How Android implements click once to return to the desktop instead of exiting the app

The specific code looks like this:

@Override  Public boolean onKeyDown (int  keycode, KeyEvent event) {    if (keycode = =  Keyevent.keycode_back) {        new  Intent (intent.action_main);        Home.setflags (intent.flag_activity_clear_top);        Home.addcategory (intent.category_home);        StartActivity (home);         return true ;    }     return Super . OnKeyDown (KeyCode, event);}

How Android implements a single click Back to return to the desktop instead of exiting the app