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? — ernest_k
– ernest_k, Commented Mar 13, 2021 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
– Turing85, Commented Mar 13, 2021 at 19:27

Nowhere Man · Accepted Answer · 2021-03-14 00:10:17Z

2

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, 2021 at 0:10

Nowhere Man

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answered Mar 13, 2021 at 19:37

Dmitrii B

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1 Comment

Shai Ace Over a year ago

Thanks. that seems to solve the problem, however I was hoping for a one liner, since its not possible I will just use this.

Yassin Hajaj · Accepted Answer · 2021-03-13 20:29:53Z

1

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."));

answered Mar 13, 2021 at 20:29

Yassin Hajaj

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Comments

Nowhere Man · Accepted Answer · 2021-03-14 00:08:10Z

0

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.")
}

answered Mar 14, 2021 at 0:08

Nowhere Man

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