Throwing exception from lambda [duplicate]

Answer 1

I wrote an extension to the Stream API which allows for checked exceptions to be thrown.

public Server send(String message) throws IOException {
    ThrowingStream.of(sessions, IOException.class)
        .parallelStream()
        .map(Session::getBasicRemote)
        .forEach(basic -> basic.sendText(message));

    return this;
}

Answer 2

The problem is indeed that all @FunctionalInterfaces used in lambdas do not allow exceptions to be thrown, save for unchecked exceptions.

One solution is using a package of mine; with it, your code can read:

sessions.parallelStream()
    .map(Session::getBasicRemote)
    .forEach(Throwing.consumer(basic -> basic.sendText(message)));
return this;

Answer 3

My approach would be to sneakily throw it from the lambda, but take care to have the send method declare it in its throws clause. Using the Exceptional class I posted here:

public Server send(String message) throws IOException {
  sessions.parallelStream()
          .map(Session::getBasicRemote)
          .forEach(basic -> Exceptional.from(() -> basic.sendText(message)).get());
  return this;
}

This way you're effectively making the compiler "look away" for just a bit, disabling its exception checking at one spot in your code, but by declaring the exception on your send method, you restore the regular behavior for all its callers.