Question

I want to show this dialog, while the thread tries to build up a connection, but the dialog will not show up when I press the button which starts this method.

public void add_mpd(View view) {
    dialog = ProgressDialog.show(MainActivity.this, "", "Trying to connect...");
    new Thread(new Runnable() {
        public void run() {
            try {
                String child;
                EditText new_mpd = (EditText) findViewById(R.id.new_mpd);
                child = new_mpd.getText().toString();
                mpd = new MPD(child);
                children.get(1).add(child);

            } catch (UnknownHostException e) {
                e.printStackTrace();
            } catch (MPDConnectionException e) {
                e.printStackTrace();
            }
        }
    }
    ).start();
    adapter.notifyDataSetChanged();
    dialog.dismiss();
}

Answer 1

It will not show up because the (blocking) work is done in another thread. That means, the start()-method of the Thread-class will not block.

Ergo, you show the Dialog, the Thread is started and the dialog is immediately dismissed (and therefore closed).

Put the call to dismiss() at the end of your run()-method and it should work just fine.

---

The above might be working for you, but you should not use the Thread-class directly. There are wrappers around it which are way more comfortable to use.

In Android, if you want to do long-term work off the UI-Thread, you should use an AsyncTask.