A simple command-line calculator

I created a simple command line calculator, and then after some reading I made a few tweaks and then remade it with a friends idea to make it more simpler. I'm wondering if this can be more optimized or if any part of the code can be done "better"?

I recently started learning Java and wanted for this first project of mine to end up the "best possible" written, so I can start learning on what is a "good code".

One of the things that was changed over time was the use of the Scanner.

• Firstly I had it create a new one on each iteration of kalkulator(); but that was obviously a unnecessary waste.

• Then I had a static scanner static Scanner scanMe = new Scanner(System.in); but somewhere I read It was better to pass it as an argument, because something that doesn't belong anywhere doesn't have place in a good code unless it is really necessary.

Here is what I ended up with so far:

"Main" Method:

public class SimpleCalculator {

public static void main(String[] args) {

    Scanner scanMe = new Scanner(System.in);

    kalkulator(scanMe);
}

"Helper" Methods:

static double scanDouble(Scanner scan){
    while (!scan.hasNextDouble()){

        System.out.println("Invalid number! Please try again.");
        scan.nextLine();
    }

    return scan.nextDouble();
}

static String scanOperator(Scanner scan){
    String In = scan.next();

    while (!(In.equals("+") || In.equals("-") || In.equals("*") || In.equals("/") || In.equals("end"))) {

        System.out.println("Invalid operator! Please select either: +,-,*,/");
        In = scan.next();
    }

    return In;
}

The Main Method:

```
public static void kalkulator(Scanner scan) {

double prviB = 0, drugiB = 0, rezultat = 0;
String operator;

System.out.println("Enter the 1st number: ");
prviB = scanDouble(scan);

System.out.println("Enter the 2nd number: ");
drugiB = scanDouble(scan);

System.out.println("Select an operator: (+,-,*,/) , or type 'end' for terminat

Nice improvement is to use Enum with behaviour. This is an example from Effective Java book :

public enum Operation {
    PLUS("+") {
        double apply(double x, double y) {
            return x + y;
        }
    },
    MINUS("-") {
        double apply(double x, double y) {
            return x - y;
        }
    },
    TIMES("*") {
        double apply(double x, double y) {
            return x * y;
        }
    },
    DIVIDE("/") {
        double apply(double x, double y) {
            return x / y;
        }
    };
    private final String symbol;

    Operation(String symbol) {
        this.symbol = symbol;
    }

    @Override
    public String toString() {
        return symbol;
    }

    abstract double apply(double x, double y);
}

StackExchange Code Review Q#128256, answer score: 3