Android从主界面退出

public class MyApplication extends Application {

    private LinkedList<Activity> list =null;

    private static  MyApplication instance;
    MyApplication(){

        list=new LinkedList<Activity>();
    }

    public static MyApplication getInstance(){

        if (instance==null){

            return instance=new MyApplication();
        }

        return  instance;
    }

    public void addActivity(Activity activity) {
        if (list != null && list.size() > 0) {
            if(!list.contains(activity)){
                list.add(activity);
            }
        }else{

            list.add(activity);
        }

    }

    public void exit() {
        if (list != null && list.size() > 0) {
            for (Activity activity : list) {
                activity.finish();
            }
        }
        System.exit(0);
    }
}