How can I throw CHECKED exceptions from inside Java 8 streams?

Question

How can I throw CHECKED exceptions from inside Java 8 streams/lambdas?

In other words, I want to make code like this compile:

public List

        

Answer 1

I think this approach is the right one:

public List getClasses() throws ClassNotFoundException {
    List classes;
    try {
        classes = Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String").map(className -> {
            try {
                return Class.forName(className);
            } catch (ClassNotFoundException e) {
                throw new UndeclaredThrowableException(e);
            }
        }).collect(Collectors.toList());
    } catch (UndeclaredThrowableException e) {
        if (e.getCause() instanceof ClassNotFoundException) {
            throw (ClassNotFoundException) e.getCause();
        } else {
            // this should never happen
            throw new IllegalStateException(e.getMessage(), e);
        }
    }
    return classes;
}

Wrapping the checked exception inside the Callable in a UndeclaredThrowableException (that’s the use case for this exception) and unwrapping it outside.

Yes, I find it ugly, and I would advise against using lambdas in this case and just fall back to a good old loop, unless you are working with a parallel stream and paralellization brings an objective benefit that justifies the unreadability of the code.

As many others have pointed out, there are solutions to this situation, and I hope one of them will make it into a future version of Java.