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.data;
  
  import edu.cmu.tetrad.util.TetradSerializable;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  
  /**
   * Implements a knowledge edge X-->Y as a simple ordered pair of strings.
   *
   * @author Joseph Ramsey
   */
  public final class KnowledgeEdge implements TetradSerializable {
      static final long serialVersionUID = 23L;
  
      /**
       * @serial
       */
      private String from;
  
      /**
       * @serial
       */
      private String to;
  
      //===============================CONSTRUCTORS=======================//
  
      /**
       * Constructs a knowledge edge for from-->to.
       */
      public KnowledgeEdge(String from, String to) {
          if (from == null || to == null) {
              throw new NullPointerException();
          }
  
          this.from = from;
          this.to = to;
      }
  
      /**
       * Generates a simple exemplar of this class to test serialization.
       */
      public static KnowledgeEdge serializableInstance() {
          return new KnowledgeEdge("X", "Y");
      }
  
      //===============================PUBLIC METHODS======================//
  
      /**
       * @return the tail node of the edge.
       */
      public final String getFrom() {
          return from;
      }
  
      /**
       * @return the head node of the edge.
       */
      public final String getTo() {
          return to;
      }
  
      /**
       * Reteurns true if (from1, to1) == (from2, to2).
       */
      public final boolean equals(Object object) {
          if (object == null) {
              return false;
          }
  
          if (!(object instanceof KnowledgeEdge)) {
              return false;
          }
  
          KnowledgeEdge pair = (KnowledgeEdge) object;
          return from.equals(pair.from) && to.equals(pair.to);
      }
  
      /**
       * @return a good hashcode.
       */
      public final int hashCode() {
          int hashCode = 31 + from.hashCode();
          return 37 * hashCode + to.hashCode();
      }
  
      public String toString() {
          return from + "-->" + to;
      }
  
      /**
       * Adds semantic checks to the default deserialization method. This method
       * must have the standard signature for a readObject method, and the body of
       * the method must begin with "s.defaultReadObject();". Other than that, any
       * semantic checks can be specified and do not need to stay the same from
       * version to version. A readObject method of this form may be added to any
       * class, even if Tetrad sessions were previously saved out using a version
       * of the class that didn't include it. (That's what the
       * "s.defaultReadObject();" is for. See J. Bloch, Effective Java, for help.
       *
       * @throws java.io.IOException
       * @throws ClassNotFoundException
       */
      private void readObject(ObjectInputStream s)
              throws IOException, ClassNotFoundException {
          s.defaultReadObject();
  
          if (from == null) {
              throw new NullPointerException();
          }
  
          if (to == null) {
              throw new NullPointerException();
          }
      }
  }
  
  
  
  
  
  