我在這個解析中做錯了什麼？

Question

我目前使用Google Gson通過reddit.com/.json解析並遇到一些麻煩。在做了一些研究之後，我發現了一種通過使用Gson解析json而不需要製作很多類的方法。我正在使用這個method。這是我到目前爲止的代碼：

import java.io.*; 
import java.net.*; 
import com.google.gson.*; 
public class Subreddits { 

    public static void main(String[] args) { 
     URL u = null; 
     try { 
      u = new URL("http://www.reddit.com/.json"); 
     } catch (MalformedURLException e) { 
      e.printStackTrace(); 
     } 
     URLConnection yc = null; 
     try { 
      yc = u.openConnection(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
     BufferedReader in = null; 
     try { 
      in = new BufferedReader(new InputStreamReader(yc.getInputStream())); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
     String json = null; 
     StringBuilder sb = new StringBuilder(); 
     try { 
      while ((json = in.readLine()) != null){ 
       sb.append(json); 
      } 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
     try { 
      in.close(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
     json = sb.toString();//String of json 
     System.out.println(json); 
     //I want to get [data][children][data][subreddit] 
     JsonParser parser = new JsonParser(); 
     JsonObject rootObj = parser.parse(json).getAsJsonObject(); 
     JsonObject locObj = rootObj.getAsJsonObject("data").getAsJsonObject("children").getAsJsonObject("data"); 

     String subreddit = locObj.get("subreddit").getAsString(); 

     System.out.println(subreddit); 
    } 

} 

Answer 1

您正在試圖獲得元素"children"爲JsonObject，但它是一個JsonArray因爲它是由[ ]包圍......

嘗試是這樣的：

JsonParser parser = new JsonParser(); 
JsonObject rootObj = parser.parse(json).getAsJsonObject(); 

//Here is the change 
JsonObject locObj = rootObj 
         .getAsJsonObject("data") 
         .getAsJsonArray("children") 
         .get(0) 
         .getAsJsonObject() 
         .getAsJsonObject("data"); 

String subreddit = locObj.get("subreddit").getAsString(); 

注：我假設你只想得到"children"數組的第一個元素的數據，因爲它似乎是你想看看你的代碼，主要看this other question of yours。

Answer 2

children對象返回您必須迭代的Array。

public static void main(String[] args) { 
    URL u = null; 
    try { 
     u = new URL("http://www.reddit.com/.json"); 
    } catch (MalformedURLException e) { 
     e.printStackTrace(); 
    } 
    URLConnection yc = null; 
    try { 
     yc = u.openConnection(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 
    BufferedReader in = null; 
    try { 
     in = new BufferedReader(new InputStreamReader(yc.getInputStream())); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 
    String json = null; 
    StringBuilder sb = new StringBuilder(); 
    try { 
     while ((json = in.readLine()) != null) { 
      sb.append(json); 
     } 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 
    try { 
     in.close(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 
    json = sb.toString();// String of json 
    System.out.println(json); 
    // I want to get [data][children][data][subreddit] 
    JsonParser parser = new JsonParser(); 
    JsonObject rootObj = parser.parse(json).getAsJsonObject(); 
    JsonArray locObj = rootObj.getAsJsonObject("data").getAsJsonArray("children"); 

    /* Iterating children object */ 
    Iterator<JsonElement> iterator = locObj.iterator(); 

    while(iterator.hasNext()){ 
     JsonElement element = iterator.next(); 
     JsonElement subreddit = element.getAsJsonObject().getAsJsonObject("data").get("subreddit"); 
     System.out.println(subreddit.getAsString()); 
    } 
} 

Answer 3

1. 你並不需要爲可拋出異常的每一個表情創建try..catch塊。
2. JsonParser.parse()方法接受Reader（例如InpustStreamReader）實例，因此您不必自行閱讀JSON。
3. root[data][children]是一個對象數組，因此您必須遍歷它們才能訪問單個對象。

4. 我相信你想讀取所有[subredit] s到某種集合，Set我按？

   public static void main(String[] args) { 
       try { 
        Set<String> subreddits = new HashSet<>(); 
        URL url = new URL("http://www.reddit.com/.json"); 
   
        JsonParser parser = new JsonParser(); 
        JsonObject root = parser.parse(new InputStreamReader(url.openConnection().getInputStream())).getAsJsonObject(); 
        JsonArray children = root.getAsJsonObject("data").getAsJsonArray("children"); 
   
        for (int i = 0; i < children.size(); i++) { 
         String subreddit = children.get(i).getAsJsonObject().getAsJsonObject("data").get("subreddit").getAsString(); 
         subreddits.add(subreddit); 
        } 
   
        System.out.println(subreddits); 
       } catch (IOException e) { 
        e.printStackTrace(); 
       } 
   } 
   

此代碼返回：

[IAmA, worldnews, technology, news, todayilearned, gaming, AskReddit, movies, videos, funny, bestof, science, WTF, politics, aww, pics, atheism, Music, AdviceAnimals]