PersistenceMigrationsSample/app/src/room2/java/com/example/android/persistence/migrations/UserRepository.java [44:64]:
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    void getUser(final LoadUserCallback callback) {
        final WeakReference<LoadUserCallback> loadUserCallback = new WeakReference<>(callback);

        // request the user on the I/O thread
        mAppExecutors.diskIO().execute(() -> {
            final User user = mUserDataSource.getUser();
            // notify on the main thread
            mAppExecutors.mainThread().execute(() -> {
                final LoadUserCallback userCallback = loadUserCallback.get();
                if (userCallback == null) {
                    return;
                }
                if (user == null) {
                    userCallback.onDataNotAvailable();
                } else {
                    mCachedUser = user;
                    userCallback.onUserLoaded(mCachedUser);
                }
            });
        });
    }
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PersistenceMigrationsSample/app/src/room/java/com/example/android/persistence/migrations/UserRepository.java [43:63]:
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    void getUser(LoadUserCallback callback) {
        final WeakReference<LoadUserCallback> loadUserCallback = new WeakReference<>(callback);

        // request the user on the I/O thread
        mAppExecutors.diskIO().execute(() -> {
            final User user = mUserDataSource.getUser();
            // notify on the main thread
            mAppExecutors.mainThread().execute(() -> {
                final LoadUserCallback userCallback = loadUserCallback.get();
                if (userCallback == null) {
                    return;
                }
                if (user == null) {
                    userCallback.onDataNotAvailable();
                } else {
                    mCachedUser = user;
                    userCallback.onUserLoaded(mCachedUser);
                }
            });
        });
    }
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