尝试访问 API 时出现空指针异常

Question

我刚刚为我目前正在开发的新闻应用程序尝试了 retrofit2，我尝试在片段类中调用我的 API 实现。但我不断收到 nullpointerexception，这是我的片段代码：

public class IgnFragment extends Fragment {


@BindView(R.id.ignNewsContainer)
RecyclerView ignNewsContainer;
Unbinder unbinder;

Call<NewsArticle> call;
public IgnFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_ign, container, false);
    unbinder = ButterKnife.bind(this, view);
    return view;

}

@Override
public void onStart() {
    super.onStart();
     call = NewsApi.getApi().getArticles("ign", "top");
    getClient();
}
public void getClient(){
    call.enqueue(new Callback<NewsArticle>() {
        @Override
        public void onResponse(Call<NewsArticle> call, Response<NewsArticle> response) {
            NewsArticle newsArticle = response.body();
            NewsAdapter newsAdapter = new NewsAdapter(newsArticle.getArticles(),getContext());
            ignNewsContainer.setAdapter(newsAdapter);
            LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getContext());
            linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
            ignNewsContainer.setLayoutManager(linearLayoutManager);
        }

        @Override
        public void onFailure(Call<NewsArticle> call, Throwable t) {
            t.printStackTrace();
            Log.e("Error","Called");
        }
    });
}

@Override
public void onDestroyView() {
    super.onDestroyView();
    unbinder.unbind();
}

这是我的 api 服务类：

public class NewsApi {
private static final String API_KEY = "________";
private static final String URL_BASE ="https://newsapi.org/v1/";


private static NewsService newsService =null;
public static NewsService getApi(){
   if(newsService == null){
       Retrofit retrofit= new Retrofit.Builder()
               .baseUrl(URL_BASE)
               .addConverterFactory(GsonConverterFactory.create())
               .client(getClient())
               .build();
        NewsService api = retrofit.create(NewsService.class);
   }
    return newsService;
}

public static OkHttpClient getClient(){
    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
    logging.setLevel(HttpLoggingInterceptor.Level.BODY);
    OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
    return  httpClient.addInterceptor(logging).build();
}

public interface  NewsService{
    @GET("articles?apiKey=" + API_KEY)
    Call<NewsArticle> getArticles(@Query("source") String source,@Query("sortBy") String sortBy);
}

}

这是我得到的错误：

10-30 20:42:01.640 32640-32640/cc.codemonkey.newsportal E/AndroidRuntime: 致命例外: main 进程：cc.codemonkey.newsportal，PID：32640 java.lang.NullPointerException：尝试在空对象引用上调用接口方法“retrofit2.Call cc.codemonkey.newsportal.service.NewsApi$NewsService.getArticles(java.lang.String, java.lang.String)” 在 cc.codemonkey.newsportal.fragments.IgnFragment.onStart(IgnFragment.java:54) 在 android.support.v4.app.Fragment.performStart(Fragment.java:2265) 在 android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1369) 在 android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1574) 在 android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1641) 在 android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:794) 在 android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2415) 在 android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2200) 在 android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2153) 在 android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:2034) 在 android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:651) 在 android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143) 在 android.support.v4.view.ViewPager.populate(ViewPager.java:1239) 在 android.support.v4.view.ViewPager.populate(ViewPager.java:1087) 在 android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1613) 在 android.view.View.measure(View.java:17442) 在 android.widget.RelativeLayout.measureChildHorizo​​ntal(RelativeLayout.java:727) 在 android.widget.RelativeLayout.onMeasure(RelativeLayout.java:463) 在 android.view.View.measure(View.java:17442) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463) 在 android.widget.FrameLayout.onMeasure(FrameLayout.java:430) 在 android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139) 在 android.view.View.measure(View.java:17442) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463) 在 android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436) 在 android.widget.LinearLayout.measureVertical(LinearLayout.java:722) 在 android.widget.LinearLayout.onMeasure(LinearLayout.java:613) 在 android.view.View.measure(View.java:17442) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463) 在 android.widget.FrameLayout.onMeasure(FrameLayout.java:430) 在 android.view.View.measure(View.java:17442) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463) 在 android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436) 在 android.widget.LinearLayout.measureVertical(LinearLayout.java:722) 在 android.widget.LinearLayout.onMeasure(LinearLayout.java:613) 在 android.view.View.measure(View.java:17442) 在 android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463) 在 android.widget.FrameLayout.onMeasure(FrameLayout.java:430) 在 com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560) 在 android.view.View.measure(View.java:17442) 在 android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001) 在 android.view.ViewRootImpl.measureHierarchy（ViewRootImpl.java:1166） 在 android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372) 在 android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054) 在 android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779) 在 android.view.Choreographer$CallbackRecord.run(Choreographer.java:767) 在 android.view.Choreographer.doCallbacks(Choreographer.java:580) 在 android.view.Choreographer.doFrame(Choreographer.java:550) 在 android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753) 在 android.os.Handler.handleCallback(Handler.java:739) 在 android.os.Handler.dispatchMessage(Handler.java:95) 在 android.os.Looper.loop(Looper.java:135) 在 android.app.ActivityThread.main(ActivityThread.java:5253) 在 java.lang.

Answer 1

你的问题就在这一行

NewsAdapter newsAdapter = new NewsAdapter(newsArticle.getArticles(),getContext());

问题是newsArticle是null，这里已经初始化了

NewsArticle newsArticle = response.body();

所以response.body() 为空，请尝试调试并查看为什么它为空！当您解决此问题时，您的解决方案应该可以工作。 此外，请参阅此 What is a NullPointerException, and how do I fix it? 以了解更多 NullPointerException 是什么。

Answer 2

如果你正在使用retrofit2，那么请在你的“新闻”类中检查你是否写得像

@SerializedName("articles")
@Expose
private List<Article> articles;

而不是

@SerializedName("article")  <------ article != articles
@Expose
private List<Article> articles;