当我的arraylist中没有项目时，CollectionsFragment无法打开并崩溃我的应用程序。
只有在已经添加了对象的情况下才能访问CollectionsFragment。
package fragments;
/*Imports*/
public class CollectionsFragment extends Fragment {
private static List<Article> mArticleList = new ArrayList<>();
private ListView lvArticles;
private CollectionAdapter mAdapter;
public CollectionsFragment() {}
public static CollectionsFragment newInstance() {
Bundle args = new Bundle();
CollectionsFragment fragment = new CollectionsFragment();
fragment.setArguments(args);
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
new MyTask().execute();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_collections, container, false);
lvArticles = (ListView) view.findViewById(R.id.lvArticles);
mAdapter = new CollectionAdapter(getContext(), R.layout.row, mArticleList);
lvArticles.setAdapter(mAdapter);
lvArticles.setEmptyView(view.findViewById(R.id.empty));
return view;
}
private class MyTask extends AsyncTask<Article, Article, Void> {
@Override
protected Void doInBackground(Article... articles) {
Article item = (Article) getArguments().getSerializable("Article");
mArticleList.add(item);
return null;
}
@Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
mAdapter.notifyDataSetChanged();
lvArticles.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Article item = mArticleList.get(position);
Intent intent = new Intent(getContext(), WebViewActivity.class);
intent.putExtra("URL", item.getUrl());
getContext().startActivity(intent);
}
});
lvArticles.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
Article item = mArticleList.get(position);
createDialog(view, item);
return true;
}
});
}
private void createDialog(View view, final Article item) {
final Dialog d = new Dialog(getContext());
d.setContentView(R.layout.dialog_collections);
d.setTitle("Remove article?");
d.setCancelable(true);
Button b = (Button) d.findViewById(R.id.button1);
b.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
mArticleList.remove(item);
mAdapter.notifyDataSetChanged();
}
});
d.show();
}
}
}
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String models.Article.getAuthor()' on a null object reference
at adapters.CollectionAdapter.getView(CollectionAdapter.java:49)
at android.widget.AbsListView.obtainView(AbsListView.java:2347)
at android.widget.ListView.makeAndAddView(ListView.java:1864)
at android.widget.ListView.fillDown(ListView.java:698)
at android.widget.ListView.fillFromTop(ListView.java:759)
at android.widget.ListView.layoutChildren(ListView.java:1659)
at android.widget.AbsListView.onLayout(AbsListView.java:2151)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1077)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.support.design.widget.HeaderScrollingViewBehavior.layoutChild(HeaderScrollingViewBehavior.java:131)
at android.support.design.widget.ViewOffsetBehavior.onLayoutChild(ViewOffsetBehavior.java:42)
at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onLayoutChild(AppBarLayout.java:1367)
at android.support.design.widget.CoordinatorLayout.onLayout(CoordinatorLayout.java:849)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.support.v4.widget.DrawerLayout.onLayout(DrawerLayout.java:1193)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15718)
at android.view.ViewGroup.layout(ViewGroup.java:5039)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2129)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1886)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1103)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5944)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:788)
at android.view.Choreographer.doCallbacks(Choreographer.java:601)
at android.view.Choreographer.doFrame(Choreographer.java:571)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:774)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Loo
package adapters;
/* Imports */
public class CollectionAdapter extends ArrayAdapter {
private List<Article> articleList;
private int resource;
private final LayoutInflater inflater;
public CollectionAdapter(Context context, int resource, List<Article> objects) {
super(context, resource, objects);
articleList = objects;
this.resource = resource;
inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
if (convertView == null) {
holder = new ViewHolder();
convertView = inflater.inflate(resource, null);
holder.author = (TextView) convertView.findViewById(R.id.a_author);
holder.title = (TextView) convertView.findViewById(R.id.a_title);
holder.desc = (TextView) convertView.findViewById(R.id.a_desc);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
holder.author.setText(articleList.get(position).getAuthor());
holder.title.setText(articleList.get(position).getTitle());
holder.desc.setText(articleList.get(position).getDescription());
return convertView;
}
private class ViewHolder {
private TextView author;
private TextView title;
private TextView desc;
}
}
答案 0 :(得分：0)
执行以下操作：
通过在View view之外声明您的onCreateView()对象类字段。
在onPostExecute()
因为你的asyncTask有可能还没有完成，你试图在你的适配器中使用mArticleList。
mAdapter = new CollectionAdapter(getContext(), R.layout.row, mArticleList);
lvArticles.setAdapter(mAdapter);
lvArticles.setEmptyView(view.findViewById(R.id.empty));
答案 1 :(得分：0)
在AsyncTask中：
@Override
protected Void doInBackground(Article... articles) {
Article item = (Article) getArguments().getSerializable("Article");
if(item!=null){
mArticleList.add(item);
}
return null;
}
在您的代码中，ArrayList添加了NULL模型类，以便您在行上获得错误
holder.author.setText(articleList.get(position).getAuthor());
缺少一个更重要的覆盖方法。
@Override
public int getCount() {
return articleList != null ? articleList.size() : 0;
}
答案 2 :(得分：0)
我不认为您的AsyncTask是必要的。代码中没有任何内容阻塞或长时间运行。
问题是你在列表中得到了空对象，我认为它来自getSerializable，因此看起来如何使用Author参数启动Fragment导致该问题。
除此之外，您可以使用mAdapter.add将对象添加到ListView中，但您应该extends ArrayAdapter<Article> {
您应该从onCreateView设置click事件侦听器。如果确实需要AsyncTask，它应该只向适配器添加数据并通知它，而不是为ListView重新分配所有侦听器
答案 3 :(得分：-1)
另外，请考虑这样做：
@Override
public int getCount() {
return articleList != null ? articleList.size() : 0;
}
这将阻止您的列表被加载，并且您将不会有空指针。