DaveNOTDavid DaveNOTDavid - 3 years ago 85
Java Question

Error parsing JSON data to an array using GSON

Before you assume that this question is a duplicate based on its title, I must say that I couldn't find any solutions out there for my specific problem. However, I'd appreciate it if you could suggest some links! That said, my problem is that I keep getting the following GSON exception:

07-24 10:12:44.713 2540-2603/com.davenotdavid.dndheadlines W/System.err: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2 path $


I understand that I'm somehow passing in a JSON object as opposed to an array, but what I don't understand is how it's not working based on the following Utility class that basically has helper methods to parse and extract JSON data from a JSON array using the OkHttpClient and GSON libraries, and then returns a list of extracted data back to the calling class:

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class QueryUtils {
public static List<Article> fetchArticleData(String requestUrl) {
String jsonResponse = "";
try {
OkHttpClient okHttpClient = new OkHttpClient();
Request request = new Request.Builder()
.url(requestUrl)
.build();
Response response = okHttpClient.newCall(request).execute();
jsonResponse = response.body().string();
} catch (Exception e) {
e.printStackTrace();
}

return extractDataFromJson(jsonResponse);
}

private static List<Article> extractDataFromJson(String jsonResponse) {
if (jsonResponse == null || jsonResponse.isEmpty()) {
return null;
}

List<Article> articlesList = new ArrayList<>();

ArticleResponse[] articleResult = new ArticleResponse[0];
try {
Gson gson = new Gson();
Type collectionType = new TypeToken<Collection<ArticleResponse>>(){}.getType();

// *** Exception is thrown here ***
Collection<ArticleResponse> enums = gson.fromJson(jsonResponse, collectionType);

articleResult = enums.toArray(new ArticleResponse[enums.size()]);
} catch (Exception e) {
e.printStackTrace();
}

if (articleResult.length != 0) {
for (int i = 0; i < articleResult.length; i++) {
ArticleResponse article = articleResult[i];
articlesList.add(new Article(
article.getTitle(),
article.getUrl(),
article.getUrlToImage(),
article.getPublishedAt()));
}
}

return articlesList;
}
}


As you'll notice that the exception is thrown in the following line of code:

Collection<ArticleResponse> enums = gson.fromJson(jsonResponse, collectionType);


Here's the ArticleResponse class I generated using GsonFormat:

public class ArticleResponse {

/**
* author : Annalee Newitz
* title : First glimpse of Steven Spielberg’s new movie, Ready Player One
* description : This trailer introduces us to a VR dystopia, and has the greatest soundtrack ever.
* url : https://arstechnica.com/the-multiverse/2017/07/first-glimpse-of-steven-spielbergs-new-movie-ready-player-one/
* urlToImage : https://cdn.arstechnica.net/wp-content/uploads/2017/07/RPO-1000x437-nbv413pszkz7s29g3blop7i6ymcv84f9mtwmvt1xxk-760x380.png
* publishedAt : 2017-07-23T17:13:56Z
*/

private String title;
private String url;
private String urlToImage;
private String publishedAt;

public String getTitle() {
return title;
}

public void setTitle(String title) {this.title = title;}

public String getUrl() {
return url;
}

public void setUrl(String url) {
this.url = url;
}

public String getUrlToImage() {
return urlToImage;
}

public void setUrlToImage(String urlToImage) {
this.urlToImage = urlToImage;
}

public String getPublishedAt() {
return publishedAt;
}

public void setPublishedAt(String publishedAt) {
this.publishedAt = publishedAt;
}
}


A sample JSON response from a network call in the Utility class is as follows:

{
"status": "ok",
"source": "ars-technica",
"sortBy": "top",
"articles": [{
"author": "Eric Berger",
"title": "Elon Musk’s Mars rocket may be about to lose half of its engines",
"description": "Downscaling the Mars booster suggests that Musk may be bending toward reality.",
"url": "https://arstechnica.com/science/2017/07/elon-musk-drops-an-important-hint-about-his-revised-mars-rocket/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/29937260386_45ba70fb85_h-1440x614-760x380.jpg",
"publishedAt": "2017-07-24T13:12:01Z"
}, {
"author": null,
"title": "Hour of Devastation review: The evil elder dragon god-pharaoh has arrived. RIP.",
"description": "Plus, a look at the major changes coming to future Magic: the Gathering sets.",
"url": "https://arstechnica.com/gaming/2017/07/magic-hour-of-devastation-review/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/401538_CN-copy-760x380.jpg",
"publishedAt": "2017-07-24T13:00:52Z"
}, {
"author": "Annalee Newitz",
"title": "Season 2 of Stranger Things looks even creepier and more intense",
"description": "The Upside Down is back, and it looks like it's about to eat the world.",
"url": "https://arstechnica.com/the-multiverse/2017/07/season-2-of-stranger-things-looks-even-creepier-and-more-intense/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/landscape-1486986380-stranger-things-mike-wheeler-2-760x380.jpg",
"publishedAt": "2017-07-23T23:20:39Z"
}, {
"author": "Annalee Newitz",
"title": "First glimpse of Steven Spielberg’s new movie, Ready Player One",
"description": "This trailer introduces us to a VR dystopia, and has the greatest soundtrack ever.",
"url": "https://arstechnica.com/the-multiverse/2017/07/first-glimpse-of-steven-spielbergs-new-movie-ready-player-one/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/RPO-1000x437-nbv413pszkz7s29g3blop7i6ymcv84f9mtwmvt1xxk-760x380.png",
"publishedAt": "2017-07-23T17:13:56Z"
}, {
"author": null,
"title": "New book explores how protesters—and governments—use Internet tactics",
"description": "The protest frontiers are changing. An entrenched researcher explains why they work.",
"url": "https://arstechnica.com/tech-policy/2017/07/twitter-and-tear-gas-book-explores-new-world-of-digital-protest/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/Screen-Shot-2017-07-22-at-1.17.15-PM-760x380.png",
"publishedAt": "2017-07-23T15:00:02Z"
}, {
"author": "Nathan Mattise",
"title": "Maybe The Americans is quietly a technophile love letter to the 1980s",
"description": "Joel Fields and Joe Weisberg talk handheld football, spy tech, mail robot, and more.",
"url": "https://arstechnica.com/the-multiverse/2017/07/maybe-the-americans-is-quietly-a-technophile-love-letter-to-the-1980s/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/AmericansFootball-600x380.jpg",
"publishedAt": "2017-07-23T14:30:15Z"
}, {
"author": "Sam Machkovech",
"title": "Dockless bike sharing lands in Seattle—and leads us down unsavory alleyways",
"description": "Now in Seattle: two services, 1,000 bikes, and a shoulder shrug at helmet laws.",
"url": "https://arstechnica.com/business/2017/07/dockless-bike-sharing-lands-in-seattle-and-leads-us-down-unsavory-alleyways/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/IMAG3645-760x380.jpg",
"publishedAt": "2017-07-23T14:00:32Z"
}, {
"author": null,
"title": "Level up: How video games evolved to solve significant scientific problems",
"description": "Science, your chance to use all that time spent gaming for the greater good.",
"url": "https://arstechnica.com/gaming/2017/07/level-up-how-video-games-evolved-to-solve-significant-scientific-problems/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/GettyImages-134429675-760x380.jpg",
"publishedAt": "2017-07-23T13:25:25Z"
}, {
"author": "John Timmer",
"title": "We’ve screwed up the coasts so badly that an invasive species is a plus",
"description": "When native species are gone, an invasive one can provide ecosystem services.",
"url": "https://arstechnica.com/science/2017/07/weve-screwed-up-the-coasts-so-badly-that-an-invasive-species-is-a-plus/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/Gracilaria-vermiculophylla-760x380.jpg",
"publishedAt": "2017-07-23T12:00:53Z"
}, {
"author": "Megan Geuss",
"title": "German energy company wants to build flow batteries in old natural gas caverns",
"description": "Research for a massive redox flow battery is underway.",
"url": "https://arstechnica.com/business/2017/07/german-energy-company-wants-to-build-flow-batteries-in-old-natural-gas-caverns/",
"urlToImage": "https://cdn.arstechnica.net/wp-content/uploads/2017/07/20170612-b4p-english-760x380.jpg",
"publishedAt": "2017-07-22T15:00:22Z"
}]
}

Answer Source

You cannot convert your jsonObject in an Array you must have a jsonArray to do this. Simply add this lines:

JsonObject jsonObject = gson.fromJson(jsonResponse, JsonObject.class);
String articlesArrayResponse = jsonObject.getAsJsonArray("articles").getAsString();

before your exception happens. Obviously you must change the argument in that line for this:

Collection<ArticleResponse> enums = gson.fromJson(articlesArrayResponse, collectionType);

I hope it helps you!

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