How to compare Mono error to a String in Java

Question

I have a method something like this :

public Mono<SomeDTO> DoAction(SomeDTO someDTOObject) {
        return findUser(someDTOObject.getUsername())
                .flatMap(existingUser -> {
                    Update update = new Update();
                   
                    return mongoTemplate.upsert(
                            Query.query(Criteria.where("username").is(someDTOObject.getUsername())),
                            update,
                            SomeDTO.class,
                            COLLECTION_NAME);                            

                }).switchIfEmpty(
                        Mono.defer(() -> {
                            return Mono.error(new Exception("User Name doesn't exist."));
                        })
                );
    }

For this, I have wriiten a testcase like this to test exception :

@Test
    public void DoAction_TestException() {
        SomeDTO someDTOObject = databaseUtil.SomeDTOMock;        

        Query query = Query.query(Criteria.where("username").regex("^"+userId+"$","i"));
        doReturn(Mono.empty()).when(mongoTemplate).findOne(query,
                SomeDTO.class, "COLLECTION_NAME");
        try {
            SomeDTO someDTOObjectResult = mongoImpl.DoAction(someDTOObject).block();
        }
        catch (Exception e) {
            String expected = "User Name doesn't exist.";
            String result = e.getMessage().toString();   ///////  this value during debugging is "java.lang.Exception:User Name doesn't exist. "
            assertEquals(expected,result);
        }
       
    }

When I run the above code , the assert is failing becuase variable result has extra string along with it. How can I remove java.lang.Exception from the result ?
I dont want to use any string functions to remove part of string.ANy help would be very helpful.

Answer 1

Eugene's answer is already showing you the correct way.

Alternate Solution #1:

Create a specific Exception class: EntityNotFoundException extends IOException or UsernameNotFoundException extends IOException and test if your result is an instance of that class.

Alternate Solution #2:

Extend your expected String expected to the String you really get.

Alternate Solution #3:

(I do not know Mono, so IF:) If Mono wraps up the Exception into another exception, you probably can reach that original exception by using e.getCause: Change your line String result = e.getMessage().toString(); to String result = e.getCause().getMessage();.

To help us find that out, simply add the line e.printStackTrace(); right after your line String expected = "User Name doesn't exist.";, and then show us what error message was printed. From there on we can help further, if the other solutions have not helped yet.