安卓中的服务类Service里面有一个方法onBind，为何不能在MainActivity中创建一个继承自Service类的对象后，主动调用这个方法呢？并且onBind方法的访问权限也是public，我在MainActivity中获取到了MyService类对象后，发现无法调用onBind方法，这是什么情况呢？

public class MyService extends Service
{

    private DownloadBinder mBinder=new DownloadBinder();

    class DownloadBinder extends Binder
    {
        public void startDownload()
        {
            Log.d("MyService","startDownload executed");
        }

        public int getProgress()
        {
            Log.d("MyService","getProgress executed");
            return 0;
        }
    }

    public MyService()
    {
    }

    @Override
    public IBinder onBind(Intent intent)
    {
        return mBinder;
    }

}