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.calculator.expression;
  
  import org.apache.commons.math3.distribution.IntegerDistribution;
  import org.apache.commons.math3.distribution.RealDistribution;
  
  import java.util.Collections;
  import java.util.List;
  
  /**
   * Represents a constant expression, that is an expression that always evaluates to the same value.
   *
   * @author Tyler Gibson
   */
  public class ConstantExpression implements Expression {
      static final long serialVersionUID = 23L;
  
      /**
       * Constant expression for PI.
       */
      public static final ConstantExpression PI = new ConstantExpression(Math.PI, "PI");//"\u03C0");
  
  
      /**
       * Constant expression for e.
       */
      public static final ConstantExpression E = new ConstantExpression(Math.E, "E");// "e");
  
      /**
       * THe value of the expression.
       */
      private double value;
  
  
      /**
       * The name of the constant or null if there isn't one. Constants with names are things
       * like PI or E.
       */
      private String name;
  
  
      /**
       * Constructs the constant expression given the value to use.
       */
      public ConstantExpression(double value) {
          this.value = value;
      }
  
  
      /**
       * Constructs the constant expression given the value and the name.
       */
      private ConstantExpression(double value, String name) {
          if (name == null) {
              throw new NullPointerException("name was null.");
          }
          this.value = value;
          this.name = name;
      }
  
      public static ConstantExpression serializableInstance() {
          return new ConstantExpression(1.2);
      }
  
      //========================== Public Methods ===============================//
  
  
      /**
       * @return the name of the constant or null if there isn't one.
       */
      public String getName() {
          return this.name;
      }
  
      /**
       * @return the constant value.
       */
      public double evaluate(Context context) {
          return this.value;
      }
  
      public String getToken() {
          return "";
      }
  
      public ExpressionDescriptor.Position getPosition() {
          return ExpressionDescriptor.Position.NEITHER;
      }
  
      public List<Expression> getExpressions() {
          return Collections.emptyList();
      }
  
      public String toString() {
          if (name == null) {
              return Double.toString(value);
          } else {
              return name;
          }
      }
  
      @Override
      public RealDistribution getRealDistribution(Context context) {
          return null;
      }
  
      public IntegerDistribution getIntegerDistribution(Context context) {
          return null;
      }
  }
  
  
  
  
  