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.ObjectStreamException;
  
  /**
   * Type-safe enum of discrete variable types. A nominal discrete variable is one
   * in which the categories are in no particular order. An ordinal discrete
   * variable is one in which the categories in a particular order. An indexical
   * discrete variable is an ordinal discrete variable for which relative
   * distances between categories can be specified.
   *
   * @author Joseph Ramsey
   */
  public final class DiscreteVariableType implements TetradSerializable {
      static final long serialVersionUID = 23L;
  
      public static final DiscreteVariableType NOMINAL =
              new DiscreteVariableType("Nominal");
      private static final DiscreteVariableType ORDINAL =
              new DiscreteVariableType("Ordinal");
  
      /**
       * The name of this type.
       */
      private transient final String name;
  
      /**
       * Protected constructor for the types; this allows for extension in case
       * anyone wants to add formula types.
       */
      private DiscreteVariableType(String name) {
          this.name = name;
      }
  
      /**
       * Generates a simple exemplar of this class to test serialization.
       */
      public static DiscreteVariableType serializableInstance() {
          return DiscreteVariableType.NOMINAL;
      }
  
      /**
       * Prints out the name of the type.
       */
      public final String toString() {
          return name;
      }
  
      // Declarations required for serialization.
      private static int nextOrdinal = 0;
      private final int ordinal = nextOrdinal++;
      private static final DiscreteVariableType[] TYPES = {NOMINAL, ORDINAL};
  
      final Object readResolve() throws ObjectStreamException {
          return TYPES[ordinal]; // Canonicalize.
      }
  }
  
  
  
  
  
  