7.1

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();

    for (String s : this)
      result.add(s);

    if (that != null)
      for (String s : that)
        result.add(s);

    return result;
  }

  /**
   * @return the intersection of the elements of this and that
   */
  public MySet intersection(MySet that)
  {
    MySet result = new MySet();

    if (that != null)
      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)
      for (String s : this)
      {
        if(!that.contains(s))
          result.add(s);
      }

    else
      for (String s : this)
        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)
    {
      for (String s : this)
        if(!that.contains(s))
          result.add(s);

      for (String s : that)
        if(!this.contains(s))
          result.add(s);
    }

    else
      for (String s : this)
        result.add(s);

    return result;
  }

  /**
   * @return a String representation of a MySet object
   */
  public String toString()
  {
    String result = "<MySet{";

    for (String s : this)
      result += (s + ",");

    result = result.substring(0, result.length()-1);
    return result + "}> ";
  }
}