Return postfix invalid expression

Question

So I have slight problem that I want to fix. I have to return the result of a postfix expression, but also have to detect an invalid postfix expression. My problem is that whenever I detect the invalid postfix expression the program still returns a value for the expression. I just want to return a message stating that the expression is invalid.

Code:

public class Evaluator {

    private static final String add = "+"; 
    private static final String sub = "-";
    private static final String mul = "*";
    private static final String div = "/";

    public static void main (String [] args) throws FileNotFoundException{
        Scanner scan = new Scanner(System.in);

        System.out.print("Input filename:"); // read file -------
        String filename = scan.nextLine();
        File file = new File(filename);
        Scanner reader = new Scanner(file);// --------------------------


        while (reader.hasNextLine()){
        String  line = reader.nextLine();
        System.out.println(line + " = " + start(line));
        }

    }

    public static int start (String line ) {
        GenericStack<Integer> stack = new GenericStack<>();

         String[] inputs = line.split(" ");
        return postfix(stack, inputs);

    }
    public static int postfix(GenericStack<Integer> stack, String[] temp) {
        int num1, num2;

        for(int i = 0; i < temp.length; i++) {
            if( temp[i].equals(add) || temp[i].equals(sub) || temp[i].equals(mul) || temp[i].equals(div) ) {
                num2 = stack.pop();
                num1 = stack.pop();
                switch(temp[i]) {
                case add: {
                    int operation = num1 + num2;
                    stack.push(operation);
                    break;
                }

                case sub: {
                    int operation = num1 - num2;
                    stack.push(operation);
                    break;
                }

                case mul: {
                    int operation = num1 * num2;
                    stack.push(operation);
                    break;
                }

                case div: {
                    int operation = num1 / num2;
                    stack.push(operation);
                    break;
                }
                }
            } else {            
                stack.push(Integer.parseInt(temp[i]));
            }

        }
        if (stack.isEmpty()) {
            System.out.print ("No value to return on the following postfix expression: ");  
        }
         if (stack.size() > 1) {
             System.out.print ("Too many operands on the following postfix expression: ");   
        }

     return stack.pop();

    }
}

Example Output:

6 5 2 3 + 8 * + 3 + * = 288
Too many operands on the following postfix expression: 6 5 2 3 + 8 * + 3 + = 48

Answer 1

Best way to do this is throw Exception

Change the postfix method throw Exception on Invalid expression and add throws Exception on method signature Like below :

public static int postfix(GenericStack<Integer> stack, String[] temp) throws Exception {
    ....
    if (stack.isEmpty()) {
        throw new Exception("No value to return on the following postfix expression: ");
    }
    if (stack.size() > 1) {
        throw new Exception("Too many operands on the following postfix expression: ");
    }
}

Change start method add throws Exception on method signature

public static int start(String line) throws Exception {
    ....
}

And in the main method add try catch block on calling method start

try {
     System.out.println(line + " = " + start(line));
} catch (Exception e) {
    System.out.println(e.getMessage());
}