Parsing nested JSON objects without keys using Gson

Question

I have a JSON with this content:

[ {
    "lines" : {
        "0" : "Hammersmith & City",
        "1" : "Circle"
    },
    "id" : "233",
    "name" : "Shepherd's Bush Market"
}, {
    "lines" :"",
    "id" : "233",
    "name" : "Shepherd's Bush Market"
}, {
    "lines" : {
        "0" : "Hammersmith & City",
        "1" : "Circle"
    },
    "id" : "233",
    "name" : "Shepherd's Bush Market"
},
, {
    "lines" : "",
    "id" : "233",
    "name" : "Shepherd's Bush Market"
  }]

Normally, I could create an object like this

public class MyObject {

    public String id;  
    public String name;
    public Line[] lines;

    public class Line {
        public String key;
        public String value;
    }
}

And the Gson serializer would handle the parsing, but in this case lines doesn't have any keys/ids. I have tried using HashMaps and Maps instead of inner classes, but it doesn't work. Is there a way I can parse this using Gson?

UPDATE:

I have changed lines from MyObject to a Map<String, String> and added some more lines to JSON response

At the moment this is the code I'm using to parse the JSON

Type listType = new TypeToken<List<MyObject>>(){}.getType();
List<MyObject> data = getGson().fromJson(str, listType);

Caused by: com.google.gson.JsonParseException: The JsonDeserializer MapTypeAdapter    failed to deserialize json object "" given the type java.util.Map<java.lang.String, java.lang.String>

After looking through the entire JSON response, it seems that lines is returned as a empty String ("") when it's not available and as a map when it is. I think this may be part of the problem

Answer 1

Use Map<String, String> instead Line[] lines.

(You don't need class Line)

It should work.

Or if your keys are integers Map<Integer, String> will work as well

[Edit]

Your json String represents list of objects: {..},{..},{..}

You need wrap it with [].

So the working json should be:

[
    {
        "lines": {
            "0": "Hammersmith & City",
            "1": "Circle"
        },
        "id": "233",
        "name": "Shepherd's Bush Market"
    },
    {
        "lines": {
            "0": "Hammersmith & City",
            "1": "Circle"
        },
        "id": "233",
        "name": "Shepherd's Bush Market"
    },
    {
        "lines": {
            "0": "Hammersmith & City",
            "1": "Circle"
        },
        "id": "233",
        "name": "Shepherd's Bush Market"
    }
]

MyObject

public class MyObject {
 public String id;

 public String name;

 public Map<String,String> lines;   
}

main method

    Gson gson = new Gson();
    Type type = new TypeToken<List<MyObject>>(){}.getType();
    List<MyObject > objList = gson.fromJson(str, type);


    assert(objList != null);  // validate not null

    for(MyObject obj : objList){

        System.out.println("id=" + obj.id + "; name=" + obj.name);
    }

Output:

id=233; name=Shepherd's Bush Market
id=233; name=Shepherd's Bush Market
id=233; name=Shepherd's Bush Market

in the loop you can extract Map as well

Answer 2

I like Maxim's solution for simplicity and +1 for him. Bur there is also other, little more complex way of doing it. In Lines class you can write keys as _0, _1

class Lines {
    private String _0;
    private String _1;
    //@Override
    //public String toString() {
    //    return "Lines [0=" + _0 + ", 1=" + _1 + "]";
    //}

}

and use it in MyObject like

class MyObject {

    private Lines lines;
    private String id;
    private String name;

    //@Override
    //public String toString() {
    //    return "Example [lines=" + lines + ", id=" + id + ", name=" + name + "]";
    //}
}

After that you will have to create FieldNamingStrategy that would remove _ from numeric key.

class MyNameStrategy implements FieldNamingStrategy {

    static Pattern numericPattern = Pattern.compile("_\\d+");

    @Override
    public String translateName(Field f) {
        if (numericPattern.matcher(f.getName()).matches()){
            return f.getName().substring(1);
        }else{
            return f.getName();
        }
    }
}

To use this strategy you need to create Gson via GsonBuilder

Gson gson = new GsonBuilder().setFieldNamingStrategy(
        new MyNameStrategy()).create();
//...
MyObject[] arr = gson.fromJson(jsonString, MyObject[].class);
System.out.println(Arrays.toString(arr));

---

Also you are right that your JSon have problem in "lines" : "". Since you are placing there object (data inside {...}) you can't change your format later to string (which is not object in JSon format).
So if you can change your JSon you should replace "lines" : "" with either

• "lines" : null
• or "lines" : {}.