Java string to object's List

Question

How is it possible to get List object from String, add new element to list and turn it back to string. I use Json and jackson, created class for list's element:

public class WeightDataJson {
        private String weightValue;
        private String dateValue;
        public WeightDataJson(String weightValue,String dateValue){
            this.weightValue=weightValue;
            this.dateValue=dateValue;
        }
        public String getWeightValue() {
            return weightValue;
        }
        public void setWeightValue(String weightValue) {
            this.weightValue = weightValue;
        }
        public String getDateValue() {
            return dateValue;
        }
        public void setDateValue(String dateValue) {
            this.dateValue = dateValue;
        }
    }

and my code for change List and String:

                String myWeightData = preferencesWeight.get();
                List<WeightDataJson> myList = new Vector<WeightDataJson>();
                ObjectMapper jsonMapper = new ObjectMapper();
                try {
                    myList = jsonMapper.readValue(myWeightData, new TypeReference<List<WeightDataJson>>(){});
                } catch (IOException e) {
                    e.printStackTrace();
                }

                WeightDataJson newWeightData = new WeightDataJson(editText.getText().toString(),"213123");
                myList.add(newWeightData);

                JSONObject myJson=new JSONObject();
                for(int i=0;i<myList.size();i++){
                    try {
                        myJson.put("weightValue",myList.get(i).getWeightValue());
                        myJson.put("dateValue",myList.get(i).getDateValue());
                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }

                String toSave ="["+myJson.toString()+"]";
                Toast.makeText(getActivity(),"zapisuje: "+toSave,Toast.LENGTH_LONG).show();
                preferencesWeight.save(toSave);
                WeightDataProvider weightDataProvider = new WeightDataProvider(addIcon,deleteIcon,editText.getText().toString(),"213123");
                weightAdapter.add(weightDataProvider);

myWeightData is 'basic' String (this is possible that it is empty), I am not sure but probably there is problem because String for jakcson has to be like [{element1},{element2}] and Json object gives {element1},{element2} so I put there String toSave ="["+myJson.toString()+"]".

If basic String is [{weightValue":"50","dateValue":"34234"}] I have an error in line myList=jsonmapper.readValue~~:

07-30 03:59:04.407 11837-11837/com.plan.aplikacjamobilna W/System.err: com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type [simple type, class com.plan.aplikacjamobilna.WeightDataJson]: can not instantiate from JSON object (missing default constructor or creator, or perhaps need to add/enable type information?)
07-30 03:59:04.407 11837-11837/com.plan.aplikacjamobilna W/System.err:     at [Source: [{"weightValue":"14","dateValue":"213123"}]; line: 1, column: 3] (through reference chain: java.util.ArrayList[0])

Is there another way to solve my problem?

Answer 1

If you want store data as String Why are you complicating things and involving other unnecessary classes and object of those ?

1. override method toString() in your class

   a) chose data(variable) delimiter1 ":"

   b) concatenate data for Class records (variables) using delimiter1

2. make constructor with String parameter to:

   a) split concatenated string with previous used delimiter1

   b) assign result [] of split to declared variables

3. make static methods to:

   a) List<> as parameter to concatenate each object toString() method produced strings with delimiter2 "%" to achieve something like this:

    "var1o1:var2o1%var1o2:var2o2%var1o3:var2o3" etc
   

   b) to split above string with:

   b.a/ delimieter2  - > then result each String pass to object to split with
   
   b.b/ delimieter1 -> then result [] of split assign to variables 
   

Answer 2

You can use Gson to switch between your type and String. Usage:

build.gradle

compile 'com.google.code.gson:gson:2.7'

From POJO to String:

Gson gson = new Gson();
String json = gson.toJson(weightData);

From String to POJO:

WeightDataJson newWeightData = gson.fromJson(json, WeightDataJson.class);

EDIT :

With ArrayList

List<WeightDataJson> list = new ArrayList<WeightDataJson>();
list.add(obj1);
list.add(obj2);
list.add(obj3);
..
Type type = new TypeToken<List<WeightDataJson>>() {}.getType();
String json = gson.toJson(list, type);
List<WeightDataJson> newWeightData = gson.fromJson(json, type);