通过（从）AsyncTask发送意图到MainActivity

Question

I looked for a couple of questions asked on Stack Overflow for sending a int to my MainActivity and displaying it on my TextView . But trying to initialize activity or context don't work.. The latest error I get is this:

FATAL EXCEPTION: AsyncTask #1 Process: com.dahlstore.jsonparsingdemo, PID: 32123 java.lang.RuntimeException: An error occurred while executing doInBackground() at android.os.AsyncTask$3.done(AsyncTask.java:309) at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354) at java.util.concurrent.FutureTask.setException(FutureTask.java:223) at java.util.concurrent.FutureTask.run(FutureTask.java:242) at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) at java.lang.Thread.run(Thread.java:818) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.app.Activity.getApplicationContext()' on a null object reference at com.dahlstore.jsonparsingdemo.JSONTask.doInBackground(JSONTask.java:63) at com.dahlstore.jsonparsingdemo.JSONTask.doInBackground(JSONTask. java:21) at android.os.AsyncTask$2.call(AsyncTask.java:295) at java.util.concurrent.FutureTask.run(FutureTask.java:237) at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) at java.lang.Thread.run(Thread.java:818) 09-17 18:31:37.015 32123-32152/com.dahlstore.jsonparsingdemo E/Surface: getSlotFromBufferLocked: unknown buffer: 0xabea80a0

Could anybody explain why I can't send my intent even though I use Activity.

/*ROW21*/      public class JSONTask extends AsyncTask<String,String, String>{

    OnDataSendToActivity dataSendToActivity;
    Activity activity;
    Intent intent;

    public JSONTask(MainActivity mainActivity) {
        dataSendToActivity = (OnDataSendToActivity)mainActivity;
    }

    public JSONTask(Activity activity){
        this.activity = activity;
    }

    @Override
    protected String doInBackground(String... params) {

        HttpURLConnection connection = null;
        BufferedReader reader = null;

        try {
            URL url = new URL(params[0]);
            connection = (HttpURLConnection) url.openConnection();
            connection.connect();

            InputStream stream = connection.getInputStream();

            reader = new BufferedReader(new InputStreamReader(stream));
            StringBuffer buffer = new StringBuffer();

            String line = "";
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }
            JSONObject parentObject = new JSONObject(buffer.toString());
            JSONObject query = parentObject.getJSONObject("query").optJSONObject("results").optJSONObject("channel").optJSONObject("item");
            String temperature = query.getJSONObject("condition").optString("temp");
            String text = query.getJSONObject("condition").optString("text");

            int code = query.getJSONObject("condition").optInt("code");
           **//ROW 63**  intent = new Intent(activity.getApplicationContext(),MainActivity.class); 

            intent.putExtra("code",code);

            return temperature + " °C " +" and "+ text;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        activity.startActivity(intent);
        dataSendToActivity.sendData(result);
    }

}

MainActivity

public class MainActivity extends AppCompatActivity implements OnDataSendToActivity{

    public TextView temperatureTextView,textView;

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

        temperatureTextView = (TextView) findViewById(R.id.temperatureTextView);
        textView = (TextView) findViewById(R.id.textView);

        new JSONTask(this).execute("https://query.yahooapis.com/v1/public/yql?q=select%20item%20from%20weather.forecast%20where%20woeid%3D906057%20and%20u%3D%27c%27&format=json");

        Intent intent = getIntent();
        if(intent!= null) {
            int code = getIntent().getIntExtra("code", 0);
            String codeToString = String.valueOf(code);
            textView.setText(codeToString);
        } else {
            Toast.makeText(MainActivity.this, "Intent is null", Toast.LENGTH_SHORT).show();
        }
    }

    @Override
    public void sendData(String str) {
        temperatureTextView.setText(str);
    }
}

UPDATED JSONTASK.JAVA

public class JSONTask extends AsyncTask<String,String, String>{
    OnDataSendToActivity dataSendToActivity;
    Context context;

    // single constructor to initialize both the context and dataSendToActivity
    public JSONTask(Context context){
        this.context = context;
        dataSendToActivity = (OnDataSendToActivity) ((Activity) context);
    }

    @Override
    protected String doInBackground(String... params) {
        HttpURLConnection connection = null;
        BufferedReader reader = null;
        StringBuffer buffer = new StringBuffer();

        try {
            URL url = new URL(params[0]);
            connection = (HttpURLConnection) url.openConnection();
            connection.connect();
            InputStream stream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));
            String line = "";
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return buffer.toString();
    }

    @Override
    protected void onPostExecute(String result) {
        try {
            JSONObject parentObject = new JSONObject(result);
            JSONObject query = parentObject.getJSONObject("query").optJSONObject("results").optJSONObject("channel").optJSONObject("item");
            String temperature = query.getJSONObject("condition").optString("temp");
            String text = query.getJSONObject("condition").optString("text");
            int code = query.getJSONObject("condition").optInt("code");
            temperature += " °C " +" and "+ text;

            Intent intent = new Intent(context, MainActivity.class);
            intent.putExtra("code", code);
            context.startActivity(intent);
            if(dataSendToActivity != null){
                dataSendToActivity.sendData(temperature);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

UPDATED MAINACTIVITY

public class MainActivity extends AppCompatActivity implements OnDataSendToActivity{

    public TextView temperatureTextView,textView;
    Intent intent;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        temperatureTextView = (TextView) findViewById(R.id.temperatureTextView);
        textView = (TextView) findViewById(R.id.textView);
        intent = getIntent();
        new JSONTask(this).execute("https://query.yahooapis.com/v1/public/yql?q=select%20item%20from%20weather.forecast%20where%20woeid%3D906057%20and%20u%3D%27c%27&format=json");

    }


    @Override
    public void sendData(String str) {
        temperatureTextView.setText(str);
    }
}

Answer 1

You are getting an error because your activity is null. This happends because you have two constructors.

// this is the constructor that is called
public JSONTask(MainActivity mainActivity) {
    dataSendToActivity = (OnDataSendToActivity)mainActivity;
}

// this is not called
public JSONTask(Activity activity){
    this.activity = activity;
}

So your activity variable is never initialized.

See my changes,

public class JSONTask extends AsyncTask<String,String, String>{
    OnDataSendToActivity dataSendToActivity;
    Context context;

    // single constructor to initialize both the context and dataSendToActivity
    public JSONTask(Context context){
        this.context = context;
        dataSendToActivity = (OnDataSendToActivity) ((Activity) context);
    }

    @Override
    protected String doInBackground(String... params) {
        HttpURLConnection connection = null;
        BufferedReader reader = null;
        StringBuffer buffer = new StringBuffer();

        try {
            URL url = new URL(params[0]);
            connection = (HttpURLConnection) url.openConnection();
            connection.connect();
            InputStream stream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));
            String line = "";
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return buffer.toString();
    }

    @Override
    protected void onPostExecute(String result) {
        try {
            JSONObject parentObject = new JSONObject(result);
            JSONObject query = parentObject.getJSONObject("query").optJSONObject("results").optJSONObject("channel").optJSONObject("item");
            String temperature = query.getJSONObject("condition").optString("temp");
            String text = query.getJSONObject("condition").optString("text");
            int code = query.getJSONObject("condition").optInt("code");
            temperature += " °C " +" and "+ text;

            if(dataSendToActivity != null){
                dataSendToActivity.sendData(temperature, code);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In your MainActivity ,

public class MainActivity extends AppCompatActivity implements OnDataSendToActivity {

    public TextView temperatureTextView,textView;

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

        temperatureTextView = (TextView) findViewById(R.id.temperatureTextView);
        textView = (TextView) findViewById(R.id.textView);

        new JSONTask(this).execute("https://query.yahooapis.com/v1/public/yql?q=select%20item%20from%20weather.forecast%20where%20woeid%3D906057%20and%20u%3D%27c%27&format=json");
    }

    @Override
    public void sendData(String str, String code) {
        temperatureTextView.setText(str);
        textView.setText(code);
    }
}

Your OnDataSendToActivity interface will become,

public interface OnDataSendToActivity {
    void sendData(String str, String code);
}

Answer 2

I think the mistake is you are using getIntExtra in the Async. Instead of that use putIntExtra to save the variable in the intent.

Put is to store the value and getIntent functions are used to get the data from the intent.

Use this line,

intent = new Intent(getApplicationContext(),MainActivity.class); 

or

intent = new Intent(YourClassName.class,MainActivity.class); 

Answer 3

You do not really have to use intent to send your "code" to the activity. In your doInBackground, put your "code" into a string variable (you need to properly parse it) then put that string as an argument to your return.

Then in postExecute(String result), the variable result should be the value your returned from doInBackground. The dataSendToActivity.sendData(result) should work fine now.