A classe User deve ter pelo menos a seguinte funcionalidade:

package xxl.core;

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;

public class User implements java.io.Serializable {
  private final String _name;
  private List<Spreadsheet> _spreadsheets = new ArrayList<>();

  public User(String name) {
    _name = name;
  }

  void add(Spreadsheet s) {
    _spreadsheets.add(s);
  }

  public List<Spreadsheet> getSpreadSheets() {
    return Collections.unmodifiableList(_spreadsheets);
  }
  
  public final String getName() {
    return _name;
  }
}