我正在学习如何为Android编写和开发应用程序，而我正在尝试使用新闻应用程序。 但是当我尝试运行应用程序时，我得到了这个错误：java.lang.NullPointerException：尝试在null上调用虚方法'java.lang.Object android.content.Context.getSystemService（java.lang.String）'对象参考。
应用程序一开始工作正常，只有一个活动。但后来我试图用浏览器对它进行打包，我修复了一堆错误，但是这个我无法找出发生了什么。我读了一些关于如何修复NullPointerException的内容，但我仍然无法修复它。
以下是代码：
请不要忘记葡萄牙语中的评论和文件，我只是错了 任何方式，无组织。 （我计划在完成代码后关注它）。
package com.example.android.news;
import android.support.v4.app.Fragment;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.Loader;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.net.Uri;
import android.support.annotation.Nullable;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.ProgressBar;
import android.widget.TextView;
import java.util.ArrayList;
import java.util.List;
public class NewsFragment extends Fragment implements android.support.v4.app.LoaderManager.LoaderCallbacks<List<News>> {
public NewsFragment(){
// Required empty public constructor
}
//Contexto do app
private Context mContext = getContext();
public static final String LOG_TAG = NewsFragment.class.getName();
/** Adapter da lista de news */
private NewsAdapters mAdapter;
/** URL para o dado da notícia (news) do dataset do The Gardian */
private static final String THE_GARDIAN_DATASET =
"https://content.guardianapis.com/search?api-key=4a8dba55-ace9-40bf-8a3c-05a1d2eebd65";
/**
* Valor constante para o ID do loader de news. Podemos escolher qualquer inteiro.
* Isto só importa realmente se você estiver usando múltiplos loaders.
*/
private static final int NEWS_LOADER_ID = 1;
/** TextView que é mostrada quando a lista é vazia */
private TextView mEmptyStateTextView;
/** ProgressBar que é mostrada enquanto o app busca dados do servidor */
private ProgressBar mLoadingSpinner;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_news , container, false);
final ArrayList<News> newses = new ArrayList<News>();
// Acha uma referência para a {@link ListView} no layout
ListView newsListView = (ListView) rootView.findViewById(R.id.list);
if (getTag() != null){
// Cria um novo adapter que pega uma lista vazia de news como entrada
mAdapter = new NewsAdapters(mContext, newses);
// Seta o adapter na {@link ListView}
// para que a lista pode ser populada na interface do usuário
newsListView.setAdapter(mAdapter);
}
//Acha uma referencia para a {@link TextView}
mEmptyStateTextView = (TextView) rootView.findViewById(R.id.empty_state_text_view);
//Acha uma referencia para a {@link ProgressBar}
mLoadingSpinner = (ProgressBar) rootView.findViewById(R.id.loading_spinner);
//Seta um emptyState {@ link emptyView} na ListView {@ link earthquakeListView}
newsListView.setEmptyView(mEmptyStateTextView);
newsListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
// Acha a news atual que foi clicado
News currentNews = mAdapter.getItem(position);
//Converte a URL da String em um objeto URI (para passar no construtor da Intent)
Uri newsUri = Uri.parse(currentNews.getmWebUrl());
// Cria um novo intent para visualizar o URI
Intent webIntent = new Intent(Intent.ACTION_VIEW, newsUri);
// Envia a intent para lançar uma nova activity
startActivity(webIntent);
}
});
//Cria um objeto da classe para monitorar a conexão com a internet
ConnectivityManager cm = (ConnectivityManager) getActivity().getSystemService(Context.CONNECTIVITY_SERVICE);
//Verifica o estado da conexão
NetworkInfo activeNetwork = cm.getActiveNetworkInfo();
boolean isConnected = activeNetwork != null && activeNetwork.isConnectedOrConnecting();
if (isConnected){
// Obtém uma referência ao LoaderManager, a fim de interagir com loaders.
LoaderManager loaderManager = getLoaderManager();
// Inicializa o loader. Passa um ID constante int definido acima e passa nulo para
// o bundle. Passa esta activity para o parâmetro LoaderCallbacks (que é válido
// porque esta activity implementa a interface LoaderCallbacks.
loaderManager.initLoader(NEWS_LOADER_ID, null, this);
} else {
mLoadingSpinner.setVisibility(View.GONE);
mEmptyStateTextView.setText(R.string.no_internet);
}
return rootView;
}
@Override
public Loader<List<News>> onCreateLoader(int id, Bundle args) {
Uri baseUri = Uri.parse(THE_GARDIAN_DATASET);
Uri.Builder uriBuilder = baseUri.buildUpon();
// Criar um novo loader para a dada URL
return new NewsLoader(getContext(), uriBuilder.toString());
}
@Override
public void onLoadFinished(Loader<List<News>> loader, List<News> news) {
//Hide the LoadingSpinner when the loading is finished
mLoadingSpinner.setVisibility(View.GONE);
//Seta o texto a ser mostrado caso não haja news a serem mostrados
mEmptyStateTextView.setText(R.string.empty_state);
// Limpa o adapter de dados de news anteriores
mAdapter.clear();
// Se há uma lista válida de {@link News}s, então os adiciona ao data set do adapter.
// Isto ativará a atualização da ListView.
if (news != null && !news.isEmpty()) {
mAdapter.addAll(news);
}
}
@Override
public void onLoaderReset(Loader<List<News>> loader) {
// Reseta o Loader, então podemos limpar nossos dados existentes.
mAdapter.clear();
}
}
和logcat：
09-29 16:17:44.553 26644-26644/com.example.android.news E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.news, PID: 26644
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at android.view.LayoutInflater.from(LayoutInflater.java:233)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:181)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:166)
at com.example.android.news.NewsAdapters.<init>(NewsAdapters.java:0)
at com.example.android.news.NewsFragment.onCreateView(NewsFragment.java:69)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192)
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:758)
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2363)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2149)
at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2103)
at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1984)
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:21051)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6459)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
at android.view.View.measure(View.java:21051)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6459)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
at android.view.View.measure(View.java:21051)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6459)
at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:393)
at android.view.View.measure(View.java:21051)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6459)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at android.view.View.measure(View.java:21051)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6459)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
at android.view.View.measure(View.java:21051)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6459)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at com.android.internal.policy.DecorView.onMeasure(DecorView.java:849)
at android.view.View.measure(View.java:21051)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2583)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1642)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1893)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1522)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7098)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:927)
at android.view.Choreographer.doCallbacks(Choreographer.java:702)
at android.view.Choreographer.doFrame(Choreographer.java:638)
09-29 16:17:44.553 26644-26644/com.example.android.news E/AndroidRuntime: at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:913)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6682)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)
我无法弄清楚出了什么问题。 如果有人可以帮助我，我会感到沮丧！
答案 0 :(得分：1)
改变这个：
class SuperType {
}
class SubType: SuperType {
}
class GenericType<TypeParameter> {
func someGenericFunction() {
print(TypeParameter.self)
}
}
let instance = GenericType<SuperType>()
let subInstance = GenericType<SubType>()
instance.someGenericFunction() // prints "SuperType"
subInstance.someGenericFunction() // prints "SubType"
let array: [GenericType<SuperType>] = [instance, subInstance] // throws error: cannot convert value of type 'SomeGenericClass<SubType>' to expected element type 'SomeGenericClass<SuperType>'
为：
//Cria um objeto da classe para monitorar a conexão com a internet
ConnectivityManager cm = (ConnectivityManager) getActivity().getSystemService(Context.CONNECTIVITY_SERVICE);
答案 1 :(得分：1)
在onCreateView()方法中将其置于顶部：
this.mContext = getActivity();
你的问题将得到解决。片段没有依赖于它们所处活动的上下文。
答案 2 :(得分：-1)
在Fragment附加到Activity之前，上下文为null。
您需要此功能
//Contexto do app
private Context mContext;
@Override
public void onAttach(Context context) {
super.onAttach(context);
this.mContext = context;
}