java user group Louvain-La-Neuve on java 8 streams

1. Java User Group - 9/2/2015
Introduction to Java 8 Streams
Louvain-La-Neuve
Belgium
Marc Tritchler

2. Some Theory
• Why Java 8
• Java 8 Streams, Lambdas

3. Java Platform is simple … but
JEEJMS, JSF, JSP, EJB, CDI, JAX-WS, JAX-RS,
JPA, JTA, JNDI, ….
JDK
JRE
JVM
JSE
JSR
JCP

4. 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)

5. 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)

6. 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

7. 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

8. 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

9. 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)

10. What is a Stream ?
sequence of elements from a source that
supports data processing operations
Ex: stream of bytes 4,5,9,0, ….

11. 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

12. 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 

13. Stream pre-requesites
• Functional programming  pass functions as
parameters
• Lambda = anonymous function
• Method references ::
• Functional Interfaces
Predicate  test()
Supplier<T>  get()
Consumer<T>  accept()
…

14. 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()

15. 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
Example
Stream.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

16. Stream termination
• forEach
• reduce
• collect
• flatMap

17. 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)

18. 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);

19. 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

20. 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 time
Stream.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));

21. 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; %sn", 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; %sn", s1, s2); return s1.compareTo(s2); })
.map(s -> { System.out.println("map: " + s); return s.toUpperCase(); })
.forEach(s -> System.out.println("forEach: " + s));

22. 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:

23. Advanced Operations
• collect()
• flatMap()
• reduce()

24. 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.

25. flatMap

26. reduce

27. A more complete example
• Create Transaction class
• Create Enum for currencies