使用retrofit2

时间：2017-03-08 11:03:36

标签： java android android-studio nullpointerexception retrofit2

使用retrofit2从API下载数据时出现NullPointerException。我在DataService类中解除了内部回调，我在片段ListFragment中调用它来下载数据。 ListFragment已实施RecyclerView并将显示该数据。现在，我想只显示两个textViews。它们的布局名称为single_data_layout。 List<Feature> features是DataResponse。任何想法，我做错了什么？

也许你知道改造equeue（）的现有回调的一些例子？ 在线人数不多。

您可以在GitHub

上找到整个项目

堆栈跟踪：

03-08 09:32:46.913 17938-17938/com.example.lwach.earthquakeapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.lwach.earthquakeapp, PID: 17938
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.lwach.earthquakeapp.apiClient.DataService.getData(retrofit2.Callback)' on a null object reference
    at com.example.lwach.earthquakeapp.ListFragment.onCreateView(ListFragment.java:33)
    at android.support.v4.app.Fragment.performCreateView(Fragment.java:2189)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299)
    at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595)
    at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:757)
    at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2355)
    at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2146)
    at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2098)
    at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1979)
    at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:626)
    at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:1268)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:1116)
    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5827)
    at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:714)
    at android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:90)
    at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1375)
    at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:784)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5827)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
    at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5827)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1435)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:721)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:612)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5827)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5827)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1435)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:721)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:612)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5827)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2897)
    at android.view.View.measure(View.java:18596)
    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2248)
    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1306)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1548)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1191)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6642)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:777)
    at android.view.Choreographer.doCallbacks(Choreographer.java:590)
    at android.view.Choreographer.doFrame(Choreographer.java:560)

ApiClient：

public interface ApiClient {

    @GET("query")
    Call<DataResponse> getData(@QueryMap Map<String, String> options);
}

ApiClientFactory：

public class ApiClientFactory {

    public static final String baseURL = "https://earthquake.usgs.gov/fdsnws/event/1/";

    public ApiClient createApiClient(){
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(baseURL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        return retrofit.create(ApiClient.class);
    }
}

的DataService：

public class DataService {

    private static final String TAG = MainActivity.class.getSimpleName();

    private final ApiClient apiClient;
    private List<Feature> features;
    private Map<String, String> map;

    public DataService() {
        apiClient = new ApiClientFactory().createApiClient();
    }

    Callback<DataResponse> responseCallback = new Callback<DataResponse>() {
        @Override
        public void onResponse(Call<DataResponse> call, Response<DataResponse> response) {
            if (response.isSuccessful()) {
                features = response.body().getFeatures();
            }
            Log.d(TAG, "Data successfully downloaded");
        }

        @Override
        public void onFailure(Call<DataResponse> call, Throwable t) {
            Log.e(TAG, t.toString());
        }
    };

    public void getData(Callback<DataResponse> callback) {

        map = new HashMap<>();
        map.put("format", "geojson");
        map.put("minsig", "700");

        apiClient.getData(map).enqueue(callback);
    }
}

的DataAdapter：

public class DataAdapter extends RecyclerView.Adapter<DataAdapter.DataViewHolder> {

    private final List<Feature> features;
    private LayoutInflater layoutInflater;
    private ListFragment listFragment;

    public DataAdapter(List<Feature> features, ListFragment listFragment) {
        this.features = features;
        this.listFragment = listFragment;
    }

    @Override
    public DataViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = layoutInflater.inflate(R.layout.single_data_layout, parent, false);
        return new DataViewHolder(view);
    }

    @Override
    public void onBindViewHolder(DataViewHolder holder, int position) {

        holder.place.setText(features.get(position).getProperties().getPlace());
        holder.alert.setText(features.get(position).getProperties().getAlert());
    }

    @Override
    public int getItemCount() {
        return features.size();
    }

    public static class DataViewHolder extends RecyclerView.ViewHolder {

        private final TextView place;
        private final TextView alert;

        public DataViewHolder(View view) {
            super(view);
            place = (TextView) view.findViewById(R.id.place_text_view);
            alert = (TextView) view.findViewById(R.id.alert_text_view);
        }
    }
}

ListFragment：

public class ListFragment extends Fragment {

    private List<Feature> features;
    private RecyclerView recyclerView;
    private DataAdapter dataAdapter;
    private DataService dataService;
    private Callback<DataResponse> callback;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_list, container, false);

        dataService.getData(callback);
        recyclerView = (RecyclerView) view.findViewById(R.id.recycler_view);
        dataAdapter = new DataAdapter(features, this);
        recyclerView.setAdapter(dataAdapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

        return view;
    }
}

2 个答案:

答案 0 :(得分：0)

您尚未初始化DataService 其中apiClient变量在其构造函数中初始化。所以直接调用不会调用它的构造函数。

使用此功能。

dataService = new DataService();
dataService.getData(callback);

在ListFragment的onCreateView中。

apiClient变量在

中初始化 
public DataService() {
        apiClient = new ApiClientFactory().createApiClient();
    }

因此，当您直接调用dataService.getData(callback);时，不会调用其构造函数

apiClient.getData(map).enqueue(callback);

将产生 nullpointerexception。因为apiClient变量为空。

答案 1 :(得分：0)

您尚未在ListFragment中初始化DataService。

在使用该对象之前对其进行初始化。

dataService = new DataService();

并致电

dataService.getData(callback);
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