Cheat sheet on Java streams

java-stream-readme.md

Java Streams

Overview

As part of Java 8 new features, lambda expressions are now supported (Project Lambda). See this very easy-to-read article from Brian Goetz State of the Lambda but more importantly focusing more on streams: State of the Lambda: Libraries Edition

Quick ways to create a stream

// From parameters
Stream<String> s = Stream.of("a", "b");

// From arrays
Arrays.stream(myArray)

// From collections
myCollections.stream()

// you also have specialized streams like IntStream, LongStream, DoubleStream, some example below:
IntStream ints = IntStream.range(0, 10_000);
IntStream rdInts = new Random().ints(1000);

Most common methods

• filter()
• mapToInt, mapToObj
• map()
• collect()
• findAny(), findFirst(), noneMatch()
• forEach()
• distinct()
• sorted() sorted(Comparator<? super T>)

Differences map() and flatmap()

These functions can be applied to

• Optional
• Stream

map()

map() returns a stream consisting of the results of applying the parameter function to the elements of the stream. This can potentially leads to producing output of type Stream<Stream<R>>, which are hard to work with. In such cases, flatmap() can be used (see below).

flatMap()

flatMap(), as it names indicates, flattens the result. Unlike map() which is only performing a transformation on each element of the stream, flatMap() also perform flattening of the inner streams in the stream resulting from the transformation.

Example:

String[] arr = { "map", "flatmap" }; 
Arrays.stream(arr).flatMap(word -> word.chars().mapToObj(o -> (char) o)).forEach(System.out::println);

This prints:

m
a
p
f
l
a
m
a
p

From stream to collection

Example, use: mystream.collect(Collectors.toList()), mystream.collect(Collectors.toCollection(ArrayList::new)

From stream to array

Example, use: mystream.toArray(String[]::new)

Reduction operations

• reduce(identity, BinaryOperator<T> accumulator)

Example:

String[] myArray = { "Where", "is", "my", "stream", "?" };
String result = Arrays.stream(myArray)
                .reduce("", (a,b) -> a + " " + b);
assert result.equals(" Where is my stream ?");               

Others...

• sum()
• average()
• count()

Parallelization

The motivation behind the stream API was mostly to enable easy parallelization

Use these methods:

• myStream.parallel()
• myCollection.parallelStream()