private void exitDialog()

    {

        AlertDialog.Builder aa=new AlertDialog.Builder(this);

        aa.setTitle("消息提示");

        aa.setMessage("确认退出？");

        aa.setIcon(R.drawable.icon);

        aa.setPositiveButton("确定",new DialogInterface.OnClickListener() {

            @Override

            public void onClick(DialogInterface dialog, int which) {

                // TODO Auto-generated method stub

                MainActivity.this.finish();

            }

        });

        aa.setNegativeButton("取消", null);

        aa.create();

        aa.show();

    }

    @Override

    public boolean onKeyDown(int keyCode, KeyEvent event)

    {

        if(keyCode == KeyEvent.KEYCODE_BACK)

        {

        exitDialog();

        }

        return true;

    }

    private static boolean isExit=false;

    private static boolean hasTask=false;

    Timer tExit=new Timer();

    TimerTask task=new TimerTask() {

        @Override

        public void run() {

            // TODO Auto-generated method stub

            isExit=false;

            hasTask=true;

        }

    };

@Override

    public boolean onKeyDown(int keyCode, KeyEvent event)

    {

        if(keyCode == KeyEvent.KEYCODE_BACK)

        {

            //exitDialog();

            if(isExit==false)

            {

                isExit=true;

                Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();

                if(!hasTask) {

                    tExit.schedule(task, 2000);

                }

            }

            else

            {

                finish();

                System.exit(0);

            }

        }

        //return true;

        return false;

    }