如果缓存为空NPE，则进行改造

Question

I add the possibility of caching and the following script happens: Everything works if the pages have been previously uploaded, but if I go without the Internet for the first time then the application crashes and throws an error

NullPointerException: Attempt to invoke interface method 'java.lang.Object[] java.util.Collection.toArray()' on a null object reference
    at java.util.ArrayList.<init>(ArrayList.java:171)
    at n.MainDetailTrainFragment$3.onResponse(MainDetailTrainFragment.java:171)

I understand the error, but how can I fix it?

do not judge strictly, I'm just learning this. here is my code^

progressBarTrain.setVisibility(View.VISIBLE);
    Log.d(TAG, "getProducts");

   final OkHttpClient client = new OkHttpClient
           .Builder()
           .cache(new Cache(getActivity().getCacheDir(), 10 * 1024 * 1024)) // 10 MB
           .addInterceptor(new Interceptor() {
               @Override public okhttp3.Response intercept(@NonNull Chain chain) throws IOException {
                   Request request = chain.request();
                   if (NetworkUtils.isNetworkAvailable(getActivity())) {
                       request = request.newBuilder().header("Cache-Control", "public, max-age=" + 60).build();
                   } else {
                       request = request.newBuilder().header("Cache-Control", "public, only-if-cached, max-stale=" + 60 * 60 * 24 * 7).build();
                   }
                   return chain.proceed(request);
               }
           })
           .build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(MY_URL)
            .client(client)
            .addConverterFactory(SimpleXmlConverterFactory.create())
            .build();

    retrofit.create(myTrainInterface.class)
            .getProducts(APP_ID, FROM_CITY, position_0, "everyday")
            .enqueue(new Callback<Responce>() {
                @SuppressLint("SetTextI18n")
                @Override
                public void onResponse(@NonNull Call<Responce> call, @NonNull Response<Responce> response) {
                    progressBarTrain.setVisibility(View.INVISIBLE);
                    if (response.body() != null) {
                        kk = response.body().products;
                    }
                    ArrayList<Responce.Item> p = new ArrayList<>(kk);
                    String s = String.valueOf(response.body());




                    adapter = new DetailTrainAdapter(p);
                    recyclerView.setAdapter(adapter);
                    Log.d(TAG, s);

                    textError.setVisibility(TextView.INVISIBLE);
                    textErrorRestart.setVisibility(TextView.INVISIBLE);
                }

                @Override
                public void onFailure(@NonNull Call<Responce> call, @NonNull Throwable throwable) {
                    Log.d(TAG, "onFailure" + throwable.getLocalizedMessage());
                    textError();
                }
            });

Answer 1

The NPE is thrown when variable kk is null . Looking at your code, you're creating a list out of your response (if not null ) and then using it to populate your UI. However, if the response is null you're not doing assigning anything to kk , so it stays with its previous values (most probably null , given the NPE).

A possible solution to avoid that is to execute all the logic of creating the adapter and setting it to your UI component only if response.body() != null , otherwise you might want to do something else (eg, display an error).