How to write testclass for custom exception

Question

How do you write test class if the exception has been thrown

   public class UserService{
        public class UserServiceException extends Exception{}
    
        public static Contact createUser(Contact applicant, Id accountid){
        if(String.isBlank(applicant.firstName){
           throw new UserServiceException('Contact firstName is null');
        }
        return applicant;
    
    }

Answer 1

In a nutshell, by using a try/catch in your unit test.

@isTest
static void testExceptionThrown() {
    // We need something to detect whether the exception has been thrown
    //   (and caught by this test) or not
    Boolean exceptionCaught = false;

    try{
        MyClass.myMethod();
    } catch(MyException e) {
        // Catching specific exceptions is generally better practice.
        // That's doubly true for unit tests.
        // So don't write "catch(Exception e)" (a.k.a. the "pokemon" catch)
        exceptionCaught = true;
    }

    // Assert that you caught the exception (because assertions are what make
    //   tests useful to us developers)
    System.Assert.isTrue(exceptionCaught, 'Didn\'t catch the expected exception');

}