Java User Group - 9/2/2015Introduction to Java 8 Streams

Louvain-La-NeuveBelgium

Marc Tritchler

Some Theory

• Why Java 8• Java 8 Streams, Lambdas

Java Platform is simple … but

JEEJMS, JSF, JSP, EJB, CDI, JAX-WS, JAX-RS, JPA, JTA, JNDI, ….

JDK

JRE

JVM

JSE

JSR

JCP

History of Java SE (simplified)http://en.wikipedia.org/wiki/Java_version_history

• SE vs EE• J2 SE 1.4 2002 (dark ages but still a lot in the wild)

• J2 SE 5.0 2004 (ancient but major enhancements)

• Java SE 6 2006 (not major enhancements)

• Java SE 7 2011 (yes, its becoming sexy)

• Java SE 8 2014 (huge changes … Oh My God)

† ( No supported anymore by Oracle)

J2 SE 1.4 (2002)

• assert• regular expressions• exception chaining• Internet Protocol version 6 (IPv6) support• non-blocking IO (named New Input/Output, NIO)• logging API• image I/O API for reading and writing images in formats like JPEG

and PNG• integrated XML parser and XSLT processor (JAXP)• integrated security and cryptography extensions (JCE, JSSE, JAAS)• Java Web Start • Preferences API (java.util.prefs)

J2 SE 1.5 (2004)• Generics• annotations; allows language constructs such as classes and methods to be tagged with additional

data, which can then be processed by metadata-aware utilities. (Specified by JSR 175.)• Autoboxing/unboxing: Automatic conversions between primitive types (such as int) and primitive

wrapper classes (such as Integer). (Specified by JSR 201.)• Enumerations: The enum keyword creates a typesafe, ordered list of values (such as Day.MONDAY,

Day.TUESDAY, etc.). Previously this could only be achieved by non-typesafe constant integers or manually constructed classes (typesafe enum pattern). (Specified by JSR 201.)

• Varargs: The last parameter of a method can now be declared using a type name followed by three dots (e.g. void drawtext(String... lines)). In the calling code any number of parameters of that type can be used and they are then placed in an array to be passed to the method, or alternatively the calling code can pass an array of that type.

• Enhanced for each loop: The for loop syntax is extended with special syntax for iterating over each member of either an array or any Iterable, such as the standard Collection classes. (Specified by JSR 201.)

• Fix the previously broken semantics of the Java Memory Model, which defines how threads interact through memory.

• Static imports

Java SE 6 (2006)• Support for older Win9x versions dropped• Scripting Language Support Generic API for tight integration with scripting

languages, and built-in Mozilla JavaScript Rhino integration• Dramatic performance improvements for the core platform and Swing.• Improved Web Service support through JAX-WS • JDBC 4.0 support.• Java Compiler API : an API allowing a Java program to select and invoke a

Java Compiler programmatically.• Upgrade of JAXB to version 2.0: Including integration of a StAX parser.

Support for pluggable annotations• Many GUI improvements, such as integration of SwingWorker in the API,

table sorting and filtering, and true Swing double-buffering (eliminating the gray-area effect). JVM improvements include: synchronization and compiler performance optimizations, new algorithms and upgrades to existing garbage collection algorithms, and application start-up performance.[27

Java SE 7• JVM support for dynamic languages Scala, …• Compressed 64-bit pointers (available in Java 6 with -XX:+UseCompressedOops)• Strings in switch• Automatic resource management in try-statement• Improved type inference for generic instance creation, aka the diamond operator <>• Simplified varargs method declaration• Binary integer literals• Allowing underscores in numeric literals• Catching multiple exception types and rethrowing exceptions with improved type checking• Concurrency utilities under JSR 166• NIO.2 New file I/O library to enhance platform independence and add support for metadata and

symbolic links.• Timsort is used to sort collections and arrays of objects instead of merge sort• Library-level support for elliptic curve cryptography algorithms• An XRender pipeline for Java 2D, which improves handling of features specific to modern GPUs• New platform APIs for the graphics features originally implemented in version 6u10 as

unsupported APIs• Enhanced library-level support for new network protocols, including SCTP and Sockets Direct

Protocol• Upstream updates to XML and Unicode

Java 8 (2014)• lambda expressions (unofficially closures) • default methods (virtual extension methods) which make multiple inheritance possible in Java.

There was an ongoing debate in the Java community on whether to add support for lambda expressions.[Sun later declared that lambda expressions would be included in Java and asked for community input to refine the feature. Supporting lambda expressions also allows to perform functional-style operations on streams of elements, such as MapReduce-inspired transformations on collections. Default methods allow an author of API to add new methods to an interface without breaking the old code using it. It also provides a way to use multiple inheritance, multiple inheritance of implementation more precisely.

• JSR 223, JEP 174: Project Nashorn, a JavaScript runtime which allows developers to embed JavaScript code within applications

• Annotation on Java Types• Unsigned Integer Arithmetic• JSR 337, JEP 120: Repeating annotations• JSR 310, JEP 150: Date and Time API• JEP 178: Statically-linked JNI libraries• JEP 153: Launch JavaFX applications (direct launching of JavaFX application JARs)• JEP 122: Remove the permanent generation• Default methods (for API dev)• Functional interfaces (new ones)

What is a Stream ?

sequence of elements from a source that supports data processing operations

Ex: stream of bytes 4,5,9,0, ….

Why Streams ?WHAT vs HOW

• Like SQL … concentrate on the what, not the how = working at higher level …

Ex: SELECT name from USERS;

vs. C program (main, fopen(), while())• Internal loop vs external

Java 8 – Streams(java.util.stream)

• Old way (Java 7): Collections• Modern way (Java 8): Collections + Streams– Collections store & access data– Streams process data (filter, group, …) in 1 or

multicore

Stream pre-requesites

• Functional programming pass functions as parameters

• Lambda = anonymous function• Method references ::• Functional Interfaces

Predicate test()

Supplier<T> get()

Consumer<T> accept()

…

Stream creations

• From collections: List & Set new methods: stream()

parallelStream()• From file, from Arrays, …• For primitive data types int, long, double– IntStream, LongStream, DoubleStream– sum() and average()– mapToInt()and mapToObj()

Stream operations

• Intermediate or terminal– Intermediate: return Streams like LEGO or

UNIX commands: they can be combined !

ex: filter, map, sorted– Terminal: don't return a Stream (void or type)

ex: forEach

ExampleStream.iterate(0, n -> n + 3).limit(10).forEach(System.out::println);

• Complete javadoc

http://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html

Stream termination

• forEach• reduce• collect• flatMap

Part 2 - Practice

Requirements– Java 8 JDK, not the JRE LOL– IDE (Eclipse, NetBeans, …)

Sources– http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/

– Java 8 In Action (Manning Editions)

Streams : simple examples// from Collections

Arrays.asList("a1", "a2", "a3").stream().findFirst().ifPresent(System.out::println);

// from Objects references

Stream.of("a1", "a2", "a3").findFirst().ifPresent(System.out::println);

// empty Stream

Stream.empty();

// IntStream

IntStream.range(1, 4).forEach(System.out::println);

// Arrays -> Stream

Arrays.stream(new int[] {1, 2, 3}).map(n -> 2 * n + 1) .average() .ifPresent(System.out::println);

// Stream -> InStream

Stream.of("a1", "a2", "a3") .map(s -> s.substring(1)).mapToInt(Integer::parseInt) .max() .ifPresent(System.out::println);

// IntStream -> Stream

IntStream.range(1, 4).mapToObj(i -> "a" + i).forEach(System.out::println);

// Stream -> InStream -> Stream (LOL)

Stream.of(1.0, 2.0, 3.0).mapToInt(Double::intValue).mapToObj(i -> "a" + i) .forEach(System.out::println);

Processing Order

• Intermediate operations laziness (= intermediate operations will only be executed when a terminal operation is present)

Ex 1: Stream.of("d2", "a2", "b1", "b3", "c")

.filter(s -> { System.out.println("filter: " + s); return true; });

NO OUTPUT

.forEach(s -> System.out.println("forEach: " + s));

SURPRIZE: filter, foreach, filter, foreach, … processing order

Ex 2: Stream.of("d2", "a2", "b1", "b3", "c")

.map(s -> { System.out.println("map: " + s); return s.toUpperCase(); })

.anyMatch(s -> { System.out.println("anyMatch: " + s); return s.startsWith("A"); });

not all elements processed

Order Matters … on large data sets

Example: 2 intermediate operations (map() and filter() and 1 terminal operation forEach())

// map & filter called 5 times, forEach 1

Stream.of("d2", "a2", "b1", "b3", "c")

.map(s -> { System.out.println("map: " + s); return s.toUpperCase(); })

.filter(s -> { System.out.println("filter: " + s); return s.startsWith("A"); })

.forEach(s -> System.out.println("forEach: " + s));

Invert map() and filter():

// map called only 1 timeStream.of("d2", "a2", "b1", "b3", "c")

.filter(s -> { System.out.println("filter: " + s); return s.startsWith("a"); })

.map(s -> { System.out.println("map: " + s); return s.toUpperCase(); })

.forEach(s -> System.out.println("forEach: " + s));

Statefull intermediate operations// sorted: 8, filter: 5, map: 1, forEach: 1

Stream.of("d2", "a2", "b1", "b3", "c")

.sorted((s1, s2) -> { System.out.printf("sort: %s; %s\n", s1, s2); return s1.compareTo(s2); }) // statefull

.filter(s -> { System.out.println("filter: " + s); return s.startsWith("a"); })

.map(s -> { System.out.println("map: " + s); return s.toUpperCase(); })

.forEach(s -> System.out.println("forEach: " + s));

filter() sorted()

// sorted: 0, filter: 5, map:1, forEach: 1

Stream.of("d2", "a2", "b1", "b3", "c")

.filter(s -> { System.out.println("filter: " + s); return s.startsWith("a"); })

.sorted((s1, s2) -> { System.out.printf("sort: %s; %s\n", s1, s2); return s1.compareTo(s2); })

.map(s -> { System.out.println("map: " + s); return s.toUpperCase(); })

.forEach(s -> System.out.println("forEach: " + s));

Reusing Streams

• Java 8 streams cannot be reused (as soon as you call any terminal operation the stream is closed)Stream<String> stream = Stream.of("d2", "a2", "b1", "b3", "c")

.filter(s -> s.startsWith("a"));

stream.anyMatch(s -> true); // ok

stream.noneMatch(s -> true); // exception

• To overcome this limitation we have to to create a new stream chain for every terminal operation we want to execute, e.g. we could create a stream supplier to construct a new stream with all intermediate operations already set up:

Advanced Operations

• collect()• flatMap()• reduce()

collect()

• Terminal operation expecting a Collector (Stream Colletion or other)

• Collector http://docs.oracle.com/javase/8/docs/api/java/util/stream/Collector.html

– Supplier, Accumulator, Combinern, Finisher

• Collectors http://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html

static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)

Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

flatMap

reduce

A more complete example

• Create Transaction class• Create Enum for currencies