Firebase Firestore Distributed Counter documentati

Question

I'm following the documentation to create a distributed counter in Firebase Firestore, however, I'm getting an error on their provided code.

// counters/${ID}
public class Counter {
    int numShards;

    public Counter(int numShards) {
        this.numShards = numShards;
    }
}

// counters/${ID}/shards/${NUM}
public class Shard {
    int count;

    public Shard(int count) {
        this.count = count;
    }
}

When running the createCounter method they've defined

public Task<Void> createCounter(final DocumentReference ref, final int numShards) {
    // Initialize the counter document, then initialize each shard.
    return ref.set(new Counter(numShards))
            .continueWithTask(new Continuation<Void, Task<Void>>() {
                @Override
                public Task<Void> then(@NonNull Task<Void> task) throws Exception {
                    if (!task.isSuccessful()) {
                        throw task.getException();
                    }

                    List<Task<Void>> tasks = new ArrayList<>();

                    // Initialize each shard with count=0
                    for (int i = 0; i < numShards; i++) {
                        Task<Void> makeShard = ref.collection("shards")
                                .document(String.valueOf(i))
                                .set(new Shard(0));

                        tasks.add(makeShard);
                    }

                    return Tasks.whenAll(tasks);
                }
            });
}

I simply get a Runtime Exception

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.enterprises.optaku.bvalyan.gametalk, PID: 8471
                  java.lang.RuntimeException: No properties to serialize found on class com.enterprises.optaku.bvalyan.gametalk.TopicVotingFragment$Counter
                      at com.google.firebase.firestore.g.zzg$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:427)
                      at com.google.firebase.firestore.DocumentReference.set(SourceFile:165)
                      at com.enterprises.optaku.bvalyan.gametalk.TopicVotingFragment.createCounter(TopicVotingFragment.java:217)
                      at com.enterprises.optaku.bvalyan.gametalk.TopicVotingFragment.lambda$null$5$TopicVotingFragment(TopicVotingFragment.java:180)

Even when I serialize the classes, I still get an exception in their code when I go to increment the counter

Shard shard = transaction.get(shardRef).toObject(Shard.class);

The compiler complains there is no no-argument constructor in the Shard class.

I'm at a loss here, as this is the only documentation I can find. Has anyone implemented this successfully and know what I could be missing here?

Answer 1

As the error message says, your Shard class has no no-argument constructor. JavaBean type classes must have a no-argument constructor in order for it to be instantiated by code that can't fully understand what the other constructors do. So, you should add a no-arg constructor to your code:

public class Shard {
    int count;

    public Shard() {}  // this constructor has no arguments

    public Shard(int count) {
        this.count = count;
    }
}

Without that constructor, the Firestore SDK doesn't have a predictable way to create instances of Shard.