Set :: binary operations

Everybody should have these. It's just a drag to rewrite it time and time again.

public class SetUtil
{
   public static <T> Set<T> and(Set<T> a, Set<T> b)
   {
      Set<T> c = new HashSet<T>();
      // c.addAll(SetUtil.or(a, b));
      // c.removeAll(SetUtil.xor(a, b));
      c.addAll(a);
      c.retainAll(b);
      return c;
   }

   public static <T> Set<T> or(Set<T> a, Set<T> b)
   {
      Set<T> c = new HashSet<T>();
      c.addAll(a);
      c.addAll(b);
      return c;
   }

   public static <T> Set<T> xor(Set<T> a, Set<T> b)
   {
      Set<T> a_minus_b = new HashSet<T>();
      a_minus_b.addAll(a);
      a_minus_b.removeAll(b);

      Set<T> b_minus_a = new HashSet<T>();
      b_minus_a.addAll(b);
      b_minus_a.removeAll(a);

      Set<T> c = new HashSet<T>();
      c.addAll(a_minus_b);
      c.addAll(b_minus_a);
      return c;
   }
}