Android 在 Android 中检查 null 或空字符串 [英] Android check null or empty string in Android

问题描述

在我尝试 AsyncTask 时，我从我的服务器获取电子邮件地址.在 onPostExecute() 中，我必须检查电子邮件地址是 empty 还是 null.我使用以下代码进行检查:

In my trying AsyncTask I get email address from my server. In onPostExecute() I have to check is email address empty or null. I used following code to check it:

if (userEmail != null && !userEmail.isEmpty()) {
    Toast.makeText(getActivity(), userEmail, Toast.LENGTH_LONG).show();
    UserEmailLabel.setText(userEmail);
}

但是在我的 Toast 中，我看到 null 被打印出来.我的完整代码:

But in my Toast I see null is printed. My full code:

private class LoadPersonalData extends AsyncTask<String, Void, Void> {
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    protected Void doInBackground(String... res) {
        List<NameValuePair> params = new ArrayList<NameValuePair>();
        params.add(new BasicNameValuePair("user_id", PrefUserName));
        params.add(new BasicNameValuePair("type", type_data));
        JSONObject json = jsonParser.makeHttpRequest(Url, "POST", params);
        String result = "";
        try {
            result = json.getString("message");
        } catch (JSONException e) {
            e.printStackTrace();
        }
        if (result.equals("success")) {
            try {
                userEmail = json.getString("email");
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        // TODO Auto-generated method stub
        super.onPostExecute(result);            
        if (userEmail != null && !userEmail.isEmpty()) {
            Toast.makeText(getActivity(), userEmail, Toast.LENGTH_LONG).show();
            UserEmailLabel.setText(userEmail);
        }
    }

如何检查null 和empty 字符串?

How can I check for null and empty string?

推荐答案

来自@Jon Skeet 的评论，实际上 String 的值是 "null".以下代码解决了它

From @Jon Skeet comment, really the String value is "null". Following code solved it

if (userEmail != null && !userEmail.isEmpty() && !userEmail.equals("null")) 

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