package  net.datastructures;
import java.util.*;

public class PostFixCalculator<E> {

    /**
     * Default constructor
     */
    public PostFixCalculator() {    
    }

    /**
     * Evaluates a postfix expression containing Integer operands
     * @param s is a postfix expression
     * @return an int giving the value of the expression
     */
    public int evaluate(String s) {
	Stack<Integer> stack = new ArrayStack<Integer>();
	StringTokenizer st = new StringTokenizer(s);
	while (st.hasMoreTokens()) {
	    String element = st.nextToken();
	    if (isNumber(element)) {
		//		stack.push(new Integer((Integer.parseInt(element))));
		stack.push((Integer.parseInt(element)));
	    }
	    else {
		ExpressionOperator op;
		Integer opnd2 = null, opnd1 = null;
		switch (element.charAt(0)) {  // Must be an operator
		    case '+': 
			op = new AdditionOperator();
			opnd2 = stack.pop();
			opnd1 = stack.pop();
			op.setOperands(opnd1, opnd2);
			stack.push(op.getValue());
			break;
		    case '-': 
			op = new SubtractionOperator();
			opnd2 = stack.pop();
			opnd1 = stack.pop();
			op.setOperands(opnd1, opnd2);
			stack.push(op.getValue());
			break;
		}
	    }
	}
	return stack.pop();
    }
    
    private boolean isNumber(String s) {
	return Character.isDigit(s.charAt(0));
    }

    public static void main(String args[]) {
	PostFixCalculator<Integer> pfc = new PostFixCalculator<Integer>();
	System.out.println(pfc.evaluate("151 60 + 92 -"));
    }
}