//clarified version of PostFixEvaluator of section 3.8
//input a string of a valid PeckPostfix expression of integers, evaluate it.

import java.util.*;    
import javax.swing.*;

public class PeckPostfix {
    public static void main(String[] args) {
    String expression;
    String token;
    String operator;
    StringTokenizer tokens;
    Boolean result=true;
    Boolean operand1=true;
    Boolean operand2=false;
    //create empty stack of pending Boolean operands
    Stack operandStack = new Stack();  
    
    expression = JOptionPane.showInputDialog( "Enter Postfix expression" );
    tokens = new StringTokenizer(expression);
    
    while (tokens.hasMoreTokens()) {
        token = tokens.nextToken();
        
        if (!isOperator(token))   //operand
            //extract boolean from String token, create Boolean object, push onto stack
            operandStack.push(new Boolean(token));  
            
        //  printStack(operandStack);   //display stack at each step. Debugging

        else {                    //token is an operator
            //pop two ints off stack.
            //convert popped Object to Boolean, extract boolean
            operand2 = ((Boolean)operandStack.pop());
            operand1 = ((Boolean)operandStack.pop());  //"first" one
            
        if (token.equals("!"))
            result = operand1 != operand2;
        else if (token.equals("&&"))
            result = operand1 && operand2;
        else if (token.equals("||"))
            result = operand1 || operand2;
            
            operandStack.push(new Boolean(result));  
                       
        }
    }
    
    //only object in stack is the answer
    result = ((Boolean)operandStack.pop());
    JOptionPane.showMessageDialog(null,expression+" = "+result);

    System.exit(0);
    }
    
    
    static boolean isOperator(String t) {
        if (t.equals("!") || t.equals("&&") || t.equals("||"))
            return true;
        else
            return false;
    }
    
    
    static void printStack( Stack s) {
    System.out.print("Stack is: ");
        //don't normally iterate over a stack.  here for illustration purposes.
        Iterator items = s.iterator();
        while (items.hasNext())
            System.out.print(items.next()+"  ");
        System.out.println();
    }
}
/*

true false &&
    postfix for: true && false = false

true false ! &&
    postfix for:  true && !false = true
    
true true ! false || ! && true false && ||
    postfix for: true && !(!true || false) || (true && false) = true
*/