Post.java

package Stack;

public class Post {

    static int evaluatePostfix(String exp)
    {
        // Create a stack
        Stack<Integer> stack = new Stack<>();
 
        // Scan all characters one by one
        for (int i = 0; i < exp.length(); i++) {
            char c = exp.charAt(i);
 
            // If the scanned character is an operand
            // (number here), push it to the stack.
            if (Character.isDigit(c))
                stack.push(c - '0');
 
            //  If the scanned character is an operator, pop
            //  two elements from stack apply the operator
            else {
                int val1 = stack.pop();
                int val2 = stack.pop();
 
                switch (c) {
                case '+':
                    stack.push(val2 + val1);
                    break;
                case '-':
                    stack.push(val2 - val1);
                    break;
                case '/':
                    stack.push(val2 / val1);
                    break;
                case '*':
                    stack.push(val2 * val1);
                    break;
                }
            }
        }
        return stack.pop();
    }
 
    public static void main(String[] args) {
        String exp = "231*+9-";
       
        // Function call
        System.out.println("postfix evaluation: "
                           + evaluatePostfix(exp));
    }
}