小记：按两次返回键退出应用程序。

private 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) { // System.currentTimeMillis()无论何时调用，肯定大于2000

                Toast.makeText(getApplicationContext(), "再按一次退出程序",
                        Toast.LENGTH_SHORT).show();
                exitTime = System.currentTimeMillis();
            } else {
                finish();
                System.exit(0);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }