long exitTime = 0;
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

        if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){
            if((System.currentTimeMillis()-exitTime) > 2000){
                Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
                exitTime = System.currentTimeMillis();
            } else {
                finishAll();
                System.exit(0);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }