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;
  
  /**
   * An Expression for a variable.
   *
   * @author Tyler Gibson
   */
  public class VariableExpression implements Expression {
      static final long serialVersionUID = 23L;
  
      /**
       * The variable that is being evaluated.
       */
      private String variable;
  
  
      public VariableExpression(String variable) {
          if (variable == null) {
              throw new NullPointerException("variable is null.");
          }
          this.variable = variable;
      }
  
      public static VariableExpression serializableInstance() {
          return new VariableExpression("a");
      }
  
      //======================== Public methods ===================//
  
      /**
       * @return the variable.
       */
      public String getVariable() {
          return this.variable;
      }
  
  
      public Double evaluateGeneric(Context context) {
          return context.getValue(variable);
      }
  
  
      public double evaluate(Context context) {
          Double value = context.getValue(variable);
  
          if (value == null) {
              throw new IllegalArgumentException(variable + " was not assigned a value.");
          }
  
          return value;
      }
  
      public String getToken() {
          return "";
      }
  
      public ExpressionDescriptor.Position getPosition() {
          return ExpressionDescriptor.Position.NEITHER;
      }
  
  
      public List<Expression> getExpressions() {
          return Collections.emptyList();
      }
  
      public String toString() {
          return variable;
      }
  
      @Override
      public RealDistribution getRealDistribution(Context context) {
          return null;
      }
  
      public IntegerDistribution getIntegerDistribution(Context context) {
          return null;
      }
  }
  
  
  
  
  