java-stream

Many Java Stream interface methods use lower bounded wildcard in parameters for example Stream<T> filter(Predicate<? super T> pred) and void forEach(Consumer<? super T> action) What is the advantage of using Predicate<? super T> over Predicate<T> here? I understand, with Predicate<? super T> as parameter, Predicate object of T and super types can be passed into the method but i can't think of a situation where a Predicate of super type needed over the specific type? For example if i have a Stream<Integer> i can pass Predicate<Integer>, Predicate<Number>, and Predicate<Object> objects as

If i have a list (~200 elements) of objects, with only few unique objects (~20 elements). I want to have only unique values. Between list.stream().collect(Collectors.toSet()) and list.stream().distinct().collect(Collectors.toList()) which is more efficient wrt latency and memory consumption ? Eugene While the answer is pretty obvious - don't bother with these details of speed and memory consumption for this little amount of elements and the fact that one returns a Set and the other a List ; there are some interesting small details (interesting IMO). Suppose you are streaming from a source that

Is there is a way to convert the following code to Java 8 Stream. final List ret = new ArrayList(values.size()); double tmp = startPrice; for (final Iterator it = values.iterator(); it.hasNext();) { final DiscountValue discountValue = ((DiscountValue) it.next()).apply(quantity, tmp, digits, currencyIsoCode); tmp -= discountValue.getAppliedValue(); ret.add(discountValue); } Java 8 streams complains about no final variable tmp ? Is there a way to solve such situations ? Local variable tmp defined in an enclosing scope must be final or effectively final First, change the code to use generics and

I have a List of List<Map<String, Object>> like this [ {"A": 50, "B": 100, "C": 200, "D": "Auction" }, { "A": 101322143.24, "B": 50243301.2, "C": 569, "D": "Sold Promissory Buyer" }, { "A": 500, "B": 1000, "C": 1500, "D": "Auction" }] I am using this stream API method to convert this list into Map finalSalesReportForSoldProperty.stream().collect(Collectors.toMap(tags -> ((String) tags.get("D")).replaceAll("[\\- ]", ""), Function.identity())); But it throws me java.lang.IllegalStateException: Duplicate key exception, because my list has duplicate keys I want to add the inner elements of

I have a Map<String, Long> map which I want to sort by the Long value in reversed order using the features of Java 8. With Google I found this thread which provides this solution Map<String, Long> sortedMap = map.entrySet().stream() .sorted(comparing(Entry::getValue)) .collect(toMap(Entry::getKey, Entry::getValue, (e1,e2) -> e1, LinkedHashMap::new)); If I want to have the order reversed in the comments it says to use comparing(Entry::getValue).reversed() instead of comparing(Entry::getValue) . However, the code doesn't work. But with this little adaption it does: Map<String, Long> sortedMap =

After having read this article , I wish to use Spring to stream database query results directly to a JSON response to ensure constant-memory usage (no greedy loading of a List in memory). Similar to what is done in the article with Hibernate, I assembled a greetingRepository object which returns a stream of the database contents based on a JdbcTemplate . In that implementation, I create an iterator over the queried ResultSet , and I return the stream as follows: return StreamSupport.stream(spliterator(), false).onClose(() -> { log.info("Closing ResultSetIterator stream"); JdbcUtils

I'm new to Streams and Reduce so I'm trying it out and have hit a problem: I have a list of counters which have a start counter and end counter. The startcounter of an item is always the endcounter of the previous. I have a list of these counters listItems which I want to loop through efficiently, filter out inactive records and then reduce the list into a new List where all the StartCounters are set. I have the following code: List<CounterChain> active = listItems.stream() .filter(e -> e.getCounterStatus() == CounterStatus.ACTIVE) .reduce(new ArrayList<CounterChain>(), (a,b) -> { b