android 退出程序提示是否退出对话框

public boolean onKeyDown(int keyCode, KeyEvent event) {
 // TODO Auto-generated method stub
 switch (keyCode) {
 case KeyEvent.KEYCODE_BACK:
  AlertDialog.Builder build=new AlertDialog.Builder(this);
  build.setTitle("注意")
    .setMessage("确定要退出吗？")
    .setPositiveButton("确定", new DialogInterface.OnClickListener() {
     
     @Override
     public void onClick(DialogInterface dialog, int which) {
      onDestroy();
      System.exit(0);
      finish();      
     }
    })
    .setNegativeButton("取消", new DialogInterface.OnClickListener() {      
     @Override
     public void onClick(DialogInterface dialog, int which) {
      // TODO Auto-generated method stub       
     }
    })
    .show();
  break;
 default:
  break;
 }
 return false; 
}