我在这个解析中做错了什么？

Question

我目前正在使用 Google Gson 解析 reddit.com/.json，但遇到了一些问题。在做了一些研究之后，我找到了一种使用 Gson 解析 json 的方法，而无需制作很多类。我正在使用这个method。到目前为止，这是我的代码：

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

您正试图将元素 "children" 获取为 JsonObject，但它是 JsonArray，因为它被 [ ] 包围...

试试这样的：

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();

注意：我假设您只想获取"children" 数组的第一个元素的数据，因为看起来它是您想要查看代码并且主要查看this other question of yours 的内容。

Answer 2

children 对象返回您必须迭代的 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. 您无需为每个可能引发异常的表达式创建try..catch 块。
2. JsonParser.parse() 方法接受 Reader（例如 InpustStreamReader）实例，因此您不必自己读取 JSON。
3. root[data][children] 是一个对象数组，因此您必须遍历它们才能访问各个对象。

4. 我相信您想将所有[subredit]s 读入某种集合中，Set 我按下？

   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();
       }
   }
   

此代码返回：

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