DatabaseException: Failed to convert value of type java.util.HashMap to String - Firebase

Question

In my project, I am implementing a login system using Firebase Authentication. Since I am using email logins, I cannot store the name of any of my users in the authentication - instead, I am storing them in the realtime database as children of the userId. So my database structure looks like this:

fireapp 
   +users
     +userID
       Name: <name>

Where each user will have a record according to their ID.

When my user logs in, they will be taken to the home screen that has a navigation drawer. In the header of this drawer, I would like to display the user's name and email. So far I have set the email using FirebaseAuth, but setting the name has proven more difficult.

This is the code I am using currently:

String userId = mAuth.getUid();
TextView userEmail = (TextView) findViewById(R.id.lblHeaderName);

FirebaseDatabase db = FirebaseDatabase.getInstance();
DatabaseReference ref = db.getReference("users/" + userId);
ref.addValueEventListener(new ValueEventListener()
{
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot)
    {
        //Retrieve the user's name and set the textview in the header accordingly
        String name = dataSnapshot.getValue(String.class);
        userName.setText(name);
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError)
    {
        //If the task is not successful, print an error message
        Toast.makeText(HomeScreen.this, "Error retrieving data from database: " + databaseError.getCode(), Toast.LENGTH_LONG).show();

    }
});

Here I am first getting the current user's ID and using that as the database path. Next I'm trying to get the name of the user and assign it to a string (since that is the only attribute I have stored for each user). finally setting it as the text of userName. When I run the code, the app crashes with this error:

2019-05-01 12:54:33.882 6447-6447/com.mobileappscw3.nrcforums E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.mobileappscw3.nrcforums, PID: 6447
    com.google.firebase.database.DatabaseException: Failed to convert value of type java.util.HashMap to String
        at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertString(com.google.firebase:firebase-database@@16.1.0:413)
        at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(com.google.firebase:firebase-database@@16.1.0:199)
        at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(com.google.firebase:firebase-database@@16.1.0:79)
        at com.google.firebase.database.DataSnapshot.getValue(com.google.firebase:firebase-database@@16.1.0:212)
        at com.mobileappscw3.nrcforums.HomeScreen$1.onDataChange(HomeScreen.java:76)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.1.0:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.1.0:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.1.0:55)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

Where HomeScreen.java:76 is this line in my code:

String name = dataSnapshot.getValue(String.class);

The tutorial that I was following used a String variable and parameter and it worked. However, every other example of this problem I have looked at uses User objects to retrieve data - however since I only have one attribute stored about the users (and this app is more proof-of-concept rather than a real working app), I would prefer not to use objects if possible. Can anyone help?

Peter Haddad · Accepted Answer · 2019-05-01 14:00:28Z

5

Change this:

String name = dataSnapshot.getValue(String.class);

into this:

String name = dataSnapshot.child("Name").getValue(String.class);

answered May 1, 2019 at 14:00

Peter Haddad

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Kkkk · Accepted Answer · 2019-05-01 14:02:40Z

3

It might help you out: Firebase "Failed to convert a value of type java.util.HashMap to int"

You might want to change String name = dataSnapshot.getValue(String.class);

to:

String name = dataSnapshot.child("users").child("userID").getValue(String.class);

answered May 1, 2019 at 14:02

Kkkk

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Comments

Frank van Puffelen · Accepted Answer · 2019-05-01 14:06:19Z

3

Some of the other answers already show you how to read a specific property from a DataSnapshot inside the onDataChange method.

If you only need a single property, an alternative would be to attach your listener to just that property:

DatabaseReference ref = db.getReference("users/" + userId + "/Name");
ref.addValueEventListener(new ValueEventListener()
{
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot)
    {
        //Retrieve the user's name and set the textview in the header accordingly
        String name = dataSnapshot.getValue(String.class);
        userName.setText(name);
    }

The advantage of this is that the listener only reads/downloads the single property that is used inside the onDataChange, so it uses less bandwidth.

answered May 1, 2019 at 14:06

Frank van Puffelen

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1 Comment

AgentNo Over a year ago

Ah, that's actually quite a good idea - I wasn't aware that you could put attributes in the path, but looking back that's common sense. As I said, this was more proof-of-concept rather than an actual deployment version, so I'm not overly fussed on performance currently. However, I will keep this in mind for actual projects - thanks very much!

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