// [Rauh 2002] 3. Auflage
// evaluation of double value expressions with binary operations only,
// given in its postfix notation

import java.util.*;
// ADT Stack -> pop(), push()
// ADT StringTokenizer -> hasMoreTokens(), nextToken()

public class Postfixrechner {

  
  public Postfixrechner (String delimiters) { 
    stack = new Stack();				// a new empty stack
    this.delimiters = delimiters;
  } // Postfixrechner
  

  public String rechneAus (String s) { 

    if (s != null && !s.equals("")) { 			// string to be evaluated is not empty
      t = new StringTokenizer(s, delimiters);
      double z1 = 0;
      double z2 = 0;

      while (t.hasMoreTokens()) {
        String token = t.nextToken();

        if ((int)token.charAt(0) < 48) { 		// operator token
          z1 = ((Double)stack.pop()).doubleValue();
          z2 = ((Double)stack.pop()).doubleValue();

          if (token.charAt(0) == '+') {			// evaluate operator token
            stack.push(new Double(z2 + z1));
          } else if (token.charAt(0) == '-') {
            stack.push(new Double(z2 - z1));
          } else if (token.charAt(0) == '*') {
            stack.push(new Double(z2 * z1));
          } else if (token.charAt(0) == '/') {
            stack.push(new Double(z2 / z1));
          } // if

        } else {					// number token
          stack.push(new Double(token));
        } // if

      } // while

      return "" + ((Double)stack.pop()).toString();	// pop the result from the stack
    
    } else {						// string to be evaluated is empty
      return "ERROR";
    } // if 

  } // rechneAus

  
  private String delimiters;
  private Stack stack;
  private StringTokenizer t;


} // class Postfixrechner