StackMachine.java

import java.util.*;

public class Main {

    private static Stack<Integer> StackMachine = new Stack<>();
    private static HashSet<Character> Operators = new HashSet<>(Arrays.asList(new Character[] {'+', '-', '*', '/'}));
    public static void main(String[] args) {

        String input = "13+62*7+*";
        /***
         *
         *  character | comment                | stack
         * -----------------------------------------------
         *            |                        | [empty]
         * '1'        | push 1 onto the stack  |
         *            |                        | 1
         * '3'        | push 3 onto the stack  |
         *            |                        | 1, 3
         * '+'        | perform addition       |
         *            |                        | 4
         * '6'        | push 6 onto the stack  |
         *            |                        | 4, 6
         * '2'        | push 2 onto the stack  |
         *            |                        | 4, 6, 2
         * '*'        | perform multiplication |
         *            |                        | 4, 12
         * '7'        | push 7 onto the stack  |
         *            |                        | 4, 12, 7
         * '+'        | perform addition       |
         *            |                        | 4, 19
         * '*'        | perform multiplication |
         *            |                        | 76
         *
         ***/
        for(int i = 0; i < input.length(); i++){
            char character = input.charAt(i);
            if(Character.isDigit(character)){
                StackMachine.push(Character.getNumericValue(character));
            }
            else if(Operators.contains(character)){
                if(StackMachine.size() < 2) {
                    continue;
                }
                int result = calculate(StackMachine.pop(), StackMachine.pop(), character);
                StackMachine.push(result);
            }
        }
        if(!StackMachine.empty()){
            System.out.println(String.valueOf(StackMachine.pop()));
        }

    }

    private static int calculate(int x, int y, char operation) {
        switch (operation){
            case '+' :
                return x + y;
            case '-' :
                return x - y;
            case '*' :
                return x * y;
            case '/':
                return x / y;
        }
        return 0;
    }
}