public class CategoriaPersist {

    private static CategoriaPersist catpersist = null;
    private ArrayList <Categoria> arrayCategorias;
    private Context contexto;
    private static final String file_name = "categorias";

    private Gson gson;

    public CategoriaPersist(Context contexto) {

        gson = new GsonBuilder().create();
        arrayCategorias = new ArrayList<>();
        loadCategory(contexto);
        this.contexto = contexto;
    }

    //Singleton - para que se pueda instanciar solo una vez
    public static CategoriaPersist instanceOf(Context contexto){

        if(catpersist == null){

            catpersist = new CategoriaPersist(contexto);
        }
    return catpersist;
    }

    public void setContext(Context contexto){

        this.contexto = contexto;
    }

    public void addCategory(Categoria cat, Context contexto){

        arrayCategorias.add(cat);
        persist(contexto);
    }

    public void delCategory(Categoria cat){

        arrayCategorias.remove(cat);
        persist(contexto);
    }

    public void persist(Context contexto){

        String categories = gson.toJson(arrayCategorias);
        File file = new File(contexto.getFilesDir(), file_name);
        FileOutputStream outputStream;
        try {
            outputStream = new FileOutputStream(file);
            outputStream.write(categories.getBytes());
            outputStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void loadCategory(Context contexto){

        File f = new File(contexto.getFilesDir(), file_name);

        if(!f.exists()){
            try{
                f.createNewFile();
            } catch (IOException ioe){

            }
        }
        try{
            FileInputStream fis = contexto.openFileInput(file_name);
            InputStreamReader isr = new InputStreamReader(fis);
            BufferedReader bufferedReader = new BufferedReader(isr);
            StringBuilder sb = new StringBuilder();
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                sb.append(line);
            }

            if(sb.toString().equals("")){
                return;
            }

            arrayCategorias = gson.fromJson(sb.toString(),
                    new TypeToken<ArrayList<Categoria>>(){}.getType());

        }catch(FileNotFoundException e){
            e.printStackTrace();
        }catch(IOException e){
            e.printStackTrace();
        }
    }

    public ArrayList <Categoria> getCategorias(){

        return arrayCategorias;
    }
}