PostfixEvaluator.java

import java.util.Stack;

public class PostfixEvaluator {

    public static int evaluatePostfix(String expression) {
        Stack<Integer> stack = new Stack<>();

        // Split the expression by spaces
        String[] tokens = expression.split(" ");

        for (String token : tokens) {
            if (isOperator(token)) {
                // Pop the top two operands
                int operand2 = stack.pop();
                int operand1 = stack.pop();

                // Apply the operator and push the result back to the stack
                int result = applyOperator(token.charAt(0), operand1, operand2);
                stack.push(result);
            } else {
                // It's a number, push it to the stack
                stack.push(Integer.parseInt(token));
            }
        }

        // The final result is at the top of the stack
        return stack.pop();
    }

    private static boolean isOperator(String token) {
        return token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/");
    }

    private static int applyOperator(char operator, int operand1, int operand2) {
        switch (operator) {
            case '+': return operand1 + operand2;
            case '-': return operand1 - operand2;
            case '*': return operand1 * operand2;
            case '/': return operand1 / operand2;
            default: throw new IllegalArgumentException("Unknown operator: " + operator);
        }
    }

    public static void main(String[] args) {
        String expression = "1 2 + 3 *";
        int result = evaluatePostfix(expression);
        System.out.println("Result of the postfix expression: " + result); // Output: 9
    }
}