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 java.io.ObjectStreamException;
  
  /**
   * A typesafe enumeration of the types of endpoints that are permitted in
   * Tetrad-style graphs: null (-), arrow (->), and circle (-o).
   *
   * @author Joseph Ramsey
   */
  public final class Endpoint implements TetradSerializable {
      static final long serialVersionUID = 23L;
  
      public static final Endpoint TAIL = new Endpoint("Tail");
      public static final Endpoint ARROW = new Endpoint("Arrow");
      public static final Endpoint CIRCLE = new Endpoint("Circle");
      private static final Endpoint STAR = new Endpoint("Star");
      public static final Endpoint NULL = new Endpoint("Null");
  
      /**
       * The name of this type.
       */
      private final transient String name;
  
      /**
       * Protected constructor for the types; this allows for extension in case
       * anyone wants to add formula types.
       */
      private Endpoint(String name) {
          this.name = name;
      }
  
      /**
       * Generates a simple exemplar of this class to test serialization.
       */
      public static Endpoint serializableInstance() {
          return Endpoint.TAIL;
      }
  
      /**
       * Prints out the name of the type.
       */
      public String toString() {
          return name;
      }
  
      // Declarations required for serialization.
      private static int nextOrdinal = 0;
      private final int ordinal = nextOrdinal++;
      public static final Endpoint[] TYPES = {TAIL, ARROW, CIRCLE, STAR, NULL};
  
      Object readResolve() throws ObjectStreamException {
          return TYPES[ordinal]; // Canonicalize.
      }
  }
  
  
  
  
  
  