java - 该类型的方法 DataOutputStream(OutputStream) 未定义

Question

我遵循了很多教程来使这个项目取得进展。现在，我正在按照教程使用 JSON 和 Jackson 库创建 Google 云消息传递服务器。

我以某种方式在互联网上的所有图书馆中找到了正确的 jackson 图书馆。但是出现了一个错误，这就是这个问题的标题。

这就是代码:

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();
        }
    }
}

我不知道如何解决这个问题，我一直在寻找答案，但没有任何答案。

Answer 1

您缺少新关键字

     //5.2 get connection stream 
    DataOutputStream wr = DataOutputStream(conn.getOutputStream());

替换为

   DataOutputStream wr = new DataOutputStream(conn.getOutputStream());