How to run a Runnable thread in Android?

The simple fix to your example is:

final Runnable r = new Runnable()
{
public void run()
{
tv.append("Hello World");
handler.postDelayed(this, 1000);
}
};

handler.postDelayed(r, 1000);

Or we can use normal thread for example (with original Runner):

Thread thread = new Thread()
{
@Override
public void run() {
try {
while(true) {
sleep(1000);
handler.post(r);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};

thread.start();

You may consider your runnable object just as a command that can be sent to the message queue for execution, and handler as just a helper object used to send that command.

More details are here http://developer.android.com/reference/android/os/Handler.html