//CS5-HMC-Sp.,00-Lab 12
//A. Khakpour, May, 00

import java.awt.*;
import java.applet.*;
import java.awt.event.*;
import java.util.*;

//==================================================================
//  This applet accepts a postfix expression from the user and evaluates
//  it.  Note that little error checking is performed. The applet 
//  may terminate if the expression is not in valid postfix form.
//==================================================================
public class Postfix4 extends Applet
{
   private PostfixGUI gui;

   //------------------------------------------------------
   //  Sets up the applet.
   //------------------------------------------------------
   public void init ()
   {
     gui = new PostfixGUI (this); 
   }  
}  

//==================================================================
//  The GUI presents a field into which the user types a postfix
//  expression, and an evaluate button to process it.
//==================================================================
class PostfixGUI
{
   private static Label instructions; 
   private static Label expressionTitle;
   private static Label resultTitle; 
   private static Button evaluateButton;
   private static Button clearButton;
   private static Label result;
   private static TextField expression;

   //------------------------------------------------------
   //  Sets up the GUI.
   //------------------------------------------------------
   public PostfixGUI (Applet app)
   {
      instructions = new Label ("Enter a postfix expression," +
      " then press enter or click the Evaluate button.", Label.CENTER);

      expressionTitle = new Label ("Expression:");
      resultTitle = new Label ("Result:");

      evaluateButton = new Button ("Evaluate");
      evaluateButton.addActionListener (new EvaluateActionListener());

      clearButton = new Button ("Clear");
      clearButton.addActionListener (new ClearActionListener());

      result = new Label ("__________");

      expression = new TextField (40);
      expression.addActionListener (new EvaluateActionListener());

      app.add (instructions);
      app.add (expressionTitle);
      app.add (expression);
      app.add (resultTitle);
      app.add (result);
      app.add (evaluateButton);
      app.add (clearButton);

      app.setSize (425, 150);
   }  
   //------------------------------------------------------
   //  Clears the text field and result label.
   //------------------------------------------------------
   public static void clearFields ()
   {
      expression.setText ("");
      result.setText ("__________");
   }
   //------------------------------------------------------
   //  Returns the expression typed by the user.
   //------------------------------------------------------
   public static String getExpression ()
   {
      return expression.getText();
   }  
   //------------------------------------------------------
   //  Sets the result label.
   //------------------------------------------------------
   public static void showResult (String answer)
   {
      result.setText (answer);
   }  
}  
//==================================================================
//  The action listener for the clear button.
//==================================================================
class ClearActionListener implements ActionListener
{
   //------------------------------------------------------
   //  Clears the text field and result label.
   //------------------------------------------------------
   public void actionPerformed (ActionEvent event)
   {
      PostfixGUI.clearFields();
   } 
}  
//==================================================================
//  The action listener for the evaluate button and text field.
//==================================================================
class EvaluateActionListener implements ActionListener
{
   //------------------------------------------------------
   //  Performs the evaluation and presents the results.
   //------------------------------------------------------
   public void actionPerformed (ActionEvent event)
   {
      String expr = PostfixGUI.getExpression();
      if (!expr.equals(""))
      {
         PostfixEvaluator evaluator = new PostfixEvaluator(expr);
         double answer = evaluator.evaluate();
         PostfixGUI.showResult (String.valueOf(answer));
      }
   }
}
//==================================================================
//  This class evaluates a postfix expression.
//==================================================================
class PostfixEvaluator
{
   private StringTokenizer tokens;
   private Stack stack;

   public PostfixEvaluator (String expr)
   {
      tokens = new StringTokenizer (expr, "+-*/ ", true);
      stack = new Stack();
   }
   public boolean isOperator (char c)
   {
      if (c == '+' || c == '-' || c == '*'|| c == '/')
         return true;
      else
         return false;
   }                      
   public double evaluate()
   {
      String d = "";
      while (tokens.hasMoreTokens())
      {
         double a=0, b=0, e=0;
         d = tokens.nextToken();
         char s = d.charAt(0);
         if(isOperator(s))
         {
            a = ((Double)stack.pop()).doubleValue();
            b = ((Double)stack.pop()).doubleValue();
         }
         switch(s)
         {
            case '+':   e = b + a;   stack.push(new Double(e));   break;
            case '-':   e = b - a;   stack.push(new Double(e));   break;
            case '*':   e = b * a;   stack.push(new Double(e));   break;
            case '/':   e = b / a;   stack.push(new Double(e));   break;
            case ' ':   break;
            default :   stack.push(new Double(d));     
         }
      }
      double r = ((Double)stack.pop()).doubleValue();
      return r;
   }  
}