save JSON data in a file and retrieve it

gson.java

Gson gson = new Gson();
Type collectionType = new TypeToken<ArrayList<PersonContainer>>() {}.getType();
ArrayList<PersonContainer> Persons = gson.fromJson(new FileReader("input.json"), collectionType);
System.out.println(gson.toJson(persons));



class PersonContainer
{
  Person person;
}

class Person
{
  public int id;
  public String name;
}

main.java

private String read(Context context, String fileName) {
    try {
        FileInputStream fis = context.openFileInput(fileName);
        InputStreamReader isr = new InputStreamReader(fis);
        BufferedReader bufferedReader = new BufferedReader(isr);
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = bufferedReader.readLine()) != null) {
            sb.append(line);
        }
        return sb.toString();
    } catch (FileNotFoundException fileNotFound) {
        return null;
    } catch (IOException ioException) {
        return null;
    }
}

private boolean create(Context context, String fileName, String jsonString){
    String FILENAME = "storage.json";
    try {
        FileOutputStream fos = context.openFileOutput(fileName,Context.MODE_PRIVATE);
        if (jsonString != null) {
            fos.write(jsonString.getBytes());
        }
        fos.close();
        return true;
    } catch (FileNotFoundException fileNotFound) {
        return false;
    } catch (IOException ioException) {
        return false;
    }

}

public boolean isFilePresent(Context context, String fileName) {
    String path = context.getFilesDir().getAbsolutePath() + "/" + fileName;
    File file = new File(path);
    return file.exists();
}


// onCreate of the Activity, you can use do the following
boolean isFilePresent = isFilePresent(getActivity(), "storage.json");
if(isFilePresent) {
   String jsonString = read(getActivity(), "storage.json");
   // do the json parsing here and do the rest of functionality of app
} else {
   boolean isFileCreated = create(getActivity, "storage.json", "{}");
   if(isFileCreated) {
     //proceed with storing the first todo  or show ui
   } else {
     //show error or try again.
   }
}