Junit test for scanner parameter filereader

What I am trying to do is to simply write a test for the calcGrade(scanner sc) function, but I don't know if it's possible to test it in this scenario since the scanner reads in grades in a textfile then returns an average of them.

public static void main(String[] args) throws FileNotFoundException {
        
        Scanner sc = new Scanner(new FileReader("./Src/Chapter3/grade.txt"));
        
        System.out.printf("The average grade is: %.2f", calcGrade(sc));
                
    }

    public static double calcGrade(Scanner sc) {
        double test1, test2, test3, test4, test5, avg;

        test1 = sc.nextDouble();
        test2 = sc.nextDouble();
        test3 = sc.nextDouble();
        test4 = sc.nextDouble();
        test5 = sc.nextDouble();
        avg = (test1 + test2 + test3 + test4 + test5) / 5.0;
        
        sc.close();
        return avg;
    }

 @Test

    public void testCalGrade() {
    double expected = 70.0;
    double actual = js.calcGrade(); //i could change the parameter here instead passed in scanner to the individual grades then test it
    assertEquals(expected, actual,0.02);
}

grade.txt

80 98 100 78 65.6

Since Scanner is a final class, you cannot provide a mock (or any fake object) instead of using a real Scanner object in the test.

Though theoretically you still could create a mock using external libraries (like PowerMock) or change the System.in field, a better way would be to make the calcGrade method more generic.
E.g. it could accept a Supplier<Double> instead of a Scanner. Here is an example.

    public static double calcGrade(Supplier<Double> valueSupplier) {
        double test1, test2, test3, test4, test5, avg;

        test1 = valueSupplier.get();
        test2 = valueSupplier.get();
        test3 = valueSupplier.get();
        test4 = valueSupplier.get();
        test5 = valueSupplier.get();
        avg = (test1 + test2 + test3 + test4 + test5) / 5.0;

        return avg;
    }

Then you can call it like this

    public static void main(String[] args) throws FileNotFoundException {

        Scanner sc = new Scanner(new FileReader("./Src/Chapter3/grade.txt"));

        System.out.printf("The average grade is: %.2f", calcGrade(sc::nextDouble));
        sc.close();
    }

If you don't know the sc::nextDouble Syntax, it's called a method reference and basically calls sc.nextDouble() when valueSupplier.get() is called.

Then you could test it like this

public class CalGradeTest {

    private int index;

    @Test
    public void testCalGrade() {
        CalGrade js = new CalGrade();
        double[] testValues = {68.0, 69.0, 70.0, 71.0, 72.0};
        double expected = 70.0;
        double actual = js.calcGrade(() -> testValues[index++]);
        assertEquals(expected, actual, 0.02);
    }
}

Scanner has multiple constructors, so you can choose a different one and use that.

See the possible constructors

Example of how you can modify your test

 @Test
 public void testCalGrade() {
    double expected = 70.0;
    double actual = js.calcGrade(new Scanner("80 98 100 78 65.6"));
    assertEquals(expected, actual,0.02);
}