code: ///////////////////////////////////////////////////////////////////////////////
  // For information as to what this class does, see the Javadoc, below.       //
  // Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,       //
  // 2007, 2008, 2009, 2010, 2014, 2015 by Peter Spirtes, Richard Scheines, Joseph   //
  // Ramsey, and Clark Glymour.                                                //
  //                                                                           //
  // This program is free software; you can redistribute it and/or modify      //
  // it under the terms of the GNU General Public License as published by      //
  // the Free Software Foundation; either version 2 of the License, or         //
  // (at your option) any later version.                                       //
  //                                                                           //
  // This program is distributed in the hope that it will be useful,           //
  // but WITHOUT ANY WARRANTY; without even the implied warranty of            //
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the             //
  // GNU General Public License for more details.                              //
  //                                                                           //
  // You should have received a copy of the GNU General Public License         //
  // along with this program; if not, write to the Free Software               //
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA //
  ///////////////////////////////////////////////////////////////////////////////
  
  package edu.cmu.tetrad.graph;
  
  import edu.cmu.tetrad.util.TetradSerializable;
  import edu.cmu.tetrad.util.TetradSerializableExcluded;
  
  /**
   * An ordered pair of objects. This does not serialize well, unfortunately.
   *
   * @author Tyler Gibson
   */
  public class OrderedPair<E> implements TetradSerializable, TetradSerializableExcluded {
      static final long serialVersionUID = 23L;
  
      /**
       * The "First" node.
       */
      private E first;
  
  
      /**
       * The "second" node.
       */
      private E second;
  
  
      public OrderedPair(E first, E second) {
          if (first == null) {
              throw new NullPointerException("First node must not be null.");
          }
          if (second == null) {
              throw new NullPointerException("Second node must not be null.");
          }
          this.first = first;
          this.second = second;
      }
  
      //============================== Public methods =============================//
  
      public E getFirst() {
          return this.first;
      }
  
      public E getSecond() {
          return this.second;
      }
  
      public int hashCode() {
          return 13 * this.first.hashCode() + 67 * this.second.hashCode();
      }
  
      public boolean equals(Object o) {
          if (!(o instanceof OrderedPair)) throw new IllegalArgumentException();
          OrderedPair<E> that = (OrderedPair<E>) o;
          return this.first.equals(that.first) && this.second.equals(that.second);
      }
  
      public String toString() {
          return "<" + this.first + ", " + this.second + ">";
      }
  
  }
  
  
  
  