How to run a Runnable thread in Android at defined intervals?

The simple fix to your example is :

handler = new Handler();

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(this);
            }
        } 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

new Handler().postDelayed(new Runnable() {
    public void run() {
        // do something...              
    }
}, 100);

I think can improve first solution of Alex2k8 for update correct each second

1.Original code:

public void run() {
    tv.append("Hello World");
    handler.postDelayed(this, 1000);
}

2.Analysis

• In above cost, assume tv.append("Hello Word") cost T milliseconds, after display 500 times delayed time is 500*T milliseconds
• It will increase delayed when run long time

3. Solution

To avoid that Just change order of postDelayed(), to avoid delayed:

public void run() {
    handler.postDelayed(this, 1000);
    tv.append("Hello World");
}