How to process input one number at a time through a while loop

My main method asks the user for an expression, an input that will be something like "5 4 +" or "6 4 + 8 /" with operands and operators separated by a space. I have a method that processes this input and gives the answer to what it equals. It loops through all the numbers inputted through a while loop that ends once scnr.hasNext() == true, or when the errorMessage isn't null. Currently, scnr.hasNext() is becoming false sooner than its supposed to, causing the loop to only read the first number, and then end. It is becoming false because I try to store scnr.nextInt() into a variable. I need for it to take just the first number, and leave the rest, which I think will keep scnr.hasNext() true. Here is my entire method

public static Number evaluateExpression(String expr) {
        int var = 0;
        String input = "";

        String tooFewOperands = "Too few operands";
        String unKnownOperator = "Unknown operator:";
        String tooManyOperands = "Too many operands.";

        Stack<Integer> stack = new Stack<>();
        String errMsg = null;

        Scanner scnr = new Scanner(expr);

        while (scnr.hasNext() && errMsg == null) {
            if (scnr.hasNextInt()) {
                var = scnr.nextInt();                           //Problematic line
                System.out.println("Operand read: " + var);
                stack.push(var);
            } else {
                input = scnr.next();
                if (checkValidOperator(input) == input.toCharArray()[0]) {
                    System.out.println("Operator read: " + input);
//                    stack.push(var);
                    if (stack.size() >= 2) {
                        execute(stack, input.toCharArray()[0]);
                    } else {
                        errMsg = tooFewOperands;
                    }
                } else {
                    errMsg = unKnownOperator + " " + input + " ";
                }
            }
            System.out.println("------ Stack state -----");
            System.out.println(stack.toString());
            System.out.println(scnr.hasNext());
        }
        if (errMsg != null) {
            System.out.println("Failed evaluation of |" + expr + "|\n" + errMsg);
            return null;
        }
        if (stack.size() > 1) {
            System.out.println("Failed evaluation of |" + expr + "|\n" + tooManyOperands + stack.toString());
            return null;
        } else {
            return stack.peek();
        }
    }

Notice that Scanner scnr is initialized with String expr, I think my teacher hinted at that allowing me to read individual tokens from the scnr. She said "Initialize a scanner using expr as a parameter to its constructor, which will allow you to read tokens from the expression." I've never initialized a Scanner with something other than System.in. She also specified that to make this method I should "Write a loop that will run as long as the scanner has input (hasNext) and error message is null." So that is what the while condition has to be.

In general When you make a Scanner object for both String types and Numerical types it occurs (or at least it has happened to me).

What I done was to use var = Integer.parseInt(scnr.nextLine()); instead.

Hope it help you.

I don't see anything wrong with the structure of your Scanner code.

The only observation would be that this line looks odd:

if (checkValidOperator(input) == input.toCharArray()[0])

What's the implementation of checkValidOperator?

This bare bones version appears to work without any issues. Compare this to your version to see if you can identify which elements might be causing the problem.

public static void main(String[] args)
{
	String expr = "6 4 + 8 /";

	Scanner scnr = new Scanner(expr);
	
	String errorMsg = null;
	
	while (scnr.hasNext() && errorMsg == null)
	{
		if (scnr.hasNextInt())
		{
			int var = scnr.nextInt();
			System.out.println("Operand: " + var);
		} 
		else
		{
			String operator = scnr.next();
			if (isOperator(operator))
			{
				System.out.println("Operator: " + operator);
			} 
			else
			{
				errorMsg = "Unknown Operator: " + operator;
			}
		}
	}
}

static boolean isOperator(String op)
{
	return op.length() == 1 && "+-*/".indexOf(op) >= 0;
}