ListView Activity OnItemClickListener TextView 空指针异常

Question

我希望当我单击特定行时，应通过意图操作将文本复制并粘贴到另一个活动。我已经实现了代码，但有一件事导致空指针异常。请帮助我。

//@Override
         // listening to single list item on click
      listview.setOnItemClickListener(new OnItemClickListener() {
          @Override
            public void onItemClick(AdapterView<?> parent, View view,
                    int position, long id) {

              // selected item
            TextView  text = (TextView) view.findViewById(R.id.title);
              String lst_txt = text.getText().toString().trim();

             System.out.println("Display text"+lst_txt ); 
              // Launching new Activity on selecting single List Item
              Intent i = new Intent(MainActivity.this, SingleListItem.class);
              // sending data to new activity
              i.putExtra("product",lst_txt );
              startActivity(i);

          }
        });

//这里TextView text = (TextView) view.findViewById(R.id.title);导致空指针异常

日志猫

12-12 07:41:25.459: E/AndroidRuntime(2326): FATAL EXCEPTION: main
12-12 07:41:25.459: E/AndroidRuntime(2326): java.lang.NullPointerException
12-12 07:41:25.459: E/AndroidRuntime(2326):     at com.example.TwitterTutorial.MainActivity$1.onItemClick(MainActivity.java:70)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.widget.AdapterView.performItemClick(AdapterView.java:298)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.widget.AbsListView.performItemClick(AbsListView.java:1100)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.widget.AbsListView$PerformClick.run(AbsListView.java:2749)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.widget.AbsListView$1.run(AbsListView.java:3423)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.os.Handler.handleCallback(Handler.java:725)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.os.Handler.dispatchMessage(Handler.java:92)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.os.Looper.loop(Looper.java:137)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at android.app.ActivityThread.main(ActivityThread.java:5039)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at java.lang.reflect.Method.invokeNative(Native Method)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at java.lang.reflect.Method.invoke(Method.java:511)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
12-12 07:41:25.459: E/AndroidRuntime(2326):     at dalvik.system.NativeStart.main(Native Method)

主要活动

public class MainActivity extends ListActivity {

private ListActivity activity;
final static String ScreenName = "bane";
final static String LOG_TAG = "rnc";
 ListView listview;
  TextView text;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

//  setContentView(R.layout.twit_list);

    // listView.setTextFilterEnabled(true);


     activity = this;
      listview = this.getListView();
    downloadTweets();

          listview.setOnItemClickListener(new OnItemClickListener() {
      @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {
          if(view != null && view.getContext() != null){

          // selected item
          //text = (TextView) view.findViewById(R.id.title);
         // String lst_txt = text.getText().toString().trim();

           String product = ((TextView) view.findViewById(R.id.targetmonths)).getText().toString();
          //System.out.println("Text"+lst_txt ); 

          // Launching new Activity on selecting single List Item
          Intent i = new Intent(MainActivity.this, SingleListItem.class);

          // sending data to new activity
          i.putExtra("product",product );
          startActivity(i);
          }

    }

    // download twitter timeline after first checking to see if there is a network connection
    public void downloadTweets() {
        ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();

        if (networkInfo != null && networkInfo.isConnected()) {
            new DownloadTwitterTask().execute(ScreenName);
        } else {
            Log.v(LOG_TAG, "No network connection available.");
        }
    }

    // Uses an AsyncTask to download a Twitter user's timeline
    private class DownloadTwitterTask extends AsyncTask<String, Void, String> {
        final static String CONSUMER_KEY = "3GsPNkJacZedXIwoajycHzkkU";
        final static String CONSUMER_SECRET = "kCsPxxjsfrwba4cSZWW0tmXvaIcWT6r5Gb2HD5VX3RDYoDGRfXG";
        final static String TwitterTokenURL = "https://api.twitter.com/oauth2/token";
        final static String TwitterStreamURL = "https://api.twitter.com/1.1/statuses/user_timeline.json?screen_name=";

        @Override
        protected String doInBackground(String... screenNames) {
            String result = null;

            if (screenNames.length > 0) {
                result = getTwitterStream(screenNames[0]);
            }
            return result;
        }

        // onPostExecute convert the JSON results into a Twitter object (which is an Array list of tweets
        @Override
        protected void onPostExecute(String result) {
            Twitter twits = jsonToTwitter(result);

            // lets write the results to the console as well
            for (Tweet tweet : twits) {
                Log.i(LOG_TAG, tweet.getText());
            }

            // send the tweets to the adapter for rendering
            ArrayAdapter<Tweet> adapter = new ArrayAdapter<Tweet>(activity, android.R.layout.simple_list_item_1, twits);
            setListAdapter(adapter);
        }

        // converts a string of JSON data into a Twitter object
        private Twitter jsonToTwitter(String result) {
            Twitter twits = null;
            if (result != null && result.length() > 0) {
                try {
                    Gson gson = new Gson();
                    twits = gson.fromJson(result, Twitter.class);
                } catch (IllegalStateException ex) {
                    // just eat the exception
                }
            }
            return twits;
        }

        // convert a JSON authentication object into an Authenticated object
        private Authenticated jsonToAuthenticated(String rawAuthorization) {
            Authenticated auth = null;
            if (rawAuthorization != null && rawAuthorization.length() > 0) {
                try {
                    Gson gson = new Gson();
                    auth = gson.fromJson(rawAuthorization, Authenticated.class);
                } catch (IllegalStateException ex) {
                    // just eat the exception
                }
            }
            return auth;
        }

        private String getResponseBody(HttpRequestBase request) {
            StringBuilder sb = new StringBuilder();
            try {

                DefaultHttpClient httpClient = new DefaultHttpClient(new BasicHttpParams());
                HttpResponse response = httpClient.execute(request);
                int statusCode = response.getStatusLine().getStatusCode();
                String reason = response.getStatusLine().getReasonPhrase();

                if (statusCode == 200) {

                    HttpEntity entity = response.getEntity();
                    InputStream inputStream = entity.getContent();

                    BufferedReader bReader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
                    String line = null;
                    while ((line = bReader.readLine()) != null) {
                        sb.append(line);
                    }
                } else {
                    sb.append(reason);
                }
            } catch (UnsupportedEncodingException ex) {
            } catch (ClientProtocolException ex1) {
            } catch (IOException ex2) {
            }
            return sb.toString();
        }

        private String getTwitterStream(String screenName) {
            String results = null;

            // Step 1: Encode consumer key and secret
            try {
                // URL encode the consumer key and secret
                String urlApiKey = URLEncoder.encode(CONSUMER_KEY, "UTF-8");
                String urlApiSecret = URLEncoder.encode(CONSUMER_SECRET, "UTF-8");

                // Concatenate the encoded consumer key, a colon character, and the
                // encoded consumer secret
                String combined = urlApiKey + ":" + urlApiSecret;

                // Base64 encode the string
                String base64Encoded = Base64.encodeToString(combined.getBytes(), Base64.NO_WRAP);

                // Step 2: Obtain a bearer token
                HttpPost httpPost = new HttpPost(TwitterTokenURL);
                httpPost.setHeader("Authorization", "Basic " + base64Encoded);
                httpPost.setHeader("Content-Type", "application/x-www-form-urlencoded;charset=UTF-8");
                httpPost.setEntity(new StringEntity("grant_type=client_credentials"));
                String rawAuthorization = getResponseBody(httpPost);
                Authenticated auth = jsonToAuthenticated(rawAuthorization);

                // Applications should verify that the value associated with the
                // token_type key of the returned object is bearer
                if (auth != null && auth.token_type.equals("bearer")) {

                    // Step 3: Authenticate API requests with bearer token
                    HttpGet httpGet = new HttpGet(TwitterStreamURL + screenName);

                    // construct a normal HTTPS request and include an Authorization
                    // header with the value of Bearer <>
                    httpGet.setHeader("Authorization", "Bearer " + auth.access_token);
                    httpGet.setHeader("Content-Type", "application/json");
                    // update the results with the body of the response
                    results = getResponseBody(httpGet);
                }
            } catch (UnsupportedEncodingException ex) {
            } catch (IllegalStateException ex1) {
            }
            return results;
        }
    }
}

用于显示列表视图的xml文件

<?xml version="1.0" encoding="utf-8"?>

<ListView xmlns:android="http://schemas.android.com/apk/res/android"
          android:id="@android:id/list"
          android:layout_width="match_parent"
          android:layout_height="wrap_content">

</ListView>

数据适配器

public class DataAdapter extends BaseAdapter {
    Context mContext; 
    private LayoutInflater mInflater;
    String targetmonths;
    //String img;
    //String targetamount; 
    String[] month;
    //String[] year;
    //String[] amount;
    public DataAdapter(Context c, String[] month) {
        this.month = month;
        //this.year = year;
        //this.amount = amount; 
        mContext = c;
        mInflater = LayoutInflater.from(c); 
    } 
    public int getCount() {
        return month.length;
    } 
    public Object getItem(int position) {
        return position;
    } 
    public long getItemId(int position) {
        return position;
    } 
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder = null;
        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.customgrid, parent, false);
            holder = new ViewHolder();
            holder.month = (TextView) convertView
                    .findViewById(R.id.targetmonths); 

            //holder.img = (ImageView) convertView.findViewById(R.id.image_id); 
        //  holder.amount = (TextView) convertView.findViewById(R.id.targetamount); 




            convertView.setTag(holder);

        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        try {

            holder.month.setText(month[position]);




    //      holder.img.setImageResource(img[position]);
        //  holder.amount.setText(amount[position]);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 
        return convertView;
    } 
    static class ViewHolder {
        TextView month;
    //  ImageView img;
    //  TextView year, amount; 
    }

}

自定义网格

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

            <TextView
                android:id="@+id/targetmonths"
                android:layout_width="0dp"
                android:layout_height="match_parent"
                android:layout_gravity="left"
                android:layout_weight="1"
                android:gravity="left|center"
                android:paddingBottom="5sp"
                android:paddingLeft="10sp"
                android:paddingRight="5sp"
                android:paddingTop="5sp"
                android:text="hello"
                android:textColor="#ffffcc"
                android:textSize="16sp">
            </TextView>
        </LinearLayout>

Answer 1

如下更改您的项目点击侦听器，看看是否可行：

更新代码

listview.setOnItemClickListener(new OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view,
                            int position, long id) {

        // selected item          
            TextView text = (TextView) view.findViewById(R.id.targetmonths);
            String lst_txt = String.valueOf(text.getText());

            System.out.println("Display text"+lst_txt );
            // Launching new Activity on selecting single List Item
            Intent i = new Intent(MainActivity.this, SingleListItem.class);
            // sending data to new activity
            i.putExtra("product",lst_txt );
            startActivity(i); 

    }
});

Answer 2

您会得到 NPE，因为当您创建该 listview 的 Adapter 时，您在此布局中传递了 android.R.layout.simple_list_item_1，没有 ID 为 title 的 TextView

所以像这样改变你的 onItemClick

//@Override
// listening to single list item on click
listview.setOnItemClickListener(new OnItemClickListener() {
  @Override
  public void onItemClick(AdapterView<?> parent, View view,
               int position, long id) {   
     // selected item
     String lst_txt = parent.getItemAtPosition(position).toString().trim();

     System.out.println("Display text"+lst_txt ); 
     // Launching new Activity on selecting single List Item
     Intent i = new Intent(MainActivity.this, SingleListItem.class);
     // sending data to new activity
     i.putExtra("product",lst_txt );
     startActivity(i);
   }
});

Grep code for android.R.layout.simple_list_item_1

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2006 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/text1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceListItemSmall"
    android:gravity="center_vertical"
    android:paddingStart="?android:attr/listPreferredItemPaddingStart"
    android:paddingEnd="?android:attr/listPreferredItemPaddingEnd"
    android:minHeight="?android:attr/listPreferredItemHeightSmall"
/>

Answer 3

你可以使用 onListItemClick 事件

protected void onListItemClick (ListView l, View v, int position, long id){
    super.onListItemClick(l, v, position, id);

        }

Answer 4

适配器中的getItem() 方法是否返回null？如果是这样，请将其更改为返回有效视图并重试。

尝试这样做并告诉我会发生什么：

        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {
          if(view != null){

          // selected item
          text = (TextView) view.findViewById(R.id.title);
          if(text != null && text.getText() != null){
              String lst_txt = text.getText().toString().trim();

              // String product = ((TextView) view.findViewById(R.id.targetmonth)).getText().toString();
              System.out.println("Text"+lst_txt ); 

              // Launching new Activity on selecting single List Item
              Intent i = new Intent(MainActivity.this, SingleListItem.class);

              // sending data to new activity
              i.putExtra("product",lst_txt );
              startActivity(i);
          }
          }

      }

另外，作为旁注，请在每个代码内注释之前添加一个新行，或者完全避免它们。