//两秒内按返回键两次退出程序    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){     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);     }