Quadratic Expression Calculator 2

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 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 calculateQuadraticFormula(double a, double b, double c) {
Set results = new HashSet();
double temp = (b b) - (4 a * c);
if (temp 0) {
results.add( (-b + Math.sqrt(temp) ) / (2 * a) );
results.add( (-b - Math.sqrt(temp) ) / (2 * a) );
}

Bugs

-
calculateQuadraticFormula() calculates wrong results for double roots. Watch your operator associativity!

if( temp == 0 ) {
    results.add(-b / 2 * a);}

-
The "exit" feature doesn't work. The prompt would never be "exit", would it?

private static double promptUserUntilValidInput(String prompt){
    if (prompt.equals("exit")) {
        sc.close();
        System.exit(0);
    }

Design issues

• Class names should be nouns, and the name should reflect their purpose. I recommend QuadraticSolver as the class name.

• The calculateQuadraticFormula() function could then be simply named solve().

• The class should not hold a Scanner as a member variable, as that would violate the Single Responsibility Principle. It can be a local variable in main() that gets passed to the prompting function. (The scanner should be created using a try-with-resources block.)

• Returning null from the solver is annoying for the caller to handle. I suggest returning an empty set if there are no real roots.

• I suggest returning two copies of the result if it is a double root, but that is a matter for debate.

Style issues

• You assign the expression (b b) - (4 a * c) to a variable named temp. Why not use a descriptive name discriminant?

-
The promptUserUntilValidInput() function could be better if you removed its tryParse() helper, I think. (Furthermore, promptUser… is redundant — who else are you going to prompt?)

Also, since there is a System.exit() hidden inside, it needs to be documented. (Actually, @rolfl's suggestion to throw an exception is better.)

/**
 * Prompts the user to enter a number, and retries until the input is a
 * valid double.  Calls System.exit(0) if the input is "exit", or if EOF
 * is encountered.
 */
private static double promptUntilValidInput(String prompt, Scanner sc) {
    do {
        try {
            System.out.print(prompt);
            String input = sc.nextLine();
            if ("exit".equals(input)) {
                System.exit(0);
            }
            return Double.parseDouble(input);
        } catch (NoSuchElementException eof) {
            System.out.println();
            System.exit(0);
        } catch (NumberFormatException retryOnBadInput) {
        }
    } while (true);
}

StackExchange Code Review Q#46322, answer score: 6