InfixToPostFixEvalution.java

import java.util.Stack;
import java.util.Scanner;

public class InfixToPostFixEvalution {

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

    private static int getPrecedence(char ch) {
        switch (ch) {
        case '+':
        case '-':
            return 1;

        case '*':
        case '/':
            return 2;

        case '^':
            return 3;
        }
        return -1;
    }

    // A utility function to check if the given character is operand
    private static boolean isOperand(char ch) {
        return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');
    }

    public static String convertToPostfix(String infix) {
        Stack<Character> stack = new Stack<Character>();
        StringBuffer postfix = new StringBuffer(infix.length());
        char c;

        for (int i = 0; i < infix.length(); i++) {
            c = infix.charAt(i);

            if (isOperand(c)) {
                postfix.append(c);
            } else if (c == '(') {
                stack.push(c);
            }
            // If the scanned character is an ‘)’, pop and output from the stack
            // until an ‘(‘ is encountered.
            else if (c == ')') {

                while (!stack.isEmpty() && stack.peek() != '(') {
                    postfix.append(stack.pop());
                }
                if (!stack.isEmpty() && stack.peek() != '(')
                    return null;
                else if(!stack.isEmpty())
                    stack.pop();
            }
            else if (isOperator(c)) // operator encountered
            {
                if (!stack.isEmpty() && getPrecedence(c) <= getPrecedence(stack.peek())) {
                    postfix.append(stack.pop());
                }
                stack.push(c);
            }
        }

        while (!stack.isEmpty()) {
            postfix.append(stack.pop());
        }
        return postfix.toString();
    }

    public static void main(String[] args) {
        String infix;

        Scanner scan = new Scanner(System.in);

        infix = scan.nextLine();
        System.out.println(convertToPostfix(infix));
    }
}