How to make the method universal using generics

Question

I'm trying to make the import From JSON method universal for working with different classes. But I'm confused about these generics.

private <T> List<T> importFromJSON(DataItems<T> dataItems) { 
  try(FileInputStream fileInputStream = context.openFileInput(fileName);
      InputStreamReader streamReader = new InputStreamReader(fileInputStream)){
  Gson gson = new Gson(); 
  dataItems = gson.fromJson(streamReader, DataItems<>.class); 
  return  dataItems.getList(); 
 } 
 catch (IOException ex){ 
  ex.printStackTrace(); 
 } 
 return null; 
} 

public class DataItems<E> { 
 
    private List<E> list; 
 
    public List<E> getList() { 
        return list; 
    } 
 
    public void setList(List<E> list) { 
        this.list = list; 
    } 
} 


public class House { 
    int number; 
    String name; 
}

json file

{"list":[{"name":"01","id":1},{"name":"02","id":2}]}

I call this method already with a specific type

DataItems<House> dataItems = new DataItems<>(); 
importFromJSON(dataItems);

Everything is readable, as a result of executing import From JSON, I get a list of objects, but only instead of the int type, for some reason there is a Double type And when trying to pull an object from this list

House house = (House) list.get(0);

получаю Exception:

java.lang.ClassCastException LinkedTreeMap cannot be cast to House

https://stackoverflow.com/questions/18397342/deserializing-generic-types-with-gson — luk2302, Sep 25 '22 at 17:37
The thing is that generics in Java are there to provide extended static time type checking. At runtime, generics are erased. So if I understand your question correctly, it is not possible to do what you are trying to do with generics in Java — ControlAltDel, Sep 25 '22 at 17:49
Instead, I recommend using the factory pattern where you can add type handlers (ie handler registers for handling of one type of message through some form of identification) — ControlAltDel, Sep 25 '22 at 17:52
Does this answer your question? [Deserializing Generic Types with GSON](https://stackoverflow.com/questions/18397342/deserializing-generic-types-with-gson) — knittl, Oct 19 '22 at 06:50

How to make the method universal using generics

0 Answers0