StackPostfixNotation.java

import java.util.*;

public class StackPostfixNotation {
    public static void main(String[] args) {
         Scanner scanner = new Scanner(System.in);
        String post = scanner.nextLine();   // Takes input with spaces in between eg. "1 21 +"
        System.out.println(postfixEvaluate(post));
    }

    // Evaluates the given postfix expression string and returns the result.
    public static int postfixEvaluate(String exp) {
	 	Stack<Integer> s = new Stack<Integer> ();
		Scanner tokens = new Scanner(exp);

		while (tokens.hasNext()) {
			if (tokens.hasNextInt()) {
				s.push(tokens.nextInt()); // If int then push to stack
			} else {    // else pop top two values and perform the operation
				int num2 = s.pop();
				int num1 = s.pop();
				String op = tokens.next();

				if (op.equals("+")) {
					s.push(num1 + num2);
				} else if (op.equals("-")) {
					s.push(num1 - num2);
				} else if (op.equals("*")) {
					s.push(num1 * num2);
				} else {
					s.push(num1 / num2);
				}

			//  "+", "-", "*", "/"
			}
		}
		return s.pop();
    }
}