How to use fromGson with arrays

Question

I have the json:

{"test": [{"param1": "param"}],
  "test2": "test",
 "test3": "test2"}

how can I get fromGson to parse the arrays in a json correctly so that it populates the arrays into an array of a particular object type.

So test should map to TestObject[] test;

test2 -> String test2;
test3 -> String test3;

I tried gson.fromJson(response, TestData.class); that gave me an error: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 664

( that the fromJson is not parsing the array in the json correctly).

I also tried going through this post: Parsing JSON array into java.util.List with Gson

But got confused.

but that did not work

Please update your feedback when other people have answered your question. — Ethan Fang, Jan 26 '14 at 03:16

Ethan Fang · Answer 1 · 2014-01-24T02:56:40.167

Answer

public class TestClass {
    public String param1;   
}

public class TestMap {
    public TestClass[] Test;
    public String test2;
    public String test3;
}

public Collection<TestMap> collectionFromJSON(String jsonString) {
    Gson gson = new Gson();
    Collection<TestMap> testMaps = gson.fromJson(jsonString, new TypeToken<Collection<TestMap>>() {
    }.getType());
    return testMaps;
}

public TestMap singleFromJSON(String jsonString) {
    Gson gson = new Gson();
    Journal testMap = gson.fromJson(jsonString, TestMap.class);
    return testMap;
}

Okay, I will explain something here.

Explain

Normally if you have a nested array of objects inside another object, you need to create a new class. Ie. if Journal has Categories, Category has two other properties, title and index.

The class will be like

class Journal {
    public Category[] categories;
    .... 
}

class Category {
    public String title;
    public Integer index;
    .... 
}

So now, the mistake is to confused Array of String and Array of Class. If the json is

{"Test", ["Test1", "Test2"...]
 "Test2": "Test3"}

You can define it as an array of String. But it is a nested JSON Object, so it has to be class again.

Sorry, I saw {"test": [{"param1": "param"}], as {"test": ["param1", "param"]. But now it is updated. — Ethan Fang, Jan 24 '14 at 02:11
As an aside, don't give answers away so easily, explain it to them and explain what they did wrong. — Sotirios Delimanolis, Jan 24 '14 at 02:19
Hehe, not too much to explain. It is the way that api is designed for handling the nested array and the experience over a dozen of failure I tried before. — Ethan Fang, Jan 24 '14 at 02:38

score 0 · Answer 2 · answered Jan 24 '14 at 01:57

0

Without seeing your class, I'm going to guess. This error

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 664

means you are trying to deserialize a String, so something like

"some string"

but what the JSON actually contained was a JSON object, something like

{"name" : "value"}

Your class is probably mapped wrong. It should be

String test2;
String test3;
TestObject[] test;

where TestObject should be

public class TestObject {
    String param1;
}

since the JSON contains an JSON string called param1.

answered Jan 24 '14 at 01:57

Sotirios Delimanolis

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should the parameters be listed in any particular order in the java class or just have the same name as the params in the json array? – user2491297 Jan 24 '14 at 01:58
@user2491297 The order doesn't matter. Gson will use reflection to retrieve them based on the names found in the JSON and the class you told it to deserialize to. – Sotirios Delimanolis Jan 24 '14 at 01:58

score -1 · Answer 3 · answered Jan 24 '14 at 01:53

-1

python has a very useful module called json

try var TestObject=json.parse()

answered Jan 24 '14 at 01:53

akiva

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Where did you get the idea that python was involved? – Sotirios Delimanolis Jan 24 '14 at 01:53

How to use fromGson with arrays

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