Using lambda to filter stream and throw an exception

Question

Currently our implementation of a base method using Set<String> clients was as follows -

if (clients.isEmpty()) {
    throw new InvalidClientException();
}

for (String client : clients) {
    if (!myMay.containsKey(client)) {
        throw new InvalidClientException(client);
    }
}

I tried converting it using lambda expression as follows -

clients.stream().filter(client -> !myMay.containsKey(client) || clients.isEmpty())
            .forEach(InvalidClientException::new);

But this seems to be not working the same way, is the parameterised constructor call a miss here?

Naman · Accepted Answer · 2016-08-28 11:45:51Z

5

First of all, if the set is empty, the lambda passed to forEach won't be executed: an empty stream is empty, and filtering it won't add any element to it. Only potentially remove some.

Second, the lambda creates an exception. But it doesn't throw it.

You can use

if (clients.isEmpty() || clients.stream().anyMatch(client -> !myMay.containsKey(client))) {
    throw new InvalidClientException();
}

EDIT: I missed the fact that you wanted to pass the (first?) client not in the set to the exception. To do that, you can do

if (clients.isEmpty()) {
    throw new InvalidClientException();
}

clients.stream()
       .filter(client -> !myMay.containsKey(client))
       .findAny() // or .findFirst()
       .ifPresent(client -> {
           throw new InvalidClientException(client);
       });

That will only work if the exception is a runtime exception, though, because you can't throw a checked exception from a Consumer. If it's a checked exception and you really want to keep it a checked exception, you can use

if (clients.isEmpty()) {
    throw new InvalidClientException();
}

Optional<String> wrongClient = 
    clients.stream()
           .filter(client -> !myMay.containsKey(client))
           .findAny();
if (wrongClient.isPresent()) {
    throw new InvalidClientException(wrongClient.get());
}

edited Aug 28, 2016 at 11:45

Naman

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answered Aug 28, 2016 at 8:09

JB Nizet

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9 Comments

Naman Over a year ago

ohk, no way to use filter while streaming and throw the exception as well?

Naman Over a year ago

also how to handle throw new InvalidClientException(client) here?

JB Nizet Over a year ago

You don't want to dilter. You want to find if an element of the stream is not in the myMay set. That's what anyMatch() does. Throwing an exception for every element not in the set doesn't make much sense, since as soon as the exception is thrown, the loop will stop.

Naman Over a year ago

i meant the parameterised call to InvalidClientException(client) where i need to pass the clients name which is invalid as well

Naman Over a year ago

the updated version doesn't seem to be working on my IDE, for the ifPresent part

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