Change ArrayList from an Object to more specific type

Question

Is it possible to change the object type of an array list i.e. from an Object ArrayList to a specific object ArrayList. I have tried using a for each. Alternatively is there a way to change the filehandling method such that it can return a specific type depending on which file it reads from without duplicating code?

My Attempt:

ArrayList<Object> librarianList = FileHandling.getLibrarianRecords(fileName);
ArrayList<Librarian> libList = new ArrayList<>();
for (Object addType: librarianList) {
libList.add(addType);
}

getLibrarianRecords code

public static ArrayList<Object> getLibrarianRecords(String filename){
                ArrayList<Object> fromFile = new ArrayList<>(); //Array of
                // existing librarians

                try{
                        FileInputStream fIS =
                                new FileInputStream(SYSTEM_PATH + filename);
                        ObjectInputStream oIS = new ObjectInputStream(fIS);
                        fromFile = (ArrayList<Object>)oIS.readObject();
                } catch (IOException ex){
                        System.out.println("Failed to read from file " + ex.getMessage());
                        ex.printStackTrace(); //Catches an IO exception.
                } catch (ClassNotFoundException ex){
                        System.out.println("Error class not found" + ex.getMessage());
                        ex.printStackTrace(); //Catches a class not found
                        // exception.
                }

                return fromFile; //Returns the array list.
        }

Answer 1

Thanks to @user3170251 for suggesting casting

ArrayList<Object> librarianList = FileHandling.getLibrarianRecords(fileName);
ArrayList<Librarian> libList = new ArrayList<>();
for (Object addType: librarianList) {
libList.add((Librarian) addType);
}

For changing the type this does work.

Answer 2

You can use this generic class definition

class ArrayListConverter<T> {
  List<T> cast;
  public ArrayListConverter(List<T> typeList){
   cast = new ArrayList<>();
   for (Object addType: typeList) {
      cast.add((T)addType);
   }

  }
  public List<T> getList(){
     return cast;
  }

}

Just do this:

ArrayListConverter<Librarian> conv = new ArrayListConverter<>(libList);
ArrayList<Librarian> libListOg  = conv.getList();

Answer 3

It is rarely a good idea to read objects from a file like this. That said all you really need to do is to cast the result of oIS.readObject() to an ArrayList<Librarian> instead of carrying it to ArrayList<Object> (as you do now) and then amend the return type of getLibrarianRecords. Oh, and naturally also the type of the local variable fromFile.

    public static ArrayList<Librarian> getLibrarianRecords(String filename){
            ArrayList<Librarian> fromFile = new ArrayList<>(); //Array of
            // existing librarians

            try{
                    FileInputStream fIS =
                            new FileInputStream(SYSTEM_PATH + filename);
                    ObjectInputStream oIS = new ObjectInputStream(fIS);
                    fromFile = (ArrayList<Librarian>)oIS.readObject();
            } catch (IOException ex){
                    System.out.println("Failed to read from file " + ex.getMessage());
                    ex.printStackTrace(); //Catches an IO exception.
            } catch (ClassNotFoundException ex){
                    System.out.println("Error class not found" + ex.getMessage());
                    ex.printStackTrace(); //Catches a class not found
                    // exception.
            }

            return fromFile; //Returns the array list.
    }

There should then be no need to loop over the list to actually do the type conversion on an element by element basis.