方法不返回任何东西

Question

I have a java class containing a method CheckForUpdate() that has a return type of String . Call to this method is initiated from my activity class . This method assigns the fetched String value to my class variable named link . Inside the if block variable link shows the fetched String . But in the return statement it returns null . Why is that?

Java Class:

public class method_class {

    Context context;

    public String link;
    boolean b;

    public FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();
    public FirebaseFirestore firestore = FirebaseFirestore.getInstance();

    //constructor
    public method_class(Context context)
    {
        this.context = context;
    }

    public String CheckForUpdate()
    {
        firestore.collection("Admin").document("appinfo")
                .get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {

                if(task.isSuccessful())
                {
                    DocumentSnapshot documentSnapshot = task.getResult();

                    String version = documentSnapshot.getString("version");

                    final String in_app_version = BuildConfig.VERSION_NAME;

                    if(!TextUtils.equals(version, in_app_version))
                    {
                        //update available
                        link = documentSnapshot.getString("link");
                    }
                    else
                    {
                        //no update
                        link = null;
                    }
                }
                else
                {
                    Toast.makeText(context, task.getException().getMessage(),Toast.LENGTH_LONG).show();
                }
            }
        });

        return link;
    }
}

Answer 1

Firebase methods are asynchronous, your method is basically returning the result before the listener fetches an event. Instead you should use the result once your onComplete fires with successful task instead of returning a String with the method

Answer 2

You are using listener to read data. So your method returns link with null value before listener finishes data reading. You should wait listener then return link.