返回鍵

 //記錄用戶首次點擊返回鍵的時間
    private long firstTime=0;

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode==KeyEvent.KEYCODE_BACK && event.getAction()==KeyEvent.ACTION_DOWN){
            if (System.currentTimeMillis()-firstTime>2000){
                Toast.makeText(MainActivity.this,"再按一次退出程序",Toast.LENGTH_SHORT).show();
                firstTime=System.currentTimeMillis();
            }else{
                finish();
                System.exit(0);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }