如何在 Android 上的 Bufferedreader 中修复 NullPointerException？

Question

Goal: Reading a HTML-file using Buffered reader in an AsyncTask on Android.

Problem: The debug log gives the following error: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference . It gives the error at the onPostExecute void. On further investigation I found out the following code: TVCode.setText(result); is throwing the NullPointerException . This means result = null . I can't find out why result = null since similar code on the normal Java, using Eclipse , is working fine.

Further info:

Android Studio: 1.3.2

JDK: 1.7.0_79-b15 amd64

Permissions:

<uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
    <uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />
    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

Gradle: minSdkVersion 15, targetSdkVersion 23, compileSdkVersion 23

Here is the code of MainActivity.java :

public class MainActivity extends AppCompatActivity {


public static String ETURL = null;
public static final String TAG = MainActivity.class.getSimpleName();



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

        TextView TVCode = (TextView) findViewById(R.id.TVCode);
        ImageButton BTSend = (ImageButton) findViewById(R.id.BTSend);
        final EditText ETURLInput = (EditText) findViewById(R.id.ETUrlInput);

        BTSend.setOnClickListener(new View.OnClickListener() {

                                      public void onClick(View v) {
                                          ETURL = ETURLInput.getText().toString();

                                          new aSync().execute(ETURLInput.getText().toString());


                                      }


                                  }
        );

}

private class aSync extends AsyncTask<String, Void, String> {

    @Override
    protected void onPreExecute() {

    }

    protected String doInBackground(String... ETURL)   {
        String line=null;
        String result="";


            try {
                URL url;

                url = new URL("" + ETURL);
                URLConnection conn = url.openConnection();


                BufferedReader br = new BufferedReader(new InputStreamReader(conn.getInputStream()));
                line=br.readLine();
                while (line!= null) {

                    result=result+line;
                    line=br.readLine();
                }

                br.close();

                return result;


            } catch (Exception e) {
               Log.e(TAG, "Error:", e);
            }


        return result;
    }

    protected void onPostExecute(String result)  {
        try {
            TextView TVCode = (TextView) findViewById(R.id.TVCode);
            TVCode.setText(result);
        }
        catch (Exception e){
            Log.e(TAG, "ERROR: ", e);
        }
    }
}

Fix:

The TextView must be declared in the onPreExecute method instead of the on onPostExecute method. Thank you all for your help and time.

Answer 1

The problem lies in the onPostExecute() . here you are trying to use findViewById() . But there is no view to be found. Hence the issue. You need to call the findViewById() on the activity or the inflated layout. If it's an activity, you need to pass the activity context to the async task and in case of fragment, you could pass in the inflated view.

More info here : Can't access "findViewById" in AsyncTask