Parsing JSON object into a Map using Gson - JsonSyntaxException

Question

I have a map property:

private Map<String, Attribute> attributes = new HashMap<>();

Attribute object looks like this:

public class Attribute{
     private String value;
     private String name;

//with constructor setters and getters
}

How do I represent attributes Map object as JSON?

I am getting a JsonSyntaxException:

Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY

when I'm trying to convert JSON object using fromJson() in the following code:

Attribute attribute = Gson().fromJson(jsonObect,Attribute.class)

My JSON object looks like this:

{
   "attributes":[
      {
         "name":"some name",
         "value":"some value"
      }
   ]
}

score 2 · Accepted Answer · answered Jul 01 '22 at 20:25

it is easy to check:

    Map<String,Attribute> attributes = new HashMap<>();
    attributes.put("key_0", new Attribute("value_0", "name_0"));// I added constructor and getter/setter methods to class Attribute
    attributes.put("key_1", new Attribute("value_1", "name_1"));
    attributes.put("key_2", new Attribute("value_2", "name_2"));
    //serialize using ObjectMapper
    ObjectMapper mapper = new ObjectMapper();
    var s = mapper.writeValueAsString(attributes);
    System.out.println(s);

output:

{
   "key_2":{
      "value":"value_2",
      "name":"name_2"
   },
   "key_1":{
      "value":"value_1",
      "name":"name_1"
   },
   "key_0":{
      "value":"value_0",
      "name":"name_0"
   }
}

Alexander Ivanchenko · Answer 2 · 2022-07-01T22:43:32.703

I am getting: "Expected BEGIN_ARRAY but was BEGIN_OBJECT" error when I'm trying to convert JSON object using fromJson() in the following code:

Attribute attribute = Gson().fromJson(jsonObect,Attribute.class)

The following JSON object doesn't match either a single Attribute object or a map Map<String, Attribute>:

{
   "attributes":[
      {
         "name":"some name",
         "value":"some value"
      }
   ]
}

Have a look carefully, the value mapped to the key "attributes" is a JSON-array.

Hence, you can parse it successfully into a map of type Map<String, List<Attribute>>

That's how it can be implemented using Gson.fromJson() and TypeToken (for more options, see this question):

public static void main(String[] args) {

    String jsonStr = """
        {
            "attributes":[
               {
                  "name":"some name",
                  "value":"some value"
               }
            ]
         }""";

    Gson gson = new Gson();
    
    Type mapStrByStr = new TypeToken<Map<String, List<Attribute>>>(){}.getType();
    Map<String, List<Attribute>> map = gson.fromJson(jsonStr, mapStrByStr);

    System.out.println(map);
    }
}


public static class Attribute{
    private String value;
    private String name;

    @Override
    public String toString() {
        return "Attribute{" +
            "value='" + value + '\'' +
            ", name='" + name + '\'' +
            '}';
    }
}

Output:

{attributes=[Attribute{value='some value', name='some name'}]}

Parsing JSON object into a Map using Gson - JsonSyntaxException

2 Answers2