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.gene.tetrad.gene.history;
  
  import edu.cmu.tetrad.util.TetradSerializable;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  
  /**
   * Holds an ordered pair (index, lag) to represent a causal parent of a factor,
   * where the factor at the given index is independently known.
   *
   * @author Joseph Ramsey jdramsey@andrew.cmu.edu
   */
  public final class IndexedParent implements TetradSerializable {
      static final long serialVersionUID = 23L;
  
      /**
       * The index of the parent.
       *
       * @serial
       */
      private final int index;
  
      /**
       * The lag of the parent.
       *
       * @serial
       */
      private final int lag;
  
      //================================CONSTRUCTORS========================//
  
      /**
       * Constructs a new index parent.
       */
      public IndexedParent(int index, int lag) {
  
          if (index < 0) {
              throw new IllegalArgumentException("Index must be >= 0: " + index);
          }
  
          if (lag < 0) {
              throw new IllegalArgumentException("Lag must be >= 0: " + lag);
          }
  
          this.index = index;
          this.lag = lag;
      }
  
      /**
       * Generates a simple exemplar of this class to test serialization.
       *
       * @see edu.cmu.TestSerialization
       * @see edu.cmu.tetradapp.util.TetradSerializableUtils
       */
      public static IndexedParent serializableInstance() {
          return new IndexedParent(0, 1);
      }
  
      //===============================PUBLIC METHODS======================//
  
      /**
       * Returns the index of the parent.
       */
      public int getIndex() {
          return this.index;
      }
  
      /**
       * Returns the lag of the parent.
       */
      public int getLag() {
          return this.lag;
      }
  
      /**
       * Returns true iff the lags and indices are equal.
       */
      public boolean equals(Object o) {
          if (o == this) {
              return true;
          }
          if (!(o instanceof IndexedParent)) {
              return false;
          }
          IndexedParent c = (IndexedParent) o;
          return c.getIndex() == this.getIndex() && c.getLag() == this.getLag();
      }
  
      /**
       * Prints out the factor index and lag.
       */
      public String toString() {
          return "IndexedParent, index = " + getIndex() + ", lag = " + getLag();
      }
  
      /**
       * 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 (index < 0) {
              throw new IllegalStateException();
          }
  
          if (lag < 0) {
              throw new IllegalStateException();
          }
  
      }
  }
  
  
  
  
  
  