//记录用户首次点击返回键的时间

    private long firstTime = 0;

    /**

     * 通过监听keyUp   实现双击返回键退出程序

     * @param keyCode

     * @param event

     * @return

     */

    @Override

    public boolean onKeyUp(int keyCode, KeyEvent event) {

        if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_UP) {

            long secondTime = System.currentTimeMillis();

            if (secondTime - firstTime > 2000) {

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

                firstTime = secondTime;

                return true;

            } else {

                System.exit(0);

            }

        }

        return super.onKeyUp(keyCode, event);

    }