1. Which of the following will not work as expected in multi-threaded environment (choose one)?

new LongAccumulator(Math::min, Integer.MAX_VALUE);  //a)
new LongAccumulator(Math::max, Integer.MIN_VALUE);  //b)
new LongAccumulator(Math::addExact, 0);             //c)
new LongAccumulator(Math::subtractExact, 0)         //d)

Answer d) - according to JavaDoc:

this class is only applicable to functions for which the order of accumulation does not matter

2. Implement the following function:

static <T> List<T> extractPresent(List<java.util.Optional<T>> opts) {
    //...
}

Answer

static <T> List<T> extractPresent(List<Optional<T>> opts) {
    final List<T> result = new ArrayList<>();
    for (Optional<T> opt : opts) {
        if(opt.isPresent()) {
            result.add(opt.get());
        }
    }
    return result;
}

static <T> List<T> extractPresent(List<Optional<T>> opts) {
    final List<T> result = new ArrayList<>();
    opts.forEach(opt -> {
        if(opt.isPresent()) {
            result.add(opt.get());
        }
    });
    return result;
}

static <T> List<T> extractPresent(List<Optional<T>> opts) {
    return opts.stream()
            .flatMap(opt -> opt.map(Collections::singletonList)
                    .orElse(Collections.emptyList())
                    .stream())
            .collect(toList());
}

static <T> List<T> extractPresent(List<Optional<T>> opts) {
    return opts.stream()
            .reduce(new ArrayList<>(), (list, opt) -> {
                opt.ifPresent(list::add);
                return list;
            }, (list1, list2) -> { list1.addAll(list2); return list1; });
}

3. Why is the following HTTP request-response pair non-idiomatic RESTful service?

> POST /user.xml?action=delete HTTP/1.1
> <id>42</id>

...

< HTTP/1.1 503 Internal server error
< {"error": "User 42 not found"

Answer

• POST used to delete, consider DELETE HTTP method
• Verb (delete) in URI
• .xml extension to signal content type, consider Accept header
• Singular resource name, plurar tend to be more popular
• ID passed in request body, consider /users/42 URI instead
• 5xx server-side error used to indicate client-side mistake (wrong ID)
• Error returned in JSON while request was in XML