code: public void getLocation(String key, LocationCallback callback) {
          DatabaseReference keyRef = this.getDatabaseRefForKey(key);
          LocationValueEventListener valueListener = new LocationValueEventListener(callback);
          keyRef.addListenerForSingleValueEvent(valueListener);
      }
docs: Gets the current location for a key and calls the callback with the current value.
  
  @param key      The key whose location to get
  @param callback The callback that is called once the location is retrieved