Question

我是android开发的菜鸟，我试图在viewpager中显示listview。但是，当我尝试运行应用程序时，由于nullpointerexception，它会在onCreate期间崩溃。这一行引起了异常：

listView.setAdapter(new UserItemAdapter(MyPagerActivity.this, R.layout.listitem, tweets));

我使用了教程here所以我不明白为什么我遇到这个问题。我是否正确实例化了listview？非常感谢任何帮助。

public class MyPagerActivity extends Activity {

///////////////////////
ArrayList<Tweet> tweets;
ProgressDialog dialog;
Handler handler; 
ImageLoader tango;


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.mypagermain);

    MyPagerAdapter adapter = new MyPagerAdapter();
    ViewPager myPager = (ViewPager) findViewById(R.id.myfivepanelpager);
    myPager.setAdapter(adapter);
    myPager.setCurrentItem(0);

    //////////////////////
    tango = new ImageLoader(MyPagerActivity.this); 
    tweets = new ArrayList<Tweet>();

    try {           
        JSONObject jArray = JSONfunctions.getJSONfromURL("http://search.twitter.com/search.json?q=gold%20silver%20news&rpp=15");
        JSONArray parsedTwitter = jArray.getJSONArray("results");       
        if(parsedTwitter.equals("") ){
            Toast.makeText(MyPagerActivity.this, "No twitter news", Toast.LENGTH_LONG).show();
        }else{
            for (int i = 0; i < parsedTwitter.length(); i++) {
            JSONObject parsedspecific = parsedTwitter.getJSONObject(i);             
        if (i == 0){                
            String from_user = parsedspecific.getString("from_user");
            String text = parsedspecific.getString("text"); 
            String profile_image_url = parsedspecific.getString("profile_image_url");  
            Tweet tweet0 = new Tweet(from_user, text, profile_image_url );
            tweets.add(tweet0);                 
            }

              }//<--for close
            }//<--else close

    } catch (Exception e) {
        Log.e("log_tag", "Error parsing data "+ e.toString());
    }

    ListView listView = (ListView) findViewById(R.id.ListViewId);  
    listView.setAdapter(new UserItemAdapter(MyPagerActivity.this, R.layout.listitem, tweets));

}



private class MyPagerAdapter extends PagerAdapter {

    public int getCount() {
        return 5;
    }

    public Object instantiateItem(View collection, int position) {

        LayoutInflater inflater = (LayoutInflater) collection.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);


        int resId = 0;
        switch (position) {
        case 0:
            resId = R.layout.news;
            break;
        case 1:
            resId = R.layout.coinshows;
            break;
        case 2:
            resId = R.layout.twitterlist;
            break;
        case 3:
            resId = R.layout.videos;
            break;
    //  case 4:
    //      resId = R.layout.farright;
    //      break;
        }

        View view = inflater.inflate(resId, null);

        ((ViewPager) collection).addView(view, 0);

        return view;
    }



}

XML导致null：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<ListView
    android:id="@+id/ListViewId"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" />

</LinearLayout>

MyPagerMain XML：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:id="@+id/tab2"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<android.support.v4.view.ViewPager
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:id="@+id/myfivepanelpager"
android:background="#234567"/>
</LinearLayout>

编辑基于Dennis Drew的回答（仍然导致nullpointer）

private class MyPagerAdapter extends PagerAdapter {

    public int getCount() {
        return 5;
    }

    public Object instantiateItem(View collection, int position) {

        LayoutInflater inflater = (LayoutInflater) collection.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);


        int resId = 0;
        switch (position) {
        case 0:
            resId = R.layout.news;
            View view0 = inflater.inflate(resId, null);
            WebView newsfeed = (WebView) findViewById(R.id.webViewnews);
            ((ViewPager) collection).addView(view0, 0);
            return view0;

        case 1:
            resId = R.layout.coinshows;
            View view1 = inflater.inflate(resId, null);
            WebView coinshows = (WebView) findViewById(R.id.webViewcoinshows);
            ((ViewPager) collection).addView(view1, 0);
            return view1;

        case 2:
            resId = R.layout.twitterlist;
            View view2 = inflater.inflate(resId, null);
            LinearLayout layout=(LinearLayout)view2.findViewById(R.id.LLtwitter);
            ListView listView = (ListView) findViewById(R.id.ListViewId);  
            listView.setAdapter(new UserItemAdapter(MyPagerActivity.this, R.layout.listitem, tweets));
            ((ViewPager) collection).addView(view2, 0);
            return view2;
            //break;
        case 3:
            resId = R.layout.videos;
            View view3 = inflater.inflate(resId, null);             
            WebView youtube = (WebView) findViewById(R.id.webViewyoutube);
            ((ViewPager) collection).addView(view3, 0);
            return view3;


        }
        return resId;


    }

Answer 1

发生了什么事情，你的findViewById在ListView上返回null，因为它在ViewPager中。在当前项目的视图被夸大后，您必须在PagerAdapter的instantiateItem中初始化ListView。在此处查看我对类似问题的回答：ViewPager findViewById

编辑：使用示例代码进行更新以澄清

public Object instantiateItem（View collection，int position）{

    LayoutInflater inflater = (LayoutInflater) collection.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);


    int resId = 0;
    switch (position) {
    case 0:
        resId = R.layout.news;
        View view0 = inflater.inflate(resId, null);
        WebView newsfeed = (WebView) findViewById(R.id.webViewnews);
        ((ViewPager) collection).addView(view0, 0);
        return view0;

    case 1:
        resId = R.layout.coinshows;
        View view1 = inflater.inflate(resId, null);
        WebView coinshows = (WebView) findViewById(R.id.webViewcoinshows);
        ((ViewPager) collection).addView(view1, 0);
        return view1;

    case 2:
        resId = R.layout.twitterlist;
        View view2 = inflater.inflate(resId, null);
        LinearLayout layout=(LinearLayout)view2.findViewById(R.id.LLtwitter);

        //Pay close attention to this modification
        //Rather than this --> ListView listView = (ListView) findViewById(R.id.ListViewId);  

        //Do this
        ListView listView = (ListView)view2.findViewById(R.id.ListViewId);
        listView.setAdapter(new UserItemAdapter(MyPagerActivity.this, R.layout.listitem, tweets));
        ((ViewPager) collection).addView(view2, 0);
        return view2;
        //break;
    case 3:
        resId = R.layout.videos;
        View view3 = inflater.inflate(resId, null);             
        WebView youtube = (WebView) findViewById(R.id.webViewyoutube);
        ((ViewPager) collection).addView(view3, 0);
        return view3;


    }
    return resId;


}

认为你的instantiateItem方法几乎就像一个sub-onCreate，在某种意义上，你必须在每个单独的寻呼机项目中膨胀的视图上调用findViewById来执行findViewById（view.findViewById）。希望这会有所帮助。

如何使用Viewpager实现ListView？

1 个答案: