amaembo Polishing pull request #183
1. flatOption -> mapPartial
2. JavaDoc, CHANGES, CHEATSHEET updated
3. Tests use stream generators
4. More efficient implementation
Latest commit 78c0490 Aug 5, 2018

StreamEx 0.6.7

Enhancing Java 8 Streams.

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This library defines four classes: StreamEx, IntStreamEx, LongStreamEx, DoubleStreamEx which are fully compatible with Java 8 stream classes and provide many additional useful methods. Also EntryStream class is provided which represents the stream of map entries and provides additional functionality for this case. Finally there are some new useful collectors defined in MoreCollectors class as well as primitive collectors concept.

Full API documentation is available here.

Take a look at the Cheatsheet for brief introduction to the StreamEx!

Before updating StreamEx check the migration notes and full list of changes.

StreamEx library main points are following:

  • Shorter and convenient ways to do the common tasks.
  • Better interoperability with older code.
  • 100% compatibility with original JDK streams.
  • Friendliness for parallel processing: any new feature takes the advantage on parallel streams as much as possible.
  • Performance and minimal overhead. If StreamEx allows to solve the task using less code compared to standard Stream, it should not be significantly slower than the standard way (and sometimes it's even faster).


Collector shortcut methods (toList, toSet, groupingBy, joining, etc.)

List<String> userNames = StreamEx.of(users).map(User::getName).toList();
Map<Role, List<User>> role2users = StreamEx.of(users).groupingBy(User::getRole);
StreamEx.of(1,2,3).joining("; "); // "1; 2; 3"

Selecting stream elements of specific type

public List<Element> elementsOf(NodeList nodeList) {
    return IntStreamEx.range(nodeList.getLength())

Adding elements to stream

public List<String> getDropDownOptions() {
    return StreamEx.of(users).map(User::getName).prepend("(none)").toList();

public int[] addValue(int[] arr, int value) {
    return IntStreamEx.of(arr).append(value).toArray();

Removing unwanted elements and using the stream as Iterable:

public void copyNonEmptyLines(Reader reader, Writer writer) throws IOException {
    for(String line : StreamEx.ofLines(reader).remove(String::isEmpty)) {

Selecting map keys by value predicate:

Map<String, Role> nameToRole;

public Set<String> getEnabledRoleNames() {
    return StreamEx.ofKeys(nameToRole, Role::isEnabled).toSet();

Operating on key-value pairs:

public Map<String, List<String>> invert(Map<String, List<String>> map) {
    return EntryStream.of(map).flatMapValues(List::stream).invert().grouping();

public Map<String, String> stringMap(Map<Object, Object> map) {
    return EntryStream.of(map).mapKeys(String::valueOf)

Map<String, Group> nameToGroup;

public Map<String, List<User>> getGroupMembers(Collection<String> groupNames) {
    return StreamEx.of(groupNames).mapToEntry(nameToGroup::get)

Pairwise differences:

DoubleStreamEx.of(input).pairMap((a, b) -> b-a).toArray();

Support of byte/char/short/float types:

short[] multiply(short[] src, short multiplier) {
    return IntStreamEx.of(src).map(x -> x*multiplier).toShortArray(); 

Define custom lazy intermediate operation recursively:

static <T> StreamEx<T> scanLeft(StreamEx<T> input, BinaryOperator<T> operator) {
        return input.headTail((head, tail) -> scanLeft(tail.mapFirst(cur -> operator.apply(head, cur)), operator)

And more!


This project is licensed under Apache License, version 2.0


Releases are available in Maven Central

Before updating StreamEx check the migration notes and full list of changes.


Add this snippet to the pom.xml dependencies section:



Add this snippet to the build.gradle dependencies section:

implementation 'one.util:streamex:0.6.7'

Pull requests are welcome.