App crashing using collection.add() with Firestore

Question

I'm a newbie on Android development, so please forgive me if this sounds quite trivial or it's too long, but I got stuck trying to add items to a collection in Firestore.

I read somewhere in a tutorial's comments section this occured below API 25 (I'm using 24), but I'm not sure why.

Here's part of my code:

FirebaseFirestore db;

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

    FirebaseFirestore firestore = FirebaseFirestore.getInstance();
    FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder()
            .setTimestampsInSnapshotsEnabled(true)
            .build();
    firestore.setFirestoreSettings(settings);

    db = FirebaseFirestore.getInstance();
}

public void sendInfo(View view){
        CollectionReference dbArtists = db.collection("artistas");
        Artist a = new Artist("0", "A", 10);
        dbArtists.add(a)
    }

sendInfo is associated to a button that works fine. The line that crashes the app is

dbArtists.add(a)

So the logcat had a warning stating:

com.esiete.anapptitle W/Firestore: (0.6.6-dev) [Firestore]: The behavior for java.util.Date objects stored in Firestore is going to change AND YOUR APP MAY BREAK.
To hide this warning and ensure your app does not break, you need to add the following code to your app before calling any other Cloud Firestore methods:

FirebaseFirestore firestore = FirebaseFirestore.getInstance();
FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder()
    .setTimestampsInSnapshotsEnabled(true)
    .build();
firestore.setFirestoreSettings(settings);

With this change, timestamps stored in Cloud Firestore will be read back as com.google.firebase.Timestamp objects instead of as system java.util.Date objects. So you will also need to update code expecting a java.util.Date to instead expect a Timestamp. For example:

// Old:
java.util.Date date = snapshot.getDate("created_at");
// New:
Timestamp timestamp = snapshot.getTimestamp("created_at");
java.util.Date date = timestamp.toDate();

Please audit all existing usages of java.util.Date when you enable the new behavior. In a future release, the behavior will be changed to the new behavior, so if you do not follow these steps, YOUR APP MAY BREAK.

Crashes still remain after adding the code as shown above. Logcat shows:

8-11-13 21:44:09.290 30030-30030/com.esiete.anapptitle D/AndroidRuntime: Shutting down VM
2018-11-13 21:44:09.294 30030-30030/com.esiete.anapptitle E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.esiete.anapptitle, PID: 30030
    java.lang.IllegalStateException: Could not execute method for android:onClick
        at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:390)
        at android.view.View.performClick(View.java:5612)
        at android.view.View$PerformClick.run(View.java:22285)
        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:6123)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385)
        at android.view.View.performClick(View.java:5612) 
        at android.view.View$PerformClick.run(View.java:22285) 
        at android.os.Handler.handleCallback(Handler.java:751) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6123) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757) 
     Caused by: java.lang.RuntimeException: No properties to serialize found on class com.esiete.anapptitle.Artist
        at com.google.firebase.firestore.g.zzg$zza.<init>(SourceFile:643)
        at com.google.firebase.firestore.g.zzg.zza(SourceFile:331)
        at com.google.firebase.firestore.g.zzg.zzb(SourceFile:152)
        at com.google.firebase.firestore.g.zzg.zza(SourceFile:1085)
        at com.google.firebase.firestore.UserDataConverter.convertPOJO(SourceFile:421)
        at com.google.firebase.firestore.CollectionReference.add(SourceFile:125)
        at com.esiete.anapptitle.RegistroActivity.enviarRegistro(RegistroActivity.java:82)
        at java.lang.reflect.Method.invoke(Native Method) 
        at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385) 
        at android.view.View.performClick(View.java:5612) 
        at android.view.View$PerformClick.run(View.java:22285) 
        at android.os.Handler.handleCallback(Handler.java:751) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6123) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757) 
2018-11-13 21:44:09.298 4088-5186/? W/ActivityManager:   Force finishing activity com.esiete.anapptitle/.RegistroActivity
2018-11-13 21:44:09.306 4088-30868/? W/DropBoxManagerService: Dropping: data_app_crash (1752 > 0 bytes)

Any help or word of advice is greatly appreciated.

EDIT: Adding Artist class code.

package com.esiete.anapptitle;

public class Artist {
    private String artistID;
    private String name;
    private int sales;

    public Artist() {
    }

    public Artist(String artistID, String name, int sales) {
        this.artistID = artistID;
        this.name = name;
        this.sales = sales;
    }
}

Answer 1

Your Artist class has no information that Firebase knows how to serialize to and from the database. In order of it to know this, the class needs to either have public fields, or public getter and setter methods.

Say that an Artist document in your database has these three fields:

artistId: "0"
name: "A"
sales: 10

That means that you need either this Java class:

public class Artist {
    public String artistId;
    public String name;
    public long sales;
}

Each public field here matches the exact field name of a document in the database, which is how the Firebase client can read/write the correct values.

Alternatively you can have this Java class:

public class Artist {
    String artistId;
    String name;
    long sales;

    public Artist() {} // Empty constructor is required
    public Artist(String artistId, String name, long sales) {
        this.artistId = artistId;
        this.name = name;
        this.sales = sales;
    }

    public String getArtistId() { return this.artistId; }
    public void setArtistId(String artistId) { this.artistId = artistId; }
    public String getName() { return this.name; }
    public void setName(String name) { this.name = name; }
    public long getRanking() { return this.ranking; }
    public void setRanking(long ranking) { this.ranking = ranking; }
}

This second class follow JavaBean naming conventions, meaning that getId/setId define a property id, which matches the exact field name in the database again.