How to test that a constructor throws an exception using JUnit 5?

Question

I'm making a Fraction API class, one of my constructors normalises a fraction by putting the numerator and denominator parameters in their lowest terms:

    public Fraction(int numerator, int denominator){
    if (denominator == 0)
        throw new ArithmeticException("Cannot divide by zero.");
    else {
        if (denominator < 0) {
            numerator = -numerator;
            denominator = -denominator;
        }
        int gcd; // Greatest Common Divisor
        int tmpNum = numerator, tmpDen = denominator;
        // Determine greatest common divisor of numerator and denominator
        while (tmpNum != 0 && tmpDen != 0) {
            int tmp = tmpDen;
            tmpDen = tmpNum % tmpDen;
            tmpNum = tmp;
        }
        gcd = Math.abs(tmpNum + tmpDen);
        this.numerator = numerator / gcd; // Assign numerator in its lowest term
        this.denominator = denominator / gcd; // Assign denominator in its lowest term

    }
}

I want to test that the constructor throws an ArithmeticException when the denominator is 0. As far as I can tell JUnit 5 does not support @Test(expected = ArithmeticException.class but uses assertThrows(). My test:

@Test
public void testZeroDenominator(){
    Fraction f;
    assertThrows(ArithmeticException.class, f = new Fraction(2, 0));
}

does not work and IntelliJ says 'Fraction is not compatible with Executable'.

How can I test that the constructor throws the exception?

Thanks

Answer 1

Here is the syntax to pass lambda for JUnit 5's Executable:

assertThrows(ArithmeticException.class, () -> new Fraction(2, 0));

You don't need to assign the result to f, because you know that the method is not going to complete.