Mapping Google Spreadsheets json response to java object returns error message

methuselah :

I am getting the following JSON output:

{
  "range": "Frontpage!E6:E15",
  "majorDimension": "ROWS",
  "values": [
    [
      "FundRequest"
    ],
    [
      "nOS"
    ],
    [
      "NEX"
    ],
    [
      "DREP"
    ],
    [
      "ChromaWay"
    ],
    [
      "Jura"
    ],
    [
      "Origo"
    ],
    [
      "Phantasma"
    ],
    [
      "NuCypher"
    ],
    [
      "Oasis Labs"
    ]
  ]
}

So I created the following wrapper:

public class Wrapper {

     private String range;
     private String majorDimension;
     private List<String> values;

     // getters and setters

}

Now I want use GsonBuilder to map the JSON to my object:

Gson gson = new GsonBuilder().create();
Wrapper w = gson.fromJson(jsonString, Wrapper.class);

but I get the following error:

java.lang.IllegalStateException: Expected a string but was BEGIN_ARRAY at line 1 column 37 path $.values[0]

How do I fix it? Is there any way for me to convert it from a List<List<String>>to a List<String>?

VelNaga :

First, You should use List List<<List<String>> instead of List<String> for values.

second, You can get List<String> from List<List<String>> Using below code,

public <T> List<T> flattenListOfListsImperatively(
    List<List<T>> nestedList) {
    List<T> ls = new ArrayList<>();
    nestedList.forEach(ls::addAll);
    return ls;
}

or using Java-8

public <T> List<T> flattenListOfListsStream(List<List<T>> list) {
    return list.stream()
      .flatMap(Collection::stream)
      .collect(Collectors.toList());    
}

Also, do a search in GSON is there any built-in way to flatten the nested collection. Please post it here if there is anything.

Edit 1: I did a quick search in GSON library and it seems possible.You have write an adapter to achieve the things. Please refer this link link2