room/runtime/src/main/java/androidx/room/RoomDatabase.java
@NonNull
        public Builder<T> addCallback(@NonNull Callback callback) {
            if (mCallbacks == null) {
                mCallbacks = new ArrayList<>();
            }
            mCallbacks.add(callback);
            return this;
        }
/**
         * Adds a {@link Callback} to this database.
         *
         * @param callback The callback.
         * @return this
         */
Adds a  Callback to this database.
@param callback The callback.
@return this
Adds a Callback to this database . @param callback The callback . @return this
@NonNull
        public Builder<T> addCallback(@NonNull Callback callback) {
            if (mCallbacks == null) {
                mCallbacks = new ArrayList<>();
            }
            mCallbacks.add(callback);
            return this;
        }
@ NonNull public Builder < T > addCallback ( @ NonNull Callback callback ) { if ( mCallbacks == null ) { mCallbacks = new ArrayList < > ( ) ; } mCallbacks . add ( callback ) ; return this ; }
Adds a {@link Callback} to this database.
Adds a { @link Callback } to this database .