Android : Display alert only once per application launch on android

I want to add a startup alert for my application,but it keeps showing up each time I go back to the main screen.How can I do it? Newbie here and thanks in regard.

 AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);
        alertDialog.setMessage("About:");
        String alert1 = "Message here " ;
        String alert2 = "Message here "  ;
        String alert3 = "Message here " ;
        alertDialog.setMessage(alert1 +"\n"+ alert2 +"\n"+ alert3); 
        AlertDialog alert = alertDialog.create();
        alert.show();