How Loop Through Again If User Inputs Wrong Word

Basically, what I want my code to do is if the user inputs anything besides "sum, quit, max, min" I want the program to loop back and ask the user the options again. I tried using a while-loop but I can't add multiple String objects within it. I'm stuck on what to do. (Also, I used if-statements instead of while so it won't continually repeat and be stuck.)

public static void main (String [] args){
        System.out.println("Enter the option sum, max, min:");
        Scanner input = new Scanner (System.in);
        user = input.nextLine();
        double [] y = theArray();


        if (user.equals("sum")){
            //do something

        }
        else if (user.equals("max")){
            //do something 
        }
        else if (user.equals("min")){
            //do something
        }
        else if (user.equals("Quit")){
            //do something
        }


    }
}