这里就不多说了，很容易

 //按键返回事件
    private long currentBackPressedTime = 0;
    private static final int BACK_PRESSED_INTERVAL = 2000;

 //按键返回事件
        public void onBackPressed() {
            //1.当前的获取的毫秒数-预定值（0）肯定大于2s  //4.当下次再按时当前值跟赋予后的设定值比较
            if (System.currentTimeMillis() - currentBackPressedTime > BACK_PRESSED_INTERVAL) {
                //2.把当前的毫秒数赋予预定值
                currentBackPressedTime = System.currentTimeMillis();
                //3.弹出框
                Toast.makeText(Home.this,"再按一次退出", Toast.LENGTH_SHORT).show();
            } else {
                //若当前值-设定值<2s 则执行:
                finish();
            }}