空指针异常android rss阅读器应用

Question

我正在关注位于http://www.androidhive.info/2012/05/android-rss-reader-application-using-sqlite-part-1/的教程，它是一个 2 部分，第二部分的链接位于底部第一个。 我输入了所有代码，然后尝试编译它，但我不断收到此错误引起的：java.lang.NullPointerException at bsu.edu.cs339.nbanews.MainActivity.onCreate(MainActivity.java:52) 这是这一行代码

lv.setOnItemClickListener(new OnItemClickListener() {

这个项目很大，有 9 个类和 9 个不同的 xml 文件，所以我没有包含它们，但 url 确实包含教程中包含的所有这些类。 出于好奇，我粘贴了他的代码，认为我可能输入错误，但在同一行上给了我同样的错误。

public class MainActivity extends Activity {

private ProgressDialog pDialog;
ArrayList<HashMap<String, String>> rssFeedList;
RSSParser rssParser = new RSSParser();
RSSFeed rssFeed;
ImageButton btnAddSite;
String[] sqliteIds;

public static String TAG_ID = "id";
public static String TAG_TITLE = "title";
public static String TAG_LINK = "link";
ListView lv;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.site_list);

    btnAddSite = (ImageButton) findViewById(R.id.btnAddSite);

    rssFeedList = new ArrayList<HashMap<String, String>>();

    new loadStoreSites().execute();
    lv = (ListView) findViewById(R.id.list);
    lv.setOnItemClickListener(new OnItemClickListener() {

        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {
            String sqlite_id = ((TextView) view.findViewById(R.id.
                    sqlite_id)).getText().toString();
            Intent in = new Intent(getApplicationContext(),
                    ListRssItemsActivity.class);
            in.putExtra(TAG_ID, sqlite_id);
            startActivity(in);
        }
    });

    btnAddSite.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(getApplicationContext(),
                    AddNewSiteActivity.class);
            startActivityForResult(i, 100);
        }
    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (resultCode == 100) {

        Intent intent = getIntent();
        finish();
        startActivity(intent);
    }
}

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo
        menuInfo) {
   if (v.getId()==R.id.list) {
       menu.setHeaderTitle("Delete");
       menu.add(Menu.NONE, 0, 0, "Delete Feed");
   }
}

@Override
public boolean onContextItemSelected(MenuItem item) {
   AdapterView.AdapterContextMenuInfo info = (AdapterView.AdapterContextMenuInfo)
           item.getMenuInfo();
    int menuItemIndex = item.getItemId();
    if(menuItemIndex == 0){
        RSSDatabaseHandler rssDb = new RSSDatabaseHandler(getApplicationContext());
        WebSite site = new WebSite();
        site.setId(Integer.parseInt(sqliteIds[info.position]));
        rssDb.deleteSite(site);
        Intent intent = getIntent();
        finish();
        startActivity(intent);
    }
    return true;
}

class loadStoreSites extends AsyncTask<String, String, String> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        pDialog = new ProgressDialog(
                MainActivity.this);
        pDialog.setMessage("Loading websites ...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(false);
        pDialog.show();
    }

    protected String doInBackground(String... args) {
        runOnUiThread(new Runnable() {
            public void run() {
                RSSDatabaseHandler rssDb = new RSSDatabaseHandler(
                        getApplicationContext());
                List<WebSite> siteList = rssDb.getAllSites();
                sqliteIds = new String[siteList.size()];

                for (int i = 0; i < siteList.size(); i++) {

                    WebSite s = siteList.get(i);
                    HashMap<String, String> map = new HashMap<String, String>();
                    map.put(TAG_ID, s.getId().toString());
                    map.put(TAG_TITLE, s.getTitle().toString());
                    map.put(TAG_LINK, s.getLink().toString());
                    rssFeedList.add(map);
                    sqliteIds[i] = s.getId().toString();
                }

                ListAdapter adapter = new SimpleAdapter(
                        MainActivity.this,
                        rssFeedList, R.layout.site_list_row,
                        new String[] { TAG_ID, TAG_TITLE, TAG_LINK },
                        new int[] { R.id.sqlite_id, R.id.title, R.id.link });

                lv.setAdapter(adapter);
                registerForContextMenu(lv);
            }
        });
        return null;
    }

    protected void onPostExecute(String args) {

        pDialog.dismiss();
    }
}
}

Answer 1

确保你的activity_main.xml有这个，

<ListView
  android:id="@+id/list"
  android:layout_width="match_parent"
  android:layout_height="wrap_content" 
  .................
  ..........
  ...... >
</ListView>

并从doInBackground移动以下代码

  ListAdapter adapter = new SimpleAdapter(
                        MainActivity.this,
                        rssFeedList, R.layout.site_list_row,
                        new String[] { TAG_ID, TAG_TITLE, TAG_LINK },
                        new int[] { R.id.sqlite_id, R.id.title, R.id.link });

                lv.setAdapter(adapter);

到onPostExecute并在lv.setAdapter(adapter)之后调用adapter.notifyDataSetChanged() lv.setAdapter(adapter) 。