set operations

import java.util.*;

class MySet extends HashSet<String> {
  private static final long serialVersionUID = 1L;

  public MySet() {
    super();
  }

  /**
   * @return the union of the elements of this and that
   */
  public MySet union(MySet that) {
    MySet result = new MySet();
    result.addAll(this);
    if (that == null || that.size() < 1) {
      return result;
    }
    result.addAll(that);
    return result;
  }

  /**
   * @return the intersection of the elements of this and that
   */
  public MySet intersection(MySet that) {
    MySet result = new MySet();
    if (that == null || that.size() < 1) {
      return result;
    }
    for (String s : this) {
      if (that.contains(s)) {
        result.add(s);
      }
    }
    return result;
  }

  /**
   * @return the difference of the elements of this and that
   */
  public MySet difference(MySet that) {
    MySet result = new MySet();
    if (that == null || that.size() < 1) {
      result.addAll(this);
      return result;
    }
    for(String s : this) {
      if (!that.contains(s)) {
        result.add(s);
      }
    }
    return result;
  }

  /**
   * @return the exclusive or (XOR) of the elements of this and that
   */
  public MySet exclusiveOr(MySet that) {
    MySet result = new MySet();
    if (that == null || that.size() < 1) {
      result.addAll(this);
      return result;
    }
    MySet intersection = this.intersection(that);
    MySet union = this.union(that);
    for (String s : union) {
      if(!intersection.contains(s)) {
        result.add(s);
      }
    }
    return result;
  }

  /**
   * @return a String representation of a MySet object
   */
  public String toString() {
    String res = "<MySet{";
    for (String s : this) {
      res += s + ",";
    }
    res = res.substring(0, res.length() - 1);
    res += "}>";
    return res;
  }
}