JarCastingjava8-utils 
The package provides helper methods for interacting with, e.g., the stream features of Java 8 and above.
Maven
<dependency>
<groupId>com.nitorcreations</groupId>
<artifactId>java8utils</artifactId>
<version>1.0.0</version>
</dependency>
Gradle
compile 'com.nitorcreations:java8utils:1.0.0'
Stream helpers
Transforming to streams:
NStreams.asStream(Optional<T>)
NStreams.asStream(Iterable<T>)
NStreams.asStream(Iterator<T>)
Concatenating multiple streams:
NStreams.concat(Stream...)
Collectors
Collecting elements of streams:
Map<Integer,Long> map = Stream.of(1, 1, 1, 3, 3, 6)
.collect(NCollectors.countingOccurrences());
// {1=3, 3=2, 6=1}
Predicates
Common predicates include, e.g.
NPredicates.is(T) // compare using ==
NPredicates.equalTo(T) // compare using #equals
NPredicates.not(Predicate<T>) // invert result of predicate
...
NPredicates.having(Function<T, S>, Predicate<S>) // for matching, e.g., properties in a stream
Example of having predicate usage:
Stream.of("foo", "foobar", "barbar", "qux")
.filter(s -> s.startsWith("f"))
.filter(havingEqual(String::length, 3))
.collect(toList()); // -> List("foo")
Includes predicates for collections, strings, and comparables:
NCollectionPredicates:
.empty() / .notEmpty()
.contains(T) / .containsAll(T...) / .containsAny(T...)
.doesNotContain()T / .doesNotContainsAllOf(T...) / .doesNotContainAnyOf(T...)
Not forgetting the optionals:
NOptionalPredicates
.empty() / .present()
.havingValue(T) / .havingValue(Predicate<T>)
.notHavingValue(T) / .notHavingValue(Predicate<T>)
Set helpers
Helpers for creating sets:
Set<V> set = NSets.asSet(Iterable<V>)
Set<V> set = NSets.asSet(Iterator<V>)
Set<V> set = NSets.asSet(V...)
Map helpers
Helpers for creating map entries:
Map.Entry<String,Integer> entry = NMappers.entryOf("a", 1);
Helpers for creating simple maps:
NMaps.mapOfEntries(entryOf("a", 1), entryOf("c", 2), ...);
NMaps.mapOf("a", 1, "c", 2, ...); // Up to ten entries
NMaps.mapping(asList("a", "bee", "cheetah"), s -> s.length()); // Map from keys to values by function
NMaps.mapOfLists(asList("a", "b", "c"), asList(1, 2, 3));
NMaps.combine(mapOf("a", 1), mapOf("b", 2, "c", 3));
Map entries as streams
A stream of Map.Entry<K, V> can be collected to a map by NCollectors.entriesToMap():
Stream<Entry<String, Integer>> stream = ...;
Map<String, Integer> map = stream.collect(NCollectors.entriesToMap());
Combining entries on duplicate keys:
Stream<Entry<String, Integer>> stream = ...;
Map<String, Integer> stream.collect(NCollectors.entriesToMap((a, b) -> a+b);
Consuming and mapping streams of entries
Entries can be consumed and manipulated in a multitude of ways:
// Just to show consumingEntry
NMaps.mapOf("a", 1, "b", 2)
.entrySet()
.forEach(consumingEntry((key, val) -> System.out.printf("%s -> %d%n", key, val)));
Further ways to manipulate entries in a stream:
Map<String, Integer> map = NMaps.mapOf("a", 1, "b", 2, "c", 3);
final Map<String, Integer> mapped = map.entrySet().stream()
.map(NMappers.mappingKey((k, v) -> k + v))
.map(NMappers.mappingValue((k, v) -> v * v))
.collect(NCollectors.entriesToMap());
// Mapped now contains { "a1"=1; "b2"=4; "c3"=9 }
Map extractors
Helper methods for consuming maps:
Stream<T> keyStream = NExtractors.doWithKeys(Map<K, ?> map, Function<K, T> fn);
Stream<T> valueStream = NExtractors.doWithValues(Map<?, V> map, Function<V, T> fn);
Keys and values to sets or lists:
NExtractors.extractKeysToSet(Map<F, ?> map, Function<F, T> fn)
NExtractors.extractKeysToList(Map<F, ?> map, Function<F, T> fn)
NExtractors.extractValuesToSet(Map<?, F> map, Function<F, T> fn)
NExtractors.extractValuesToList(Map<?, F> map, Function<F, T> fn)
