passing static method as parameter in Java

I am afraid your tests are of low quality.

The problems that should be fixed immediately include

1. UserCredentialsValidator.usernameValidation(username, userList); The method shouldn't take the second argument. The place from where that list is retrieved should be concealed from the API consumer.

2. List<String> correctEmails = Arrays.asList(...) and List<String> correctUsernames = Arrays.asList(...) should be removed. You'd better make the tests parameterised with @ParameterizedTest and @ValueSource.

3. I'd rather remove the System.out.println statements. They make little sense in tests.

---

@ParameterizedTest
@ValueSource(strings = {"[email protected]", "[email protected]"})
void testUserEmailValidationWithValidUserEmailShouldPass(String validUserEmail) {
    boolean isValid = UserCredentialsValidator.emailValidator(validUserEmail);
    assertTrue(isValid);
}

@ParameterizedTest
@ValueSource(strings = {"username", "123username"})
void testUserNameValidationWithValidUserNameShouldPass(String validUserName) {
    boolean isValid = UserCredentialsValidator.usernameValidation(validUserName);
    assertTrue(isValid);
}

Now there is nothing to reduce.