Handler, Message - sending messages to different threads

public Handler messageHandler = new Handler() {
    @Override    public void handleMessage(Message msg) {
        Log.i("arrive", msg.arg1 + " <<< msg.what == 0");        

        long runTime = System.currentTimeMillis() - startTime;        

        numOfHorsesArrived++;         

        textView[msg.arg1].setText(String.format("第 %d 名\n%d.%03d (sec.)", numOfHorsesArrived, runTime/1000, runTime%1000));
        if(numOfHorsesArrived >= numOfHorses)
        {
            ready.setEnabled(true);            

            Log.i("arrive", "ready.setEnabled(true);");        

        }
    }
};

Message m = handler.obtainMessage();

m.what = selected_opt;
m.arg1 = x_pos;
m.arg2 = y_pos;

handler.sendMessage(m);