Junit test not working for sumoverarray method Java

I am trying to run a test for my sumoverarray method but I keep getting a failure saying AssertionError. Here's my method.

public class SumOverArray {

	public static int IterateAndSum(int[] arr) {
		int sum=0;
		for (int i:arr) {
			sum=sum+i;
		}
		return sum;
		
	}
}

Here is my junit test

@Test
    public void test3() {
        int[] arr = { 1, 2, 3 };
        assertNotEquals(0, SumOverArray.IterateAndSum(arr));
        try {
            SumOverArray.IterateAndSum(null);
            assertTrue(true);
        } catch (NullPointerException e) {
            fail();
        }
        int [] arr2 = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
        assertEquals(78, SumOverArray.IterateAndSum(arr2));
        int[] arr3 = { -1, -2, -3, -4, -5, -6, -7, -8, -9 };
		assertEquals(-45, SumOverArray.IterateAndSum(arr3));
		int[] arr4 = { 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80 };
		assertEquals(1095, SumOverArray.IterateAndSum(arr4));

    }

The behaviour is expected. Looking at the second part of the test method:

try {
    SumOverArray.IterateAndSum(null);
    assertTrue(true);
} catch (NullPointerException e) {
    fail();
}

We call SumOverArray.IterateAndSum with null and expect no NullPointerException to be thrown. But looking at the implementation, we do not perform a null-check in method SumOverArray.IterateAndSum and thus try to iterate over null. This, ultimately, leads to a NullPointerException to be thrown. The catch-block in the test is entered and the test fails.

All other tests would pass, if this test would pass.

<kbd>Ideone demo</kbd>

If we, however, want to assert that a NullPointerException is thrown when null is passed as parameter, then we can use Assertions.assertThrows(...):

final NullPointerException exception = assertThrows(
    NullPointerException.class,
    () -> SumOverArray.IterateAndSum(null));
// validate exception here if necessary

Some remarks on the code:

• Method names in Java should always start with a lowercase letter (IterateAndSum -> iterateAndSum)
• A test should test one thing and one thing only. Thus, the one test method above should be written as four tests.
• The last three tests are redundant, one of them is sufficient.
• The explicit fail() on an exception is superfluous and can be omitted.
• Semantically, the first test is also covered by the last three tests. Thus this test is also redundant.
• I would suggest to define a value to return if null is passed as argument, 0 seems sensible.

This leaves us with the following two tests:

@Test
@DisplayName("should return 0 if null is passed as argument")
public void shouldReturnZerofNullIsPassedAsParameter() {
    // GIVEN: nothing

    // WHEN
    final int actual = SumOverArray.iterateAndSum(null);

    // THEN
    assertEquals(0, actual);
}

// Alternative test, if we want to assure that a NPE is thrown if null is passed
@Test
@DisplayName("should throw NPE if null is passed as argument")
public void shouldThrowNullPointerExceptionIfNullIsPassedAsParameter() {
    // GIVEN: nothing

    // WHEN & THEN
    assertThrows(
        NullPointerException.class,
        () -> SumOverArray.iterateAndSum(null));
}

@Test
@DisplayName("should return 78 if 1, 2 , ... , 12 are summed")
void shouldReturnCorrectResult() {
    // GIVEN
    final int[] arr = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };

    // WHEN
    final int actual = SumOverArray.iterateAndSum(arr);

    // THEN
    assertEquals(78, actual);
}