Converting string to JSONArray, and then to list

Question

I have String "[{...}]" and I want convert it to JSONArray, and then to List list = new List I was trying this solution, and this is my code

public void RestoreData()
{
    String toConvert = "[{...}]" // I don't place full String, but it's typical JSONArray, but in String
    ArrayList<myClass> listdata = new ArrayList<myClass>();
    JsonObject jsonObject = new JsonObject();
    JSONArray jArray = (JSONArray)jsonObject;
    if (jArray != null) {
        for (int i=0;i<jArray.length();i++){
            listdata.add(jArray.getString(i));
        }
    }
}

And when I'm trying to compile this I get 2 errors:

In JSONArray jArray = (JSONArray)jsonObject; I get error 'Incovertible types; cannot cast 'org.json.JSONObject' to 'org.json.JSONArray'.
And second: in listdata.add(jArray.getString(i)); I get error 'unhandled exception org.json.JSONException'.

I'm new in Java and I work with Json for the first time.

EDIT A small truncated example of the Json string:

[{"lootArmorGain":0,"lootBlockGain":0,"lootCost":93500,"lootCritGain":2,"lootCritPowerGain":0,"lootDamageAbsorptionGain":0,"lootDamageGain":0, }]

yes some part of string so that we can check it, if it is valid data or not. — Alok, May 05 '18 at 17:41
Please give a small (truncated) example of the string you say is a "... typical JSONArray". — Barns, May 05 '18 at 17:48
Now we need to know what `myClass` is. By-the-way in java naming convention methods do not use capitalization... however, Classes are capitalized. — Barns, May 05 '18 at 17:51
I made a mistake. It's Object Array, and this is fragment of this String:[{"lootArmorGain":0,"lootBlockGain":0,"lootCost":93500,"lootCritGain":2,"lootCritPowerGain":0,"lootDamageAbsorptionGain":0,"lootDamageGain":0, }] — Hubizonek, May 05 '18 at 17:51
myClass is a class created by me: public class myClass() { public myClass(....) { /*code */ } /* code */ } — Hubizonek, May 05 '18 at 17:53

Levi Moreira · Accepted Answer · 2018-05-05T18:29:23.843

3

I think what you want is:

public void RestoreData()
{
   try{
    String toConvert = "[{...}]" // I don't place full String, but it's typical JSONArray, but in String
    ArrayList<MyClass> listdata = new ArrayList<MyClass>();
    JSONArray jsonArray = new JSONArray(toConvert); 

    if (jsonArray != null) {
       Gson gson = new Gson();
        for (int i=0;i<jsonArray.length();i++){
            String json = jsonArray.getJSONObject(i).toString();
            MyClass obj = gson.fromJson(json, MyClass.class);
            listdata.add(obj);
        }
    }
    }catch(JSONException e){
          e.printStackTrace();
    }
}

To convert from JSONOject to your custom class use GSON, see above.

 compile 'com.google.code.gson:gson:2.8.4'

Note that your custom class need to have an empty constructor as well as getters and setters in order to make gson work.

edited May 05 '18 at 18:29

answered May 05 '18 at 17:39

Levi Moreira

11,917
4
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i got again "unhandled exception org.json.jsonexception". on this code "JSONArray(toConvert)" – Hubizonek May 05 '18 at 17:44
it means your String is probably a not valid json array as you said – Levi Moreira May 05 '18 at 17:45
Ohhh yea, sorry, i made mistake.. look at my question now. I want to conver to ArrayList – Hubizonek May 05 '18 at 17:48
Now my code handles the exception, can you run it and tell us what the stack trace tells you the JSONExeption message? – Levi Moreira May 05 '18 at 17:51
I can run this, and it working, but now, how to convert this JSONArray to ArrayList ? – Hubizonek May 05 '18 at 18:00
I see, take a look at the edit, you'll need to use Gson – Levi Moreira May 05 '18 at 18:03
'cannot resolve method 'fromJson(org.json.JSONObject'' on 'jsonArray.getJSONObject(i)', and '';'expected' on point between MyClass and class, and after class on ')' 'Indentifier expected' – Hubizonek May 05 '18 at 18:12
instructions in for was made, but size of listdata = 0 :/ (btw i didn't vote down u xD) – Hubizonek May 05 '18 at 18:23
1

No problem, some people don't understand that if they downvote someone it is polite to explain why :/ – Levi Moreira May 05 '18 at 18:25
Could your post MyClass? It needs to have an empty constructor and getters and setters in order to Gson to work properly – Levi Moreira May 05 '18 at 18:28
Great :) Glad I could help you :) – Levi Moreira May 05 '18 at 18:30
1

I agree @LeviAlbuquerque. I just don't understand people down voting valid answers without at least saying why it was down voted. Maybe that person will eventually get up some courage to explain themselves! An Up vote to compensate. – Barns May 05 '18 at 18:32

score 1 · Answer 2 · answered May 05 '18 at 18:17

I think best approach will be using Google Gson Library.

String toConvert = "[{...}]"
Type listType = new TypeToken<List<myClass>>() {}.getType();
List<myClass> yourList = new Gson().fromJson(toConvert, listType);

You dont need to get each position manually.

score 0 · Answer 3 · edited May 05 '18 at 17:50

0

For simplicity you can also use Jackson api

ObjectMapper objectMapper = new ObjectMapper();
TypeFactory typeFactory = objectMapper.getTypeFactory();
List<SomeClass> someClassList = objectMapper.readValue(jsonString,typeFactory.constructCollectionType(List.class, SomeClass.class));

edited May 05 '18 at 17:50

Levi Moreira

11,917
4
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answered May 05 '18 at 17:47

Naveen

830
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19

How to use this? I must write something build.gradle? what? – Hubizonek May 05 '18 at 18:02
you must add dependency of jackson api in your build.gralde ( just like another dependency and use the above code ) – Naveen May 05 '18 at 18:03

Converting string to JSONArray, and then to list

3 Answers3