Решение квадратного уравнения через теорему Виета

public class Solver {
    public static Pair<Integer, Integer> solveSquareEquation(final int a, final int b, final int c) throws SolverException {
 
        if ((b % a != 0) || (c % a != 0))
            throw new SolverException(String.format("Cannot solve equation %dx^2 + %dx + %d = 0. ", a, b, c) + "Reason: " +
                    "equation cannot be reduced");
 
        int p = b / a;
        int q = c / a;
        int sign1 = 1;
        int sign2 = 1;
 
        if (q < 0)
            sign1 = -1;
        else if ((q > 0) && (p > 0)) {
            sign1 = -1;
            sign2 = -1;
        }
 
        q = Math.abs(q);
 
        Map<Integer, Integer> divisors = obtainAllDivisors(q);
 
        Pair<Integer, Integer> result = null;
 
        Iterator iterator = divisors.entrySet().iterator();
        while ((iterator.hasNext()) && (result == null)) {
 
            Map.Entry entry = (Map.Entry) iterator.next();
 
            int divisor = (int) entry.getValue();
            int x1 = divisor * sign1;
            int x2 = divisors.get(q / divisor) * sign2;
 
            if (x1 + x2 == -p)
                result = new ImmutablePair<>(x1, x2);
        }
 
        if (result == null)
            throw new SolverException(String.format("Cannot solve equation %dx^2 + %dx + %d = 0. ", a, b, c) + "Reason: " +
                    "equation has no integer roots");
 
        return result;
    }
 
    private static Map<Integer, Integer> obtainAllDivisors(final int n) {
        Map<Integer, Integer> result = new HashMap<>();
 
        final int limit = (int)Math.sqrt(n);
 
        for (int i = 1; i <= limit; i++) {
            if (n % i == 0) {
 
                if (n / i != i)
                    result.put(n / i, n / i);
 
                result.put(i, i);
            }
        }
 
        return result;
    }
}

public class SolverException extends Exception {
    public SolverException(String message) {
        super(message);
    }
 
    public SolverException(String message, Throwable cause) {
        super(message, cause);
    }
}

public class SolverTest {
    @Rule
    public ExpectedException expectedException = ExpectedException.none();
 
    /*
        tests x^2 + 7x + 10 = 0 equation with [-2; -5] roots
     */
    @Test
    public void testEq1() throws SolverException {
        Pair<Integer, Integer> expected = new ImmutablePair<>(-2, -5);
        Pair<Integer, Integer> actual = Solver.solveSquareEquation(1, 7, 10);
        assertEquals(expected, actual);
    }
 
    /*
        tests x^2 + 3x - 10 = 0 equation with [-5; 2] roots
     */
    @Test
    public void testEq2() throws SolverException {
        Pair<Integer, Integer> expected = new ImmutablePair<>(-5, 2);
        Pair<Integer, Integer> actual = Solver.solveSquareEquation(1, 3, -10);
        assertEquals(expected, actual);
    }
 
    /*
        tests 3x^2 - 30x + 75 = 0 equation with [5; 5] roots
     */
    @Test
    public void testEq3() throws SolverException {
        Pair<Integer, Integer> expected = new ImmutablePair<>(5, 5);
        Pair<Integer, Integer> actual = Solver.solveSquareEquation(3, -30, 75);
        assertEquals(expected, actual);
    }
 
    /*
        tests x^2 + x - 30 = 0 equation with [-6; 5] roots
     */
    @Test
    public void testEq4() throws SolverException {
        Pair<Integer, Integer> expected = new ImmutablePair<>(5, -6);
        Pair<Integer, Integer> actual = Solver.solveSquareEquation(1, 1, -30);
        assertEquals(expected, actual);
    }
 
    /*
        tests x^2 - 30x + 700 = 0 equation with no integer roots
     */
    @Test
    public void testEq5() throws SolverException {
        final int a = 1;
        final int b = -30;
        final int c = 700;
        final String expectedMessage = String.format("Cannot solve equation %dx^2 + %dx + %d = 0. ", a, b, c) + "Reason: " +
                "equation has no integer roots";
        expectedException.expect(SolverException.class);
        expectedException.expectMessage(expectedMessage);
        Solver.solveSquareEquation(a, b, c);
    }
 
    /*
        tests 3x^2 - 7x + 3 = 0 equation is unreducable
     */
    @Test
    public void testEq6() throws SolverException {
        final int a = 3;
        final int b = -7;
        final int c = 3;
        final String expectedMessage = String.format("Cannot solve equation %dx^2 + %dx + %d = 0. ", a, b, c) + "Reason: " +
                "equation cannot be reduced";
        expectedException.expect(SolverException.class);
        expectedException.expectMessage(expectedMessage);
        Solver.solveSquareEquation(a, b, c);
    }
}