Android 如何通过上下文菜单从sqlitedatabase的listview on touch事件中删除记录

我想从sqlite数据库中删除数据。我已经将记录或数据从sqlitedab显示到列表视图。现在，当用户单击列表视图中显示的特定记录时，我尝试删除该记录。当用户单击特定记录时，我使用上下文菜单的更新和删除选项。我的问题是，我没有成功地在该上下文菜单中应用删除选项，并且无法删除该特定记录

我的代码如下：

 public class update_page extends Activity 
{

    DatabaseHandler db = new DatabaseHandler(this);
    public String tableName = db.TABLE_CONTACTS;
    public String databaseName = db.DATABASE_NAME;
    public String taskID = db.KEY_ID;

    public String task_name = db.KEY_TASK;
    public String dt = db.KEY_DATETIME;
    private ArrayList<String> results = new ArrayList<String>();
    private ArrayList<String> results2 = new ArrayList<String>();

    Intent op_intent;
    SQLiteDatabase database;
    ListView myview;
    Cursor c;
    String o_name,o_no1;
    //String o_id1 = null;
     HttpClient client;
    Integer op_id,status;
    String url;
    Context ctx;
    Intent myintent;

    final MainActivity act = new MainActivity();
    protected Object adapter;

     @Override
    public void onCreate(Bundle savedInstanceState)
        {

            super.onCreate(savedInstanceState);
            setContentView(R.layout.update_list);
            final ProgressDialog myPd_ring=ProgressDialog.show(update_page.this, "Please wait", "Login please wait..", true);
            myPd_ring.setCancelable(true);
            myPd_ring.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            new Thread() {

                public void run() {

                try{

                sleep(1000);

                } catch (Exception e) {

                Log.e("tag", e.getMessage());

                }

                // dismiss the progress dialog

                myPd_ring.dismiss();

                }

                }.start();


            myview = (ListView)this.findViewById(R.id.list);
            ctx=this;





            database = act.getInstance().openOrCreateDatabase("contactsManager",
            SQLiteDatabase.CREATE_IF_NECESSARY, null);       
            Toast.makeText(getApplicationContext(), "Database is open", 1500).show();


       //  database=this.openOrCreateDatabase("contactsManager", SQLiteDatabase.CREATE_IF_NECESSARY,null);

            final String[] columnsone={"_id","task_name","date_time"};
            String[] columnstwo={"task_name","date_time"};
            int to[] = {R.id.lbl_task,R.id.lbl_datetime}; 
            c = database.query("contacts", columnsone, null, null, null, null, null);
            status=c.getCount();

            Toast.makeText(getApplicationContext(), "totle task :" +status, 1500).show();



             if(c!=null)
               {

                Toast.makeText(getApplicationContext(), "in courser", 1500).show();
                SimpleCursorAdapter adapter=new 
                        SimpleCursorAdapter(update_page.this,R.layout.update_listview, c, columnstwo, to);
                Toast.makeText(getApplicationContext(), "records are in list", 1500).show();

                myview.setAdapter(adapter);


                        myview.setOnItemLongClickListener(new OnItemLongClickListener() 
                        {

                            public boolean onItemLongClick(AdapterView<?> arg0,
                                    View record, int arg2, long arg3)
                            {
                                update_page.this.registerForContextMenu(record);

                                return false;
                            }

                        });
                }
             }
            //@Override 
            public boolean onContextItemSelected(MenuItem item)
            {
                AdapterContextMenuInfo menuInfo = (AdapterContextMenuInfo)item.getMenuInfo();
                int choice = item.getItemId();
                switch(choice)
                {
                case 1:
                    Toast.makeText(getApplicationContext(), "Update Task", 2000).show();

                    Intent op_intent = new Intent(update_page.this,MainActivity.class).addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                    update_page.this.startActivity(op_intent);

                    break;

                case 2:



                    database = act.getInstance().openOrCreateDatabase("contactsManager",
                    SQLiteDatabase.CREATE_IF_NECESSARY, null);       
                    AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(ctx);
                    // set title
                    alertDialogBuilder.setTitle("are you sure to delete this record  !!");
                    // set dialog message
                    alertDialogBuilder
                            .setMessage("Click cancle to exit!")
                            .setCancelable(false)
                            .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                                                 public void onClick(DialogInterface dialog, int whichButton) {
                                                 DatabaseHandler db=new DatabaseHandler(getApplicationContext());

                                                 db.deleteContact(arg3+"");
                                                 list.remove(position);
                                                 update_page.this.adapter.notifyDataSetChanged();
                                                }
                                              });

                        /*  .setNegativeButton("Cancle",
                                    new DialogInterface.OnClickListener() 
                                    {
                                        public void onClick(DialogInterface dialog,
                                                int id) {
                                            // if this button is clicked, just close
                                            // the dialog box and do nothing
                                            dialog.cancel();
                                            myintent = new Intent(update_page.this,MainActivity.class);
                                            startActivity(myintent);
                                        }
                                    });
                            */

                    // create alert dialog
                    AlertDialog alertDialog = alertDialogBuilder.create();

                    // show it
                    alertDialog.show();





                    break;




                }

                return super.onContextItemSelected(item);
            }


            @Override
            public void onCreateContextMenu(ContextMenu menu, View v,
                    ContextMenuInfo menuInfo)
            {
                menu.add(0,1,0,"Update Task");
                menu.add(0,2,0,"Delete");

                super.onCreateContextMenu(menu, v, menuInfo);
            }


    }

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提前谢谢！！实际上，我想知道当用户单击特定数据时，如何确定该数据的位置。根据这一点，我想删除sqlite db中的记录。任何建议都可以接受！！