JSON Array With GSON Throws Error "Expected BEGIN_OBJECT but was STRING"

Question

I am trying to convert a JSON array to a Java object, but I am having problems understanding how to use GSON.

The JSON array looks like this:

"[
  {
    "category": "1",
    "checks": [
      {
        "check": "1.1",
        "penaltypoints": "1.1",
        "keypoint": "1.1"
      },
      {
        "check": "1.2",
        "penaltypoints": "1.2",
        "keypoint": "1.2"
      }
    ]
  },
  {
    "category": "2",
    "checks": [
      {
        "check": "2.1",
        "penaltypoints": "2.1",
        "keypoint": "2.1"
      },
      {
        "check": "2.2",
        "penaltypoints": "2.2",
        "keypoint": "2.2"
      }
    ]
  }
]"

My corresponding Java classes are:

class Category {
    public String description;
    public List<Check> checks;
}

class Check {
    public String description;
    public float penaltyPoints;
    public KeyPoint keypoint;
}

class KeyPoint {
    public String description;
}

And this is how I called GSON:

Gson gson = new Gson();         
Category categoriesArray[] = gson.fromJson(jsonString, Category[].class);

At the moment it is throwing up the following error:

Expected BEGIN_OBJECT but was STRING at line 1 column 125

I am new to GSON and am having problems understanding how it works. Can anyone please help me understand what I am doing wrong?

Sotirios Delimanolis · Accepted Answer · 2014-06-11 16:10:18Z

3

You're expecting this

    "keypoint": "2.1"

to be mapped to

public KeyPoint keypoint;
...
class KeyPoint {
    public String description;
}

In Java-JSON conversions, a POJO is meant to map to a JSON object and vice versa. Here, you're trying to map a JSON String to a POJO. That won't work by by default.

Either write and register your own TypeAdapter with Gson that will do this conversion or change your JSON to

"keypoint" : {
     "description" : "2.1"
}

answered Jun 11, 2014 at 16:10

Sotirios Delimanolis

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1 Comment

codefi Over a year ago

Thanks, it seems that was the problem. I had it mapped differently in my mind! I will accept your answer when the timer lets me :).

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