Aggregate runtime exceptions in Java 8 streams

Question

Let\'s say I have a method which throws a runtime exception. I\'m using a Stream to call this method on items in a list.

class ABC {

    public voi         


        

Answer 1

In this simple case where the doStuff method is void and you only care about the exceptions, you can keep things simple:

myObjs.stream()
    .flatMap(o -> {
        try {
            ABC.doStuff(o);
            return null;
        } catch (RuntimeException ex) {
            return Stream.of(ex);
        }
    })
    // now a stream of thrown exceptions.
    // can collect them to list or reduce into one exception
    .reduce((ex1, ex2) -> {
        ex1.addSuppressed(ex2);
        return ex1;
    }).ifPresent(ex -> {
        throw ex;
    });

However, if your requirements are more complicated and you prefer to stick with the standard library, CompletableFuture can serve to represent "either success or failure" (albeit with some warts):

public static void doStuffOnList(List myObjs) {
    myObjs.stream()
            .flatMap(o -> completedFuture(o)
                    .thenAccept(ABC::doStuff)
                    .handle((x, ex) -> ex != null ? Stream.of(ex) : null)
                    .join()
            ).reduce((ex1, ex2) -> {
                ex1.addSuppressed(ex2);
                return ex1;
            }).ifPresent(ex -> {
                throw new RuntimeException(ex);
            });
}