AsyncTask - error occured while executing doInBackground()

Question

I have problem with AsyncTask in Android. I try to insert data from accelerometr into database, when user push Start. Normally, when user push the button Start, application hanged after 5 second. So I try put this function in AsyncTask, but i have problem with compilation:

01-21 17:44:58.515: E/AndroidRuntime(1131): FATAL EXCEPTION: AsyncTask #1
01-21 17:44:58.515: E/AndroidRuntime(1131): java.lang.RuntimeException: An error occured while executing doInBackground()
01-21 17:44:58.515: E/AndroidRuntime(1131):     at android.os.AsyncTask$3.done(AsyncTask.java:299)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.util.concurrent.FutureTask.setException(FutureTask.java:219)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.util.concurrent.FutureTask.run(FutureTask.java:239)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.lang.Thread.run(Thread.java:841)
01-21 17:44:58.515: E/AndroidRuntime(1131): Caused by: java.lang.NullPointerException
01-21 17:44:58.515: E/AndroidRuntime(1131):     at pl.pawelfrydrych.flyingball.MyTask.insertData(MyTask.java:37)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at pl.pawelfrydrych.flyingball.MyTask.doInBackground(MyTask.java:18)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at pl.pawelfrydrych.flyingball.MyTask.doInBackground(MyTask.java:1)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at android.os.AsyncTask$2.call(AsyncTask.java:287)
01-21 17:44:58.515: E/AndroidRuntime(1131):     at java.util.concurrent.FutureTask.run(FutureTask.java:234)
01-21 17:44:58.515: E/AndroidRuntime(1131):     ... 4 more

MyTask class:

    import java.util.Date;

    import android.content.ContentValues;
    import android.os.AsyncTask;
    import android.util.Log;

    public class MyTask extends AsyncTask<Void, Void, Void> {

        private MainActivity main;
        public Database myDBAdapter;
        private int abc;

        @Override
        protected Void doInBackground(Void... arg0) {
            try {
   //18 line//          insertData();
            } catch (InterruptedException e) {
                Log.d("appname","błąd w doInBackground");
                e.printStackTrace();
            }
            return null;
        }

    public void insertData() throws InterruptedException{

            ContentValues values = new ContentValues();
            Date date = new Date();


        while(true){
            long millis = System.currentTimeMillis();
            abc++;

            values.put(Database.ID, 1+abc);
            values.put(Database.LEFT_POSITION, main.xPosition);
            values.put(Database.RIGHT_POSITION, main.yPosition);
            values.put(Database.GPS, main.GPSposition);
            values.put(Database.TIME, date.toString());


            if(myDBAdapter.db != null){

                myDBAdapter.db.insert("baza", null, values);
                Thread.sleep(1000 - millis % 1000);

            }else{
                Log.d(Database.DB_NAME,"db is null");
            }

        }

        }
    }

MainActivity class:

 public void onClick(View v) {

                switch(v.getId()){
                case R.id.bStart:


                myDBAdapter = new Database(this).open();
                        new MyTask().execute();

                case R.id.bStop:

                    myDBAdapter.close();
                    break;

                }

            }

Answer 1

It looks like you get a NPE here

if(myDBAdapter.db != null){

in insertData() because you never initialize myDBAdapter. You declare it in your task

public Database myDBAdapter;

but you never initialize it before trying to use it.

It actually looks like you have an Activity variable and an AsyncTask variable so when you declare it in your task it is a local variable which isn't initialized. You can re-initialize that variable but if they are the same then it would be better to delete it and make sure it is a member variable if the task is an inner class of the Activity. If its a separate file then you will need to initialize it or pass a reference to a constructor of the AsyncTask class.

Answer 2

It doesn't look like you are initializing private MainActivity main; with anything. You try to use it before it is initialized.