Android & Firebase Async tasks

I have a class Java that I overwritten the toString() method inside this method before returning I consult the fireBase and fill in an object. My problem is the return of toString() ends up running before the firebase has filled the object. How can I tell the class to wait for the object to be filled before it returns

public class exom(){
 private String ID;
 private String horaInicio;
 private String horaFim;
 private String IDEvento;
 // Objeto que deve vir do firebase
 Objeto obj;
  //Getters and setters....
   @Override
   public String toString() {
     FireDB fireDB = new FireDB(null);
     fireDB.getMdatabase().addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
           for (DataSnapshot child : dataSnapshot.getChildren()) {
            obj = child.getValue(Objeto.class);
           }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

     return horaInicio + " - " + horaFim + " - "+ obj.getNome();
   }
}