title: The method DataOutputStream(OutputStream) is undefined for the type
body: <p>I followed a lot of tutorials to make progress with this project. Now I am following a tutorial to create a Google cloud messaging server using JSON and Jackson library.</p>
  
  <p>I somehow got the right Jackson library of all the libraries on the internet. But an error appeared which is the title of this question.</p>
  
  <p>This is that code:</p>
  
  <pre><code>import java.io.BufferedReader;
  import java.io.DataOutputStream;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.net.HttpURLConnection;
  import java.net.MalformedURLException;
  import java.net.URL;
  
  import org.codehaus.jackson.map.ObjectMapper;
  
  
  public class POST2GCM {
  
      public static void post(String apiKey, Content content){
          try{
              //1. url
              URL url = new URL("https://android.googleapis.com/gcm/send");
              //2. open connection
              HttpURLConnection conn = (HttpURLConnection) url.openConnection();
              //3. specify POST method
              conn.setRequestMethod("POST");
              //4.set the headers
              conn.setRequestProperty("Content-Type", "application/json");
              conn.setRequestProperty("Authorization", "key="+apiKey);
  
              conn.setDoOutput(true);
              //5. add json data into POST request body
              //5.1 use jackson object mapper to convert contnet object into JSON
              ObjectMapper mapper = new ObjectMapper();
              //5.2 get connection stream 
              DataOutputStream wr = DataOutputStream(conn.getOutputStream());
              //5.3 copy content "JSON" into
              mapper.writeValue(wr, content);
              //5.4 send the request
              wr.flush();
              //5.5
              wr.close();
              //6. get the response
              int responseCode = conn.getResponseCode();
              System.out.println("\nSending 'POST' request to URL: "+url);
              System.out.println("Response Code: "+responseCode);
  
              BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
              String inputLine;
              StringBuffer response = new StringBuffer();
  
              while((inputLine = in.readLine()) != null){
                  response.append(inputLine);
              }
              in.close();
              //7. print result
              System.out.println(response.toString());
          }catch(MalformedURLException e){
              e.printStackTrace();
          }catch(IOException e){
              e.printStackTrace();
          }
      }
  }
  </code></pre>
  
  <p>I don't know how to fix this one, I've looked for answers but there isn't any answer.</p>