Java 8: How do I work with exception throwing methods in streams?

This question may be a little old, but because I think the "right" answer here is only one way which can lead to some issues hidden Issues later in your code. Even if there is a little Controversy, Checked Exceptions exist for a reason.

The most elegant way in my opinion can you find was given by Misha here Aggregate runtime exceptions in Java 8 streams by just performing the actions in "futures". So you can run all the working parts and collect not working Exceptions as a single one. Otherwise you could collect them all in a List and process them later.

A similar approach comes from Benji Weber. He suggests to create an own type to collect working and not working parts.

Depending on what you really want to achieve a simple mapping between the input values and Output Values occurred Exceptions may also work for you.

If you don't like any of these ways consider using (depending on the Original Exception) at least an own exception.

You might want to do one of the following:

• propagate checked exception,
• wrap it and propagate unchecked exception, or
• catch the exception and stop propagation.

Several libraries let you do that easily. Example below is written using my NoException library.

// Propagate checked exception
as.forEach(Exceptions.sneak().consumer(A::foo));

// Wrap and propagate unchecked exception
as.forEach(Exceptions.wrap().consumer(A::foo));
as.forEach(Exceptions.wrap(MyUncheckedException::new).consumer(A::foo));

// Catch the exception and stop propagation (using logging handler for example)
as.forEach(Exceptions.log().consumer(Exceptions.sneak().consumer(A::foo)));

You need to wrap your method call into another one, where you do not throw checked exceptions. You can still throw anything that is a subclass of RuntimeException.

A normal wrapping idiom is something like:

private void safeFoo(final A a) {
    try {
        a.foo();
    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }
}

(Supertype exception Exception is only used as example, never try to catch it yourself)

Then you can call it with: as.forEach(this::safeFoo).

If all you want is to invoke foo, and you prefer to propagate the exception as is (without wrapping), you can also just use Java's for loop instead (after turning the Stream into an Iterable with some trickery):

for (A a : (Iterable<A>) as::iterator) {
   a.foo();
}

This is, at least, what I do in my JUnit tests, where I don't want to go through the trouble of wrapping my checked exceptions (and in fact prefer my tests to throw the unwrapped original ones)