buferredReader

public void otworz(){

        FileChooser chooser = new FileChooser();
        this.file = chooser.showOpenDialog(poleTekstowe.getScene().getWindow());

        if(file != null) {
            try(BufferedReader re = new BufferedReader(new FileReader(this.file.getAbsolutePath()))) {

                String linia;

                while(true){
                    linia = re.readLine();
                    if(linia == null){
                        break;
                    }
                    poleTekstowe.setText(linia);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
   }