调用自定义适配器时的空点异常：Android

时间：2016-12-31 07:49:07

标签： java android listview android-fragments nullpointerexception

我想在ListView中创建Fragment。我为数据类型为ArrayList的对象数组创建了一个自定义适配器。

但是，当我启动包含此Article片段的活动时，应用会崩溃。

错误

 12-31 12:01:09.746 25620-25620/com.example.root.talaash E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.root.talaash, PID: 25620
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.root.talaash/com.example.root.talaash.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2355)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418)
    at android.app.ActivityThread.access$800(ActivityThread.java:167)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1326)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5309)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
    at com.example.root.fragments.ArticleFragment.onCreateView(ArticleFragment.java:40)
    at android.support.v4.app.Fragment.performCreateView(Fragment.java:2087)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1113)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1295)
    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:801)
    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1682)
    at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:388)
    at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:607)
    at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:181)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1236)
    at android.app.Activity.performStart(Activity.java:6068)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2318)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418) 
    at android.app.ActivityThread.access$800(ActivityThread.java:167) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1326) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5309) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703) 

文章类（包：com.example.root.aditional）

public class Article {
    String title;
    String author;
    String category;
    String date;
    String url;
    Boolean tag = false;

    public Article(String title, String author, String category, String date, String url) {
        this.title = title;
        this.author = author;
        this.category = category;
        this.date = date;
        this.url = url;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }

    public String getCategory() {
        return category;
    }

    public String getDate() {
        return date;
    }

    public void setTag(Boolean tag) {
        this.tag = tag;
    }

    public String getUrl() {
        return url;
    }
}

ArticleAdaper类（包：com.example.root.adapters）

public class ArticleAdapter<A> extends ArrayAdapter<Article>{

    private static class ViewHolder {
        TextView title, author, category, date;
    }

    public ArticleAdapter(Context context, ArrayList<Article> articles) {
        super(context, R.layout.row_article, articles);
    }

    @NonNull
    @Override
    public View getView(int position, View convertView, @NonNull ViewGroup parent) {
        ViewHolder viewHolder = new ViewHolder();
        if (convertView == null) {
            convertView = LayoutInflater.from(this.getContext()).inflate(R.layout.row_article, parent, false);

            viewHolder.title = (TextView)convertView.findViewById(R.id.titleText);
            viewHolder.author= (TextView)convertView.findViewById(R.id.authorText);
            viewHolder.category= (TextView)convertView.findViewById(R.id.categoryText);
            viewHolder.date = (TextView)convertView.findViewById(R.id.dateText);

            convertView.setTag(viewHolder);
        } else {
            viewHolder = (ViewHolder)convertView.getTag();
        }

        Article item = getItem(position);
        if(item!= null){
            viewHolder.title.setText(item.getTitle());
            viewHolder.author.setText(item.getAuthor());
            viewHolder.category.setText(item.getCategory());
            viewHolder.date.setText(item.getDate());
        }

        return convertView;
    }
}

我在阅读这个问题的第一个答案的帮助下写了上面的课： How to use ArrayAdapter<myClass>

包含listView（package：com.example.root.project）

的片段 
public class ArticleFragment extends Fragment {

    public ArticleFragment() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        ArrayList<Article> articles = new ArrayList<>(6);
        articles.add(new Article("Rose", "Sample Author", "Sample category", "Sample date", "Sample url"));

        ListAdapter articleAdapter = new ArticleAdapter(this.getContext(), articles);
        ListView listView = (ListView)container.findViewById(R.id.articleListView);
        listView.setAdapter(articleAdapter);

        /*ListView listView = (ListView)findViewById(R.id.articleListView);*/
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_article, container, false);
    }
}

我对Android开发非常陌生，我想详细解释我哪里出错了以及我该怎么做才能解决它。

4 个答案:

答案 0 :(得分：1)

  

当应用程序尝试使用时抛出NullPointerException   具有空值的对象引用。

首先返回您的观点。

不要

Sub finddata()
    With Sheets("Sheet1")
        .Range("P3:X37").ClearContents
        With .Range("B1", .Cells(.Rows.count, 2).End(xlUp)) '<--| reference column "B" range from row 1 (header) down to last not empty row
            .AutoFilter field:=1, Criteria1:=Application.Transpose(.Parent.Range("K8:K30").Value), Operator:=xlFilterValues '<--| filter on all K8:K30 values
            If Application.WorksheetFunction.Subtotal(103, .Cells) > 1 Then '<--| if any filtered cell found
                .Offset(1, -1).Resize(.Rows.count - 1, 3).SpecialCells(xlCellTypeVisible).Copy '<-- copy filtered range offsetted one column to the right and resized to three columns
                .Parent.Cells(.Rows.count, "P").End(xlUp).Offset(1, 0).PasteSpecial xlPasteFormulasAndNumberFormats '<--| paste special
            End If
        End With
        .AutoFilterMode = False
    End With
End Sub

待办事项

return inflater.inflate(R.layout.fragment_article, container, false);

然后传递 getActivity（）

实施例

return your view ;

答案 1 :(得分：0)

您正试图在错误CreateProcessAsUser内找到ListView。试试这个 - 

ViewGroup

答案 2 :(得分：0)

您需要先在ListView中对视图进行充气，然后您需要找到container而不是onCreateView。最后将视图作为public class ArticleFragment extends Fragment { public ArticleFragment() { // Required empty public constructor } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_article, container, false); ArrayList<Article> articles = new ArrayList<>(6); articles.add(new Article("Rose", "Sample Author", "Sample category", "Sample date", "Sample url")); ListAdapter articleAdapter = new ArticleAdapter(this.getContext(), articles); ListView listView = (ListView) v.findViewById(R.id.articleListView); listView.setAdapter(articleAdapter); // Return the inflated Fragment here return v; } } 中的最后一个return语句返回。

{{1}}

答案 3 :(得分：0)

试试这个：

    @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    View view = inflater.inflate(R.layout.fragment_article, container, false);
    ArrayList<Article> articles = new ArrayList<>(6);
    articles.add(new Article("Rose", "Sample Author", "Sample category", "Sample date", "Sample url"));

    ListAdapter articleAdapter = new ArticleAdapter(this.getContext(), articles);
    ListView listView = (ListView)view.findViewById(R.id.articleListView);

    listView.setAdapter(articleAdapter);

    /*ListView listView = (ListView)findViewById(R.id.articleListView);*/
    // Inflate the layout for this fragment
    return view;
}

您没有正确获取列表视图

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