asynchronous - Can't cancel Async task in android

Question

I need to cancel my asyncthread . In my application I am doing some heavy calculations, and I want to give user ability to cancel calculations(and then retry). I read on forums, that you can't just stop task from what is it doing, and that you need to check if task isCancelled=true inside your DoinBackground code. But that doesn't work for me.

Task itself is working great and it outputs correct data if I leaved it to end on itself.

In my App first I call function naredi_pdf_start(view), then when the task is running, if I call close_pdf1(view), it gives me an error.(I am changing views and app can't find my pdf_text1 Textview when calling publishProgress- null pointer exception). I really dont know how to use task.cancel(true) method (in my case: start_pdf.cancel(true))).

Here is my code:

String progress_pdf;
naredi_pdf start_pdf;

public void naredi_pdf_start(View view) {
    start_pdf=new naredi_pdf();
    start_pdf.execute();
}

public void close_pdf1(View view) {

    if(start_pdf!=null) {
        Log.v("not null","not null");

        start_pdf.cancel(true);
        setContentView(R.layout.other_view); //This is where 
                                             //I don't have TextView pdf_text1
    }
}

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

    protected Void doInBackground( Void... ignoredParams ) {
        progress_pdf="Calculating Statistical Data";

        //A LOT OF CODING

        for(int i = 0; i < 1; i++) {
            if(isCancelled()) {
                break;
            }
            else {
                publishProgress("Calculating team statistics");  
            }
        }

        //MORE OF CODING              

        for (int i = 0; i < 1; i++) {
            if (isCancelled()) {
                break;
            }
            else {
                publishProgress("Calculating player's BIO");  
            }
        }

        //MORE OF CODING       

        for (int i = 0; i < 1; i++) {
            if (isCancelled()) {
                break;
            }
        else {
                publishProgress("Calculating player's individual performance");
            }
        }

        return null; 
    }

    protected void onPostExecute( Void array ) {
        //saving to database
    }

    protected void onProgressUpdate(String... values) {
        progress_pdf=values[0]+"
"+progress_pdf;

        if (isCancelled())  {

        }
        else {
            TextView pdf_text1 = (TextView) findViewById (R.id.pdf_text1);
            pdf_text1.setText(progress_pdf);
            // dialog(view);
        }
    }
}

Answer 1

The return statement cancels the execution of the doInBackground method, not break.