evaluate a mathematical expression

Question:

You are give a piece of mathematical expression, e.g., 3 * 4 + 5, return the value of that expression.

public class EvaluateDeluxe {

    // result of applying binary operator op to two operands val1 and val2
    public static double eval(String op, double val1, double val2) {
        if (op.equals("+")) return val1 + val2;
        if (op.equals("-")) return val1 - val2;
        if (op.equals("/")) return val1 / val2;
        if (op.equals("*")) return val1 * val2;
        throw new RuntimeException("Invalid operator");
    }
    
    public HashMap<String, Integer> priorityInfo(){
        // precedence order of operators
        HashMap<String, Integer> precedence = new HashMap<String, Integer>();
        precedence.put("(", 0);   // for convenience with algorithm
        precedence.put(")", 0);  
        precedence.put("+", 1);   // + and - have lower precedence than * and /
        precedence.put("-", 1);
        precedence.put("*", 2);
        precedence.put("/", 2);
        
        return precedence;
    }
    
    
    public double calculate(String[] stdIn, HashMap<String, Integer> precedence){

        Stack<String> ops  = new Stack<String>();
        Stack<Double> vals = new Stack<Double>();
        
        for (int i = 0; i < stdIn.length; i++) {
            String s = stdIn[i];
            // token is a value
            if (!precedence.containsKey(s)) {
                vals.push(Double.parseDouble(s));
                continue;
            }
            // token is an operator
            while (true) {
                // the last condition ensures that the operator with higher precedence is evaluated first
                if (ops.isEmpty() || s.equals("(") || (precedence.get(s) > precedence.get(ops.peek()))) {
                    ops.push(s);
                    break;
                }
                // evaluate expression
                String op = ops.pop();

                // but ignore left parentheses
                if (op.equals("(")) {
                    assert s.equals(")");
                    break;
                }
                // evaluate operator and two operands and push result onto value stack
                else {
                    double val2 = vals.pop();
                    double val1 = vals.pop();
                    vals.push(eval(op, val1, val2));
                }
            }
        }

        // finished parsing string - evaluate operator and operands remaining on two stacks
        while (!ops.isEmpty()) {
            String op = ops.pop();
            double val2 = vals.pop();
            double val1 = vals.pop();
            vals.push(eval(op, val1, val2));
        }
        return vals.pop();
    }
}

The code is from:  http://algs4.cs.princeton.edu/13stacks/EvaluateDeluxe.java.html