Need to continue filtering Java stream when map throws exception after filter and findFirst

Question

I have this code:

return myList
    .stream()
    .filter(Objects::nonNull)
    .filter(listItem -> {
        try {
            return listItem.matchesCondition();
        } catch (Exception e) {
            // log error
            return false;
        }
    })
    .findFirst()
    .map(listItem -> {
        try {
            return listItem.getResult();
        } catch (Exception e) {

            // IF THIS HAPPENS, HOW CAN WE ADVANCE TO THE NEXT ITEM IN THE STREAM.
            // I'M ASSUMING WE CAN NOT SINCE THE STREAM WAS TERMINATED BY findFirst.
            // BUT HOW CAN I WRITE THIS IN A DIFFERENT WAY TO ACHIEVE THAT BEHAVIOR?

            // log error
            return null;
        }
    });

The issue is, if the first matching listItem throws an exception in the map function, an empty Optional is returned.

But instead, I want to continue testing the remaining items in the list, and try to map the next one that matches the filter.

How can I do that with streams and lambdas?

I can convert to this more imperative code but would like to find a functional solution.

for (MyListItem myList : listItem) {
    try {
        if (listItem.matchesCondition()) {
            return Optional.of(listItem.getResult());
        }
    } catch (Exception e) {

        // SWALLOW THE EXCEPTION SO THE NEXT ITEM IS TESTED

        // log error
    }
}

return Optional.empty();

Thilo · Accepted Answer · 2017-06-22 23:47:52Z

10

You have to move the findFirst to the very end, because it reduces the stream to just (at most) a single element and you cannot "go back" and get more after that.

return myList
    .stream()
    .filter(Objects::nonNull)
    .filter( ... )
    .map( ... ) // returns null on Exception, so filter again
    .filter(Objects::nonNull)
    .findFirst()

answered Jun 22, 2017 at 23:47

Thilo

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3 Comments

Holger Over a year ago

Alternatively, instead of .map(to null on exception ).filter(Objects::nonNull) you may use .flatMap(listItem -> { try { return Stream.of(listItem.getResult()); } catch(Exception e) { return null; } }). The flatMap operation will treat null like Stream.empty().

Thilo Over a year ago

@Holger. I thought about that (or the similar flatMap over Option.of). But it is just as noisy as the filter(::nonNull). Would be nice to have something like Scala's .flatMap(Try(....).toOption) to just ignore exceptions in the stream.

Holger Over a year ago

Well, you could make a utility method doing exactly that.

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