gist.java

// The user already has an account and is signed in 
FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();

if (user != null) {
            mDatabase.child("userData").runTransaction(new com.google.firebase.database.Transaction.Handler() {
                @Override
                public com.google.firebase.database.Transaction.Result doTransaction(com.google.firebase.database.MutableData mutableData) {
                    Object value = mutableData.child(user.getUid()).getValue();
                    System.out.println("New value : " + value);
                    if (value == null) {
                        mDatabase.child("userData").child(user.getUid()).push().setValue(1);
                        return com.google.firebase.database.Transaction.success(mutableData);
                    }
                    return null;
                }

                @Override
                public void onComplete(DatabaseError databaseError, boolean b, com.google.firebase.database.DataSnapshot dataSnapshot) {
                    System.out.println("[DEBUG]: inside onComplete!");
                    System.out.println("[DEBUG]: message" + databaseError.getMessage());
                    System.out.println("[DEBUG]: details" + databaseError.getDetails());
                    System.out.println("[DEBUG]: code" + databaseError.getCode());
                    System.out.println("[DEBUG]: boolean b" + b);
                    System.out.println("[DEBUG]: DataS dataSnapshot" + dataSnapshot);
                }
            });
        }