Throwing a custom exception with lambda expression in java

Question

I was looking around for a while but could not seem to find the right answer. I am trying to reduce my code since I have a project and want to use lambda expressions. I am new to lambda and still practicing it, I came across "EXCEPTION HANDLING" with lambda in general, however, for my specific problem I could not seem to find.

I have a whole project to fix but here is an example of a code I want to reduce.

Code:

for (Customer allCustomers : cust.getAllCustomers()) {
    if (customer.getEmail().equals(allCustomers.getEmail()))
        throw new CustomerAlreadyExists(
                "ERROR: Cannot add customer. email is already in use.");

}

What I have tried:

cust.getAllCustomers()
    .stream()
    .filter(x -> x.getEmail().equals(customer.getEmail())
    .forEach(throw new CustomerAlreadyExists(
            "ERROR: Cannot add customer. email is already in use."));

My problem seems to be with throwing the exception. (Will not compile) otherwise I think the expression is fine. is there a way to throw an exception with a one liner?

thanks. Still learning.

`Iterable.forEach` takes a `Consumer`, whose `accept` method does not have any declared checked exception. Your code would work if `CustomerAlreadyExists` is a runtime exception. If you need to throw a checked exception, read through [How can I throw CHECKED exceptions from inside Java 8 streams?](https://stackoverflow.com/questions/27644361/how-can-i-throw-checked-exceptions-from-inside-java-8-streams) — ernest_k, Mar 13 '21 at 19:27
"*My problem seems to be with throwing the exception.*" - What does that mean? Does it not compile? Does it not behave as expected? I would take a different approach: stream, filter, find first, map to exception (don't throw it), check presence and if present throw. This, however, will not be a one-liner. — Turing85, Mar 13 '21 at 19:27

score 1 · Answer 1 · edited Mar 14 '21 at 00:10

1

You can do it like that:

cust.getAllCustomers().stream()
    .filter(x -> x.getEmail().equals(customer.getEmail())
    .forEach(s -> {throw new CustomerAlreadyExists("ERROR: Cannot add customer. email is already in use.");});

edited Mar 14 '21 at 00:10

Nowhere Man

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answered Mar 13 '21 at 19:37

Dmitrii B

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Thanks. that seems to solve the problem, however I was hoping for a one liner, since its not possible I will just use this. – Shai Ace Mar 13 '21 at 19:59

score 1 · Answer 2 · answered Mar 13 '21 at 20:29

I would approach the problem differently and more in line with the throwing of an exception.

Negate your filter condition
Use findAny() and orElseThrow()

cust.getAllCustomers()
    .stream()
    .map(Customer::getEmail)
    .filter(Predicate.not(customer.getEmail()::equals))
    .findAny()
    .orElseThrow(() -> throw new CustomerAlreadyExists(
            "ERROR: Cannot add customer. email is already in use."));

score 0 · Accepted Answer · answered Mar 14 '21 at 00:08

It's a late answer but perhaps it's worth to use Stream::anyMatch to check the condition:

String email = customer.getEmail();
if (cust.getAllCustomers()
    .stream()
    .map(Customer::getEmail)
    .anyMatch(email::equals)) {
    throw new CustomerAlreadyExists("ERROR: Cannot add customer. email is already in use.")
}

Throwing a custom exception with lambda expression in java

3 Answers3