public void onClick(View view)
{
     retrieve_data();
}

public void retrieve_data()
    {
        databasereference = db.getReference("User");
        names = new ArrayList<>();
        arrayAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, names);
        databasereference.addValueEventListener(new ValueEventListener() {

           // User u1,u2;

           // String s1,s2;
            public void onDataChange(DataSnapshot datasnapshot) {
                try {
                    names.clear();
                    for (DataSnapshot ds : datasnapshot.getChildren()) {
                      User u1=(User) ds.child("name").getValue(User.class);
                    User u2 =(User) ds.child("contact").getValue(User.class);
                   names.add(u1);
                   names.add(u2);                        
                   }
                   l1.setAdapter(arrayAdapter);

                } catch (Exception e) {
                    // Toast.makeText(this,"failed",Toast.LENGTH_SHORT).show();
                    e.printStackTrace();
                }
        }
            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }
}

User.java

public class User
{
    String name;
    String contact;
    String id;
    String tmp;

    public User() {
    }


    public User(String name,String contact,String id) {
        this.name = name;
        this.contact = contact;
        this.id = id;
    }

    public User(String tmp)
    {
        this.tmp=tmp;
    }
    public User(String name,String id) {
        this.name = name;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getContact() {
        return contact;
    }

    public void setContact(String contact) {
        this.contact = contact;

    }

}

database structure in firebase

Exception details:

 com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.firebase_smpl.User
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertBean(CustomClassMapper.java:436)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(CustomClassMapper.java:232)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(CustomClassMapper.java:80)
    at com.google.firebase.database.DataSnapshot.getValue(DataSnapshot.java:203)
    at com.example.firebase_smpl.MainActivity3$1.onDataChange(MainActivity3.java:118)
    at com.google.firebase.database.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:75)
    at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(EventRaiser.java:55)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5628)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:853)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:737)

someone,please help me. I am stuck here.

Note: I changed this line : User u1=(User) ds.child("name").getValue(User.class); To this line: User u1=ds.getValue(User.class); according to suggestion but it is giving me above error:

Firebase list output

Please edit your question add your database structure as a JSON file or at least a screenshot. — Alex Mamo, Nov 11 '20 at 14:36
yes i have added image of database structure please check it out — Bhagyashri Patwardhan, Nov 15 '20 at 06:18

score 0 · Answer 1 · answered Nov 11 '20 at 10:22

0

It should be like...

public class User implements Serializable

User user = ds.getValue(User.class); // ds will be your whatever datasnapshot node

answered Nov 11 '20 at 10:22

Rushit Vaishnani

276
2
6

still it isn't working . It is throwing same exception.I have edited code again check it out. – Bhagyashri Patwardhan Nov 15 '20 at 06:19

score 0 · Accepted Answer · answered Nov 15 '20 at 12:14

0

You are getting the following error:

com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.firebase_smpl.User

Most likely at this line of code:

User u1=(User) ds.child("name").getValue(User.class);

And this is because ds.child("name") returns a String and not a User object. If you need to get each User object under your User node, then please change the above line of code to:

User u1 = ds.getValue(User.class);

Where a casting is not needed at all. If you only need to get the name, then please change it to:

String name = ds.child("name").getValue(String.class);

answered Nov 15 '20 at 12:14

Alex Mamo

130,605
17
163
193

hello ! now i am facing new problem which i have added in queestion. – Bhagyashri Patwardhan Nov 17 '20 at 02:33
When it comes to a new issue, it's best to post a new question. However, you get that behavior because you display the addresses from the memory. To solve this, simply override toString() method in your User class, to return, for example, the name. – Alex Mamo Nov 17 '20 at 08:29
how to do that? can you please tell? – Bhagyashri Patwardhan Nov 19 '20 at 08:34
You can check [this](https://stackoverflow.com/questions/10734106/how-to-override-tostring-properly-in-java) out. – Alex Mamo Nov 19 '20 at 10:28

DatabaseException in firebase android

2 Answers2