After taking some great advice on this question, I'm hoping for some more feedback on my quadratic expression calculator.This is my code as stands:public class Quadratic_Equations {    private final static Scanner sc = new Scanner(System.in);    public static void main(String[] args) {        while (true) {            System.out.println(Please enter a, b and c or enter \exit\ at any time to exit);            double a = promptUserUntilValidInput(Please enter a:);            double b = promptUserUntilValidInput(Please enter b:);            double c = promptUserUntilValidInput(Please enter c:);            System.out.println(formatOutput(calculateQuadraticFormula(a, b, c)));            System.out.println();        }    }    private static String formatOutput(Set<Double> resultsFromEquation) {        if (resultsFromEquation == null)            return These numbers do not compute - they produce an illegal result.;        else {            return resultsFromEquation.toString();        }    }    private static double promptUserUntilValidInput(String prompt){        if (prompt.equals(exit)) {            sc.close();            System.exit(0);        }        boolean valid = false;        Double value = new Double(0.0);        while(!valid){            System.out.println(prompt);            value = tryParse(sc.nextLine());            if (value != null)                valid = true;        }        return value;    }    private static Double tryParse(String text){        try{            return Double.parseDouble(text);        }        catch (NumberFormatException e){            return null;        }    }    public static Set<Double> calculateQuadraticFormula(double a, double b, double c) {        Set<Double> results = new HashSet<Double>();        double temp = (b * b) - (4 * a * c);        if (temp < 0)            return null;        if( temp == 0 ) {            results.add(-b / 2 * a);}         else if( temp > 0) {            results.add( (-b + Math.sqrt(temp) ) / (2 * a) );            results.add( (-b - Math.sqrt(temp) ) / (2 * a) );        }        return results;    }}Before I go on, there are a few things I should point out.I'm aware this could be better if the quadratic stuff was in a class of its own - however, for the purposes of this simple exercise I'm happy skipping that.My main issues are:static Scanner. Some people on previous question suggested making this a private variable, but then it needs to get passed to everything. Is this worth it?Does it look okay to close the scanner (and exit the program) where I do?Do other methods look single-responsibility-ish enough?
Quadratic Expression Calculator 2
java;beginner