In Android Activities and Services, most callbacks are run on the main thread. This makes it simple to update the UI, but running processor- or I/O-heavy tasks on the main thread can cause your UI to pause and become unresponsive (official documentation on what then happens).

You can remedy this by putting these heavier tasks on a background thread.

One way to do this is using an AsyncTask, which provides a framework to facilitate easy usage of a background Thread, and also perform UI Thread tasks before, during, and after the background Thread has completed its work.

Methods that can be overridden when extending AsyncTask:


public class MyCustomAsyncTask extends AsyncTask<File, Void, String> {

    protected void onPreExecute(){
        // This runs on the UI thread before the background thread executes.
        // Do pre-thread tasks such as initializing variables. 
        Log.v("myBackgroundTask", "Starting Background Task");  

    protected String doInBackground(File... params) {
        // Disk-intensive work. This runs on a background thread.
        // Search through a file for the first line that contains "Hello", and return
        // that line.
        try (Scanner scanner = new Scanner(params[0])) {
            while (scanner.hasNextLine()) {
                final String line = scanner.nextLine();
                publishProgress(); // tell the UI thread we made progress

                if (line.contains("Hello")) {
                    return line;
            return null;

    protected void onProgressUpdate(Void...p) {
        // Runs on the UI thread after publishProgress is invoked
        Log.v("Read another line!")

    protected void onPostExecute(String s) {
        // This runs on the UI thread after complete execution of the doInBackground() method
        // This function receives result(String s) returned from the doInBackground() method.
        // Update UI with the found string.
        TextView view = (TextView) findViewById(;
        if (s != null) {
        } else {
            view.setText("Match not found.");



MyCustomAsyncTask asyncTask = new MyCustomAsyncTask<File, Void, String>();
// Run the task with a user supplied filename.

or simply:

new MyCustomAsyncTask().execute(userSuppliedFilename);


When defining an AsyncTask we can pass three types between < > brackets.

Defined as <Params, Progress, Result> (see Parameters section)

In the previous example we’ve used types <File, Void, String>:

AsyncTask<File, Void, String>
// Params has type File
// Progress has unused type
// Result has type String

[Void](<>) is used when you want to mark a type as unused.