按两次退出应用

 /**
     * 按两次退出按钮退出程序
     */
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
            // System.currentTimeMillis()无论何时调用，肯定大于2000
            if ((System.currentTimeMillis() - exitTime) > 2000) {
                Toast.makeText(this, "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);
    }