java为什么在println时即使有值显示也会出现空指针异常?
这是类a的一个函数,我将我的值传递给一个类名,并确认它的寄存器函数
private void registerOrder() {
confirm.register(id);
}
这是课堂确认。在这门课上,我计划在服务器内部发布数据。url中没有错误。我已经打印了这个代码“System.out.println”(“wei”+getPostDataString(postDataParams));”我得到了我的价值。但系统显示为空指针。代码的其他部分有错误吗
public class confirm {
private static final String REGISTER_URL =
"http://192.168.43.214/apexStore2/confirm.php";
public static void register(String id) {
class RegisterUser extends AsyncTask<String, Void, String> {
ProgressDialog loading;
@Override
protected void onPreExecute() {
super.onPreExecute();
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
loading.dismiss();
}
@Override
protected String doInBackground(String... params) {
HashMap<String, String> data = new HashMap<>();
data.put("product_id",params[0]);
String result = sendPost(REGISTER_URL,data);
return result;
}
}
RegisterUser ru = new RegisterUser();
ru.execute(String.valueOf(id));
}
public static String sendPost(String requestURL,
HashMap<String, String> postDataParams) {
URL url;
String response = " ";
try {
url = new URL(requestURL);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type",
"application/x-www-form-urlencoded");
conn.setRequestProperty("Content-Length", "" +
Integer.toString(getPostDataString(postDataParams).getBytes().length));
conn.setRequestProperty("Content-Language", "en-US");
conn.setReadTimeout(15000);
conn.setConnectTimeout(15000);
conn.setDoInput(true);
conn.setDoOutput(true);
OutputStream os = conn.getOutputStream();
BufferedWriter writer = new BufferedWriter(
new OutputStreamWriter(os, "UTF-8"));
writer.write(getPostDataString(postDataParams));
System.out.println("wei" + getPostDataString(postDataParams));
writer.flush();
writer.close();
os.close();
int responseCode=conn.getResponseCode();
if (responseCode == HttpsURLConnection.HTTP_OK) {
BufferedReader br=new BufferedReader(new
InputStreamReader(conn.getInputStream()));
response = br.readLine();
}
else {
response="Error Registering";
}
} catch (Exception e) {
e.printStackTrace();
}
return response;
}
private static String getPostDataString(HashMap<String, String> params)
throws UnsupportedEncodingException {
StringBuilder result = new StringBuilder();
boolean first = true;
for(Map.Entry<String, String> entry : params.entrySet()){
if (first)
first = false;
else
result.append("&");
result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
result.append("=");
result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
}
return result.toString();
}
}
# 1 楼答案
读取代码时,如果调用
confirm.register(id)
,则会出现空指针异常如果要以静态方式使用类,则需要将其声明为最终类:
对你来说,这应该管用。不过,您也可以通过创建类的实例来解决问题,而无需使用最终标识符:
我还看到你的班名以小写字母开头。建议创建以大写字母开头的类名
如果它不能解决你的错误,请在这里填写错误信息
# 2 楼答案
您有一个实例变量,它从未分配给:
它被用于