java - Using GSON to parse array with multiple types

Question

I wish to use GSON to parse the following json:

[
    [
        "hello",
        1,
        [2]
    ],
    [
        "world",
        3,
        [2]
    ]
]

So, that's 1 array, containing 2 arrays. The 2 inner arrays are themselves arrays, comprised of String, int, array types.

I'm unsure how I can using Java classes to model the array which has 3 different types (String, int, array). I start with:

// String json just contains the aforementioned json string.

ArrayList<ArrayList<XXX>> data = new ArrayList<ArrayList<XXX>>();

Type arrayListType = new TypeToken<ArrayList<ArrayList<XXX>>>(){}.getType();

data = gson.fromJson(json, arrayListType);

But what should be where the 'XXX' are? I think it should be an array, but it should be an array with 3 different data types. So how can I use Java to model this?

Can any help? Thank you.

Answer 1

Gson has special handling for deserializing some single-component arrays into a non-array type. For example, int data = gson.fromJson("[3]", int.class); would assign the int value 3 to data.

Of course, deserializing a single-component array into a non-array type is not required. For example, the previous example could be deserialized as int[] data = gson.fromJson("[3]", int[].class);.

Gson will also often deserialize a non-String value into a String, when asked. Applying this to the first example, String data = gson.fromJson("[3]", String.class); works just as well.

Note that it does not work to tell Gson to deserialize the first example as type Object. Object data = gson.fromJson("[3]", Object.class); results in a parse exception complaining that [3] is not a primitive.

Applied to the example in the original question above, if it's acceptable to treat all of the values as Strings, then deserialization becomes simple.

// output:
// hello 1 2 
// world 3 2 

public class Foo
{
  static String jsonInput = 
    "[" +
      "["hello",1,[2]]," +
      "["world",3,[2]]" +
    "]";

  public static void main(String[] args)
  {
    Gson gson = new Gson();
    String[][] data = gson.fromJson(jsonInput, String[][].class);
    for (String[] data2 : data)
    {
      for (String data3 : data2)
      {
        System.out.print(data3);
        System.out.print(" ");
      }
      System.out.println();
    }
  }
}

Unfortunately, with Gson I've not been able to figure out a simple deserialization approach that would allow for "better" binding to more specific and mixed types in an array, since Java doesn't provide a syntax for defining a mixed type array. For example, the preferred type of the collection in the original question might be List<List<String, int, List<int>>>, but that's not possible to define in Java. So, you gotta be content with List<List<String>> (or String[][]), or turn to an approach with more "manual" parsing.

(Yes, Java allows a type declaration of List<List<Object>>, but Object is not a specific enough type to meaningfully deserialize to. Also, as discussed, attempting to deserialize [3] to Object results in a parse exception.)

---

Small Update: I recently had to deserialize some sloppy JSON that included a structure not too dissimilar from that in the original question. I ended up just using a custom deserializer to create a object from the messy JSON array. Similar to the following example.

// output: 
// [{MyThreeThings: first=hello, second=1, third=[2]}, 
//  {MyThreeThings: first=world, second=3, third=[4, 5]}]

import java.lang.reflect.Type;
import java.util.Arrays;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;

public class FooToo
{
  static String jsonInput =
      "[" +
          "["hello",1,[2]]," +
          "["world",3,[4,5]]" +
      "]";

  public static void main(String[] args)
  {
    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(MyThreeThings.class, new MyThreeThingsDeserializer());
    Gson gson = gsonBuilder.create();
    MyThreeThings[] things = gson.fromJson(jsonInput, MyThreeThings[].class);
    System.out.println(Arrays.toString(things));
  }
}

class MyThreeThings
{
  String first;
  int second;
  int[] third;

  MyThreeThings(String first, int second, int[] third)
  {
    this.first = first;
    this.second = second;
    this.third = third;
  }

  @Override
  public String toString()
  {
    return String.format(
        "{MyThreeThings: first=%s, second=%d, third=%s}",
        first, second, Arrays.toString(third));
  }
}

class MyThreeThingsDeserializer implements JsonDeserializer<MyThreeThings>
{
  @Override
  public MyThreeThings deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
      throws JsonParseException
  {
    JsonArray jsonArray = json.getAsJsonArray();
    String first = jsonArray.get(0).getAsString();
    int second = jsonArray.get(1).getAsInt();
    JsonArray jsonArray2 = jsonArray.get(2).getAsJsonArray();
    int length = jsonArray2.size();
    int[] third = new int[length];
    for (int i = 0; i < length; i++)
    {
      int n = jsonArray2.get(i).getAsInt();
      third[i] = n;
    }
    return new MyThreeThings(first, second, third);
  }
}

The Gson user guide does cover handling deserialization of collections of mixed types with a similar example as this in the "Serializing and Deserializing Collection with Objects of Arbitrary Types" section.