How to evaluate a postfix expression ?

Postfix notation represents a algebraic expression.
These are evaluated faster than infix notation, as ( ) are not required.

Algorithm - using Stack

• For each element in postfix string
• if element is an operand
• push element into Stack => Operand
• else  // Operator
• Pop from Stack => Operand 1
• Pop from Stack => Operand 2
• Evaluate value using both operands and element (Operator) => value
• Push the value to Stack
• Pop the element of Stack => Final result

Example

String postfix = "2 3 1 * + 9 -"

element	oprnd1	oprnd2	value	stack
2				2
3				2,3
1				2,3,1
*	3	1	3*1	2,3
+	3	2	3+2	5
9				5,9
-	5	9	5-9	-4

Java code
public static int evaluatePostfix(String postfixStr) {
  String[] postfixArr = postfixStr.split("");
  Stack<Integer> stack = new Stack<Integer>();

  for(String element : postfixArr) {
      boolean isOperator = isOperator(element);
      if(isOperator) {
         int value1 = stack.pop();

         int value2 = stack.pop();
         int value = calculate(value2, value1, element);
         stack.push(value);
      else {
         stack.push(Integer.valueOf(element));
      }

  }

  return stack.pop();
}

private static int calculate(int a, int b, String operator){
  int result = 0;
  if("+".equals(operator)) {
    result = (a+b);
  else if("-".equals(operator)) {
    result = (a-b);

  else if("*".equals(operator)) {
    result = (a*b);

  else if("/".equals(operator)) {
    result = (a/b);

  
  return result;

}

private static boolean isOperator(String element) {
  if("+".equals(element) || "-".equals(element) 
         || "/".equals(element) || "*".equals(element)) {
     return true;
  }
  return false;
}