PostfixCalculator.java

import java.util.Stack;

class PostfixCalculator {
    public static int evaluatePostfix(String expression) {
        Stack<Integer> stack = new Stack<>();
        String[] tokens = expression.split(" ");

        for (String token : tokens) {
            if (token.matches("-?\\d+")) {
                stack.push(Integer.parseInt(token));
            } else {
                int operand2 = stack.pop();
                int operand1 = stack.pop();
                switch (token) {
                    case "+":
                        stack.push(operand1 + operand2);
                        break;
                    case "-":
                        stack.push(operand1 - operand2);
                        break;
                    case "*":
                        stack.push(operand1 * operand2);
                        break;
                    case "/":
                        stack.push(operand1 / operand2);
                        break;
                }
            }
        }

        return stack.pop();
    }
}