Gson Expected BEGIN_ARRAY but was STRING at line 1 column 62

Question

I have the following class :

final class CFS {
    public Map<String, String> files = new HashMap<String, String>();
    public List<String> directories = new ArrayList<String>();
}

And this code which should parse the json :

CFS cfs = JStorage.getGson().fromJson(JSON_STRING, CFS.class);

Where

JSON_STRING = "{\"directories\" : [\"folder1\", \"folder1/folder2\"], \"files\" : [{\"folder1\" : \"file.txt\"}, {\"folder1/folder2\" : \"file.cfg\"}]}"

JSON is:

{
  "directories": ["folder1", "folder1/folder2"],
  "files": [
    {
      "folder1": "file.txt"
    }, 
    {
      "folder1/folder2": "file.cfg"
    }
  ]
}

The error I'm getting is: Expected BEGIN_ARRAY but was STRING at line 1 column 62

But I have no idea why, the json is valid according to jsonlint.

Any idea on why I am getting this error?

You have a JSON object that contains two arrays, and a Java object that contains an array (List) and a Map. — Brian Roach, May 05 '13 at 01:47

score 10 · Answer 1 · answered Sep 23 '18 at 23:32

If someone is getting this error in AndroidStudio :

Try two things:

Roll back to last working conditions. (Revert if you use VCS).
In build options Clean project and rebuild. (Worked for me.)

I'm fairly new to android. Excuse any mistakes if committed. Suggestions are welcome :)

score 4 · Accepted Answer · edited May 23 '17 at 10:30

Your JSON is valid - but your mapping class isn't (parts of it don't match). In particular, the files property of your class cannot be mapped as a Map<String, String> from the given JSON. It's hard to recommend an alternate structure for storing the data without seeing a larger sample, but in general you can follow this guide when mapping between JSON structures and Java classes. This JSON:

"files": [
    {
        "folder1": "file.txt"
    }, 
    {
        "folder1/folder2": "file.cfg"
    }
]

represents an array containing objects, where each object is best represented as a map. So in essence, a list of maps. Consequently your Java object should be:

public class CFS {
    private List<Map<String, String>> files = new ArrayList<Map<String, String>>(
            4);
    private List<String> directories = new ArrayList<String>(4);

    // Constructors, setters/getters
}

Note that I've corrected your properties by making them private and adding getters/setters. With the above defined class your program should work just fine.

final Gson gson = new GsonBuilder().create();
final CFS results = gson.fromJson(json, CFS.class);
Assert.assertNotNull(results);
Assert.assertNotNull(results.getFiles());
System.out.println(results.getFiles());

Produces:

[{folder1=file.txt}, {folder1/folder2=file.cfg}]

If you find yourself needing to retain the current CFS structure though, you would need to manually parse the JSON into it.

score 2 · Answer 3 · answered May 05 '13 at 09:48

As already Brain noted you have a array of objects so you need to convert this by your own with a custom deserializer.

Here is an example implementation:

public class q16380367 {
    final class CFS {
        public HashMap<String, String> files = new HashMap<String, String>();
        public ArrayList<String> directories = new ArrayList<String>();
    }

    public static void main(String[] args) {
        new q16380367();
    }

    public q16380367() {
        String JSON_STRING = "{\"directories\" : [\"folder1\", \"folder1/folder2\"], \"files\" : [{\"folder1\" : \"file.txt\"}, {\"folder1/folder2\" : \"file.cfg\"}]}";
        Gson gson = new GsonBuilder().registerTypeAdapter(
                new TypeToken<HashMap<String, String>>() {
                }.getType(), new CfsDeserializer()).create();
        CFS foo = gson.fromJson(JSON_STRING, CFS.class);
    }

    private final class CfsDeserializer implements
            JsonDeserializer<HashMap<String, String>> {
        @Override
        public HashMap<String, String> deserialize(JsonElement json,
                Type typeOfT, JsonDeserializationContext context)
                throws JsonParseException {
            HashMap<String, String> data = new HashMap<String, String>();
            JsonArray list = json.getAsJsonArray();
            for (JsonElement e : list) {
                Set<Entry<String, JsonElement>> entries = e.getAsJsonObject()
                        .entrySet();
                for (Entry<String, JsonElement> entry : entries) {
                    data.put(entry.getKey(), entry.getValue().getAsString());
                }
            }
            return data;
        }
    }
}

Gson Expected BEGIN_ARRAY but was STRING at line 1 column 62

3 Answers3

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