Android的Activity以后台隐藏

public class MainActivity extends Activity {
     /** Called when the activity is first created. */
     NotificationManager nm;
     Notification notifi;
     PendingIntent pi;
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         nm = (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);
         notifi = new Notification(R.drawable.ic_launcher,"",System.currentTimeMillis());
         Intent intent = new Intent(this,MainActivity.class);
         intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP|Intent.FLAG_ACTIVITY_NEW_TASK);
         pi = PendingIntent.getActivity(this, 0, intent, 0);
         notifi.setLatestEventInfo(this,"别当心", "哥还在", pi);
         notifi.flags  = Notification.FLAG_INSISTENT|Notification.FLAG_ONGOING_EVENT;
         setContentView(R.layout.main);
         
     }
 
     @Override
     public void onBackPressed() {
         // TODO Auto-generated method stub
         //super.onBackPressed();
         AlertDialog.Builder builder = new AlertDialog.Builder(this);
         builder.setIcon(R.drawable.ic_launcher);
         builder.setTitle("是否退出");
         //builder.setMessage("");
         builder.setPositiveButton("退出", new DialogInterface.OnClickListener() {
             
             @Override
             public void onClick(DialogInterface dialog, int which) {
                 // TODO Auto-generated method stub
                 finish();
                 android.os.Process.killProcess(android.os.Process.myPid());
             }
         });
         builder.setNeutralButton("隐藏", new DialogInterface.OnClickListener() {
             
             @Override
             public void onClick(DialogInterface dialog, int which) {
                 // TODO Auto-generated method stub
                 Intent intent = new Intent(Intent.ACTION_MAIN);
                 intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                 intent.addCategory(Intent.CATEGORY_HOME);
                 MainActivity.this.startActivity(intent);
                 
             }
         });
         builder.setNegativeButton("取消", new DialogInterface.OnClickListener() {
             
             @Override
             public void onClick(DialogInterface dialog, int which) {
                 // TODO Auto-generated method stub
                 
             }
         });
         
         builder.create().show();
     }
     
     
     
 
     @Override
     public void finish() {
         // TODO Auto-generated method stub
         super.finish();
         nm.cancel(R.drawable.ic_launcher);
     }
 
     @Override
     protected void onDestroy() {
         // TODO Auto-generated method stub
         super.onDestroy();
         Log.e("MainActivity", "onDestroy");
         nm.cancelAll();
     }
 
     @Override
     protected void onStop() {
         // TODO Auto-generated method stub
         super.onStop();
         nm.notify(R.drawable.ic_launcher, notifi);
     }

 感谢大神:http://www.cnblogs.com/bvin/archive/2012/08/18/2645584.html