Reverse Polish Notation Evaluation

ReversePolishEval.java

import java.util.Stack;

public class HelloWorld
{
  public static void main(String[] args)
  {
    String[] arr = {"4", "13", "5", "/", "+"};
    Eval eval = new Eval();
    
    int n = eval.evalRPN(arr);
    
    System.out.println(String.format("Evaluated: %-4d", n));
      
  }
  
  
}
// this will become its own file too (and these can be in any order)
public class Eval
{
    public int evalRPN(String[] tokens) {
      int retVal = 0;
      Stack<String> stkNumbers= new Stack<String>();
      String operators = "+-*/";
      
      for (String s: tokens){
        if (!operators.contains(s)){
          stkNumbers.push(s);
        }
        else{
         int a = Integer.valueOf(stkNumbers.pop());
         int b = Integer.valueOf(stkNumbers.pop());  
         char c = s.charAt(0);
         switch (c) {
           case '+':
           	stkNumbers.push(String.valueOf(a + b));
           break;
           
           case '-':
           	stkNumbers.push(String.valueOf(b - a));
           break;
           
           case '*':
           	stkNumbers.push(String.valueOf(a * b));
           break;
           
           case '/':
           	stkNumbers.push(String.valueOf(b / a));
           break;
         }
          
        
        }
        
      }
      retVal = Integer.valueOf(stkNumbers.pop());
      return retVal;
        
    }
}