请问cannot resolve symbol DownImgAsyncTask 如何解决

以下是源码，报错信息总是显示cannot resolve symbol DownImgAsyncTask

public class MainActivity extends AppCompatActivity {
EditText username;
EditText password;
EditText secretcode = null;
Button getCode = null;
Button logIn = null;
ImageView codeImage = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    getCode = (Button)findViewById(R.id.getCode);
    codeImage = (ImageView)findViewById(R.id.codeImage);
    logIn = (Button)findViewById(R.id.logIn);
    username = (EditText)findViewById(R.id.username);
    password = (EditText)findViewById(R.id.password);
    secretcode = (EditText)findViewById(R.id.secretcode);




    getCode.setOnClickListener(new OnClickListener()
    {
        @Override
        public void onClick(View v) {
            codeImage.setVisibility(View.VISIBLE);

            String imgURL = "http://210.42.121.241/servlet/GenImg";
            new DownImgAsyncTask().execute(imgURL);
        }
    });
}
private String getURLResponse(String urlString) {
    HttpURLConnection conn = null;
    InputStream is = null;
    String resultData = "";
    try {
        URL url = new URL(urlString);
        conn = (HttpURLConnection)url.openConnection();
        conn.setDoInput(true);
        conn.setDoOutput(true);
        conn.setRequestMethod("POST");
        final String code = secretcode.getText().toString();
        String content = "id=542452452&pwd=********&xdvfb"+code;
        OutputStream outputStream = conn.getOutputStream();
        outputStream.write(content.toString().getBytes("UTF-8"));
        outputStream.close();
        is = conn.getInputStream();
        InputStreamReader isr = new InputStreamReader(is);
        BufferedReader bufferedReader = new BufferedReader(isr);
        String cookie = conn.getHeaderField("Set-Cookie");
        String inputLine = "";
        while((inputLine = bufferedReader.readLine())!=null) {
            resultData += inputLine+"\n";
        }
        System.out.println(resultData);
    }catch (MalformedURLException e) {
        e.printStackTrace();
    }catch (IOException e) {
        e.printStackTrace();
    }finally {
        if (is != null) {
            try {
                is.close();
            }catch (IOException e) {
                e.printStackTrace();
            }
        }
        if (conn != null) {
            conn.disconnect();
        }
    }
}