Android - switch to activity without restarting it

I am programming a chat program for android.

I have the contact list as one activity and the chat windows as a second activity. I use startActivity to switch to the chat activity, but the chat activity gets reloaded every time. Therefore the screen gets cleared.

Is there a way to switch to a running activity without having to restart it?

private Intent myIntent = null;

…

    if (myIntent == null)
        myIntent = new Intent(HanasuAndroidActivity.activity, ChatWindow.class);

    this.startActivity(myIntent);