Convert JSON String to 2D array

Define a class that reflects your data structure.

class Data {
    private List<String[]> reels;
}

Then use that in the call to Gson.toJson:

Data data = GSON.fromJson(jsonString, Data.class);

Related questions:

• Gson Json parser Array of Arrays
  
• Converting JSON data to Java object

import java.util.List;

import com.google.gson.Gson;

public class Main {

    public static void main(String[] args) {
        String jsonString = "{\"reels\":[[\"BL\",\"77\",\"BL\",\"1B\",\"BL\",\"3B\",\"BL\",\"1B\",\"BL\",\"77\",\"BL\",\"2B\",\"BL\",\"1B\",\"BL\",\"LG\",\"BL\",\"1B\",\"BL\",\"3B\",\"BL\",\"2B\"],[\"BL\",\"77\",\"BL\",\"1B\",\"BL\",\"3B\",\"BL\",\"1B\",\"BL\",\"77\",\"BL\",\"2B\",\"BL\",\"1B\",\"BL\",\"LG\",\"BL\",\"1B\",\"BL\",\"3B\",\"BL\",\"2B\"],[\"BL\",\"77\",\"BL\",\"1B\",\"BL\",\"3B\",\"BL\",\"1B\",\"BL\",\"77\",\"BL\",\"2B\",\"BL\",\"1B\",\"BL\",\"LG\",\"BL\",\"1B\",\"BL\",\"3B\",\"BL\",\"2B\"]]}";
        Gson GSON = new Gson();
        Data data = GSON.fromJson(jsonString, Data.class);
        System.out.println(data);
    }
    
    class Data {
        private List<String[]> reels;
        public String toString() {
            String retval = "[";
            for (String[] str : reels) {
                retval +="[";
                for (String inStr : str) {
                    if (retval.charAt(retval.length()-1)!='[')
                        retval +=",";
                    retval +="\\\""+inStr+"\\\"";
                }
                retval +="]";
            }
            retval +="]";
            return retval;
        }
    }
}

I think you should be able to do it by reading the JSON string as JsonObject and then reading JsonObject's reel key as JsonArray into the target two dimensional array:

Gson gson = new Gson();
// Read JSON string as JsonObject
JsonObject jsonObject = gson.fromJson(reels, JsonObject.class);
// Read reels array key
JsonArray jsonArray = jsonObject.getAsJsonArray("reels");

// Iterate reels array and assign to result array
String[][] result = new String[3][22];
for (int i = 0; i < jsonArray.size(); i++) {
    JsonArray reelJsonArr = jsonArray.get(i).getAsJsonArray();
    for (int j = 0; j < reelJsonArr.size(); j++) {
        result[i][j] = reelJsonArr.get(j).getAsString();
    }
}

If you don't want to do manual convertion of JsonArray to your String array, you should provide your own POJO with respect to JSON string as mentioned by @geocodezip in his answer.