Stream API from Java 8 rewritten on iterators for Java 7 and below.

• Functional interfaces (Supplier, Function, Consumer etc);
• Stream/IntStream/LongStream/DoubleStream (without parallel processing, but with a variety of additional methods and with custom operators);
• Optional/OptionalBoolean/OptionalInt/OptionalLong/OptionalDouble classes;
• Exceptional class - functional way to deal with exceptions;
• Objects from Java 7.

Stream.of(/* array | list | set | map | anything based on Iterator/Iterable interface */)
    .filter(..)
    .map(..)
    ...
    .sorted()
    .forEach(..);
Stream.of(value1, value2, value3)...
IntStream.range(0, 10)...

Example project: https://github.com/aNNiMON/Android-Java-8-Stream-Example

Unlike Java 8 streams, Lightweight-Stream-API provides the ability to apply custom operators.

Stream.of(...)
    .custom(new Reverse<>())
    .forEach(...);

public final class Reverse<T> implements UnaryOperator<Stream<T>> {

    @Override
    public Stream<T> apply(Stream<T> stream) {
        final Iterator<? extends T> iterator = stream.getIterator();
        final ArrayDeque<T> deque = new ArrayDeque<T>();
        while (iterator.hasNext()) {
            deque.addFirst(iterator.next());
        }
        return Stream.of(deque.iterator());
    }
}

You can find more examples here.

In addition to backported Java 8 Stream operators, the library provides:

• filterNot - negated filter operator

  // Java 8
  stream.filter(((Predicate<String>) String::isEmpty).negate())
  // LSA
  stream.filterNot(String::isEmpty)

• select - filters instances of the given class

  // Java 8
  stream.filter(Integer.class::isInstance)
  // LSA
  stream.select(Integer.class)

• withoutNulls - filters only not null elements

  Stream.of("a", null, "c", "d", null)
      .withoutNulls() // [a, c, d]

• sortBy - sorts by extractor function

  // Java 8
  stream.sorted(Comparator.comparing(Person::getName))
  // LSA
  stream.sortBy(Person::getName)

• groupBy - groups by extractor function

  // Java 8
  stream.collect(Collectors.groupingBy(Person::getName)).entrySet().stream()
  // LSA
  stream.groupBy(Person::getName)

• chunkBy - partitions sorted stream by classifier function

  Stream.of("a", "b", "cd", "ef", "gh", "ij", "klmnn")
      .chunkBy(String::length) // [[a, b], [cd, ef, gh, ij], [klmnn]]

• sample - emits every n-th elements

  Stream.rangeClosed(0, 10)
      .sample(2) // [0, 2, 4, 6, 8, 10]

• slidingWindow - partitions stream into fixed-sized list and sliding over the elements

  Stream.rangeClosed(0, 10)
      .slidingWindow(4, 6) // [[0, 1, 2, 3], [6, 7, 8, 9]]

• takeWhile / dropWhile - introduced in Java 9, limits/skips stream by predicate function

  Stream.of("a", "b", "cd", "ef", "g")
      .takeWhile(s -> s.length() == 1) // [a, b]
  Stream.of("a", "b", "cd", "ef", "g")
      .dropWhile(s -> s.length() == 1) // [cd, ef, g]

• scan - iteratively applies accumulation function and returns Stream

  IntStream.range(1, 6)
      .scan((a, b) -> a + b) // [1, 3, 6, 10, 15]

• indexed - adds an index to every element, result is IntPair

  Stream.of("a", "b", "c")
      .indexed() // [(0 : "a"), (1 : "b"), (2 : "c")]

• filterIndexed / mapIndexed / takeWhileIndexed / takeUntilIndexed / dropWhileIndexed / reduceIndexed / forEachIndexed - indexed specialization of operators

  Stream.of("a", "b", "c")
      .mapIndexed((i, s) -> s + Integer.toString(i)) // [a0, b1, c2]

No more ugly try/catch in lambda expressions.

// Java 8
stream.map(file -> {
    try {
        return new FileInputStream(file);
    } catch (IOException ioe) {
        return null;
    }
})
// LSA
stream.map(Function.Util.safe(FileInputStream::new))

Download latest release or grab via Maven:

<dependency>
  <groupId>com.annimon</groupId>
  <artifactId>stream</artifactId>
  <version>1.1.9</version>
</dependency>

or Gradle:

dependencies {
  ...
  compile 'com.annimon:stream:1.1.9'
  ...
}

Also included version for Java ME. Checkout javame branch.

For use lambda expressions in Java 6, Java 7 or Android, take a look at Retrolambda repository.