Infix to Postfix Conversion in Java

import java.util.Stack;

public class InfixToPostfix {

    private static boolean isOperand(char ch) {
        return Character.isLetterOrDigit(ch);
    }

    private static boolean isOperator(char ch) {
        return ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == '^';
    }

    private static int precedence(char op) {
        switch (op) {
            case '^': return 3;
            case '*':
            case '/': return 2;
            case '+':
            case '-': return 1;
            default:  return -1;
        }
    }

    // ^ is right-associative, others are left-associative
    private static boolean isRightAssociative(char op) {
        return op == '^';
    }

    public static String convert(String infix) {
        Stack ops = new Stack<>();
        StringBuilder postfix = new StringBuilder();

        for (int i = 0; i < infix.length(); i++) { char ch = infix.charAt(i); // Ignore spaces if (ch == ' ') continue; if (isOperand(ch)) { postfix.append(ch); } else if (ch == '(') { ops.push(ch); } else if (ch == ')') { while (!ops.isEmpty() && ops.peek() != '(') { postfix.append(ops.pop()); } // Pop '(' if (!ops.isEmpty() && ops.peek() == '(') ops.pop(); else throw new IllegalArgumentException("Mismatched parentheses"); } else if (isOperator(ch)) { while (!ops.isEmpty() && isOperator(ops.peek())) { char top = ops.peek(); // Pop if top has higher precedence // Or same precedence and current is left-associative if (precedence(top) > precedence(ch) ||
                        (precedence(top) == precedence(ch) && !isRightAssociative(ch))) {
                        postfix.append(ops.pop());
                    } else {
                        break;
                    }
                }
                ops.push(ch);
            } else {
                throw new IllegalArgumentException("Invalid character found: " + ch);
            }
        }

        while (!ops.isEmpty()) {
            if (ops.peek() == '(') throw new IllegalArgumentException("Mismatched parentheses");
            postfix.append(ops.pop());
        }

        return postfix.toString();
    }

    public static void main(String[] args) {
        String infix = "a*(b+c)/d";
        String postfix = convert(infix);

        System.out.println("Infix   : " + infix);
        System.out.println("Postfix : " + postfix);
    }
}