双击返回退出程序

private long mExitTime;

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

        if (keyCode == KeyEvent.KEYCODE_BACK) {

            if ((System.currentTimeMillis() - mExitTime) > 2000) {
                Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
                mExitTime = System.currentTimeMillis();
            } else {
                System.exit(0);//正常退出App
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }