Java SE 8

Java SE 8 presentation give at the LJC meeting in London on August 28th, 2014.

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Java 8:
Create The Future
Simon Ritter
Head of Java Technology Evangelism
Oracle Corp.

Twitter: @speakjava

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The following is intended to outline our general product
direction. It is intended for information purposes only, and may
not be incorporated into any contract. It is not a commitment
to deliver any material, code, or functionality, and should not
be relied upon in making purchasing decisions.

The development, release, and timing of any features or
functionality described for Oracle’s products remains at the
sole discretion of Oracle.

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Java SE 8 New Features
(Other Than Lambdas and Streams)

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Annotations On Java Types
! Annotations can currently only be used on type declarations
– Classes, methods, variable definitions
! Extension for places where types are used
– e.g. parameters
! Permits error detection by pluggable type checkers
– e.g. null pointer errors, race conditions, etc
public void process( @notnull List data) {…}

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Access To Parameter Names At Runtime
! Mechanism to retrieve parameter names of methods and constructors
–  At runtime via core reflection
! Improved code readability
– Eliminate redundant annotations
! Improve IDE capabilities
– Auto-generate template code
! Method and Constructor now inherit from new Executable class
– getParameters() returns array of Parameter objects
– Name, type, annotations for each parameter

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Concurrency Updates
! Scalable update variables
– DoubleAccumulator, DoubleAdder, etc
– Multiple variables avoid update contention
– Good for frequent updates, infrequent reads
! ConcurrentHashMap updates
– Improved scanning support, key computation
! ForkJoinPool improvements
– Completion based design for IO bound applications
– Thread that is blocked hands work to thread that is running

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Parallel Array Sorting
! Additional utility methods in java.util.Arrays
– parallelSort (multiple signatures for different primitives)
! Anticipated minimum improvement of 30% over sequential sort
– For dual core system with appropriate sized data set
! Built on top of the fork-join framework
– Uses Doug Lea’s ParallelArray implementation
– Requires working space the same size as the array being sorted

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Date And Time APIs
! A new date, time, and calendar API for the Java SE platform
! Supports standard time concepts
– Partial, duration, period, intervals
– date, time, instant, and time-zone
! Provides a limited set of calendar systems and be extensible to others
! Uses relevant standards, including ISO-8601, CLDR, and BCP47
! Based on an explicit time-scale with a connection to UTC

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Unicode 6.2
! Java SE 7 support Unicode 6.0
! Changes in Unicode 6.1 (February, 2012)
– Add 11 new blocks to java.lang.Character.UnicodeBlock
– Add 7 new scripts to java.lang.Character.UnicodeScript
– Support over 700 new characters in java.lang.Character, String,
and other classes
! Changes in Unicode 6.2 (September, 2012)
– Support a new Turkish currency sign (U+20BA)

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HTTP URL Permissions
! New type of network permission
– Grant access in terms of URLs, rather than IP addresses
! Current way to specify network permissions
– java.net.SocketPermission
– Not restricted to just HTTP
– Operates in terms of IP addresses only
! New, higher level capabilities
– Support HTTP operations (POST, GET, etc)
– Build on limited doPrivileged feature

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Compact Profiles
Approximate static footprint goals
Compact1 Profile
Compact2 Profile
Compact3 Profile
Full JRE 54Mb
30Mb
16Mb
11Mb

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Nashorn JavaScript Engine
! Lightweight, high-performance JavaScript engine
– Integrated into JRE
! Use existing javax.script API
! ECMAScript-262 Edition 5.1 language specification compliance
! New command-line tool, jjs to run JavaScript
! Internationalised error messages and documentation

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Retire Rarely-Used GC Combinations
! Rarely used
– DefNew + CMS
– ParNew + SerialOld
– Incremental CMS
! Large testing effort for little return
! Will generate deprecated option messages
– Won’t disappear just yet

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Remove The Permanent Generation
! No more need to tune the size of it
! Current objects moved to Java heap or native memory
– Interned strings
– Class metadata
– Class static variables
! Part of the HotSpot, JRockit convergence
Permanently

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Lambdas And Streams

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2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
1.4 5.0 6 7 8
java.lang.Thread
java.util.concurrent
(jsr166)
Fork/Join Framework
(jsr166y)
Project Lambda Concurrency in Java
Phasers, etc
(jsr166)

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The Problem: External Iteration
List<Student> students = ...
double highestScore = 0.0;
for (Student s : students) {
if (s.getGradYear() == 2011) {
if (s.getScore() > highestScore) {
highestScore = s.getScore() ;
}
}
}
• Our code controls iteration
• Inherently serial: iterate from
beginning to end
• Not thread-safe because
business logic is stateful
(mutable accumulator
variable)

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Internal Iteration With Inner Classes
! Iteration handled by the library
! Not inherently serial – traversal
may be done in parallel
! Traversal may be done lazily – so
one pass, rather than three
! Thread safe – client logic is
stateless
! High barrier to use
– Syntactically ugly
More Functional, Fluent
List<Student> students = ...
double highestScore = students.
filter(new Predicate<Student>() {
public boolean op(Student s) {
return s.getGradYear() == 2011 ;
}
}).
map(new Mapper<Student,Double>() {
public Double extract(Student s) {
return s.getScore();
}
}).
max();

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Internal Iteration With Lambdas
SomeList<Student> students = ...
double highestScore = students.
filter(Student s -> s.getGradYear() == 2011 ).
map(Student s -> s.getScore()).
max();

• More readable
• More abstract
• Less error-prone

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Lambda Expressions
! Lambda expressions represent anonymous functions
– Same structure as a method
! typed argument list, return type, set of thrown exceptions, and a body
– Not associated with a class
! We now have parameterised behaviour, not just values
Some Details
double highestScore = students.
filter(Student s -> s.getGradYear() == 2011 ).
map(Student s -> s.getScore() )
max();

What
How

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Lambda Expression Types
• Single-method interfaces are used extensively in Java
– Definition: a functional interface is an interface with one abstract method
– Functional interfaces are identified structurally
– The type of a lambda expression will be a functional interface
! Lambda expressions provide implementations of the abstract method
interface Comparator<T> { boolean compare(T x, T y); }
interface FileFilter { boolean accept(File x); }
interface Runnable { void run(); }
interface ActionListener { void actionPerformed(…); }
interface Callable<T> { T call(); }

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Local Variable Capture
• Lambda expressions can refer to effectively final local variables from
the enclosing scope
• Effectively final: A variable that meets the requirements for final variables
(i.e., assigned once), even if not explicitly declared final
• Closures on values, not variables
void expire(File root, long before) {
root.listFiles(File p -> p.lastModified() <= before);
}

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What Does ‘this’ Mean For Lambdas?
• ‘this’ refers to the enclosing object, not the lambda itself
• Think of ‘this’ as a final predefined local
• Remember the Lambda is an anonymous function
class SessionManager {
long before = ...;

void expire(File root) {
// refers to ‘this.before’, just like outside the lambda
root.listFiles( File p -> checkExpiry(p.lastModified(), this.before) );
}

boolean checkExpiry(long time, long expiry) { ... }
}

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Type Inference
! The compiler can often infer parameter types in a lambda expression
! Inferrence based on the target functional interface’s method signature
! Fully statically typed (no dynamic typing sneaking in)
– More typing with less typing
List<String> list = getList();
Collections.sort(list, (String x, String y) -> x.length() - y.length() );
Collections.sort(list, (x, y) -> x.length() - y.length() );
static T void sort(List<T> l, Comparator<? super T> c );

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Method References
• Method references let us reuse a method as a lambda expression
• Also work for constructors
FileFilter x = File f -> f.canRead() ;
FileFilter x = File::canRead;

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Library Evolution

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Library Evolution Goal
! Requirement: aggregate operations on collections
– New methods required on Collections to facilitate this





! This is problematic
– Can’t add new methods to interfaces without modifying all implementations
– Can’t necessarily find or control all implementations
int heaviestBlueBlock = blocks.
filter(b -> b.getColor() == BLUE).
map(Block::getWeight).
reduce(0, Integer::max);

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Solution: Default Methods
• Specified in the interface
• From the caller’s perspective, just an ordinary interface method
• Provides a default implementation
• Default only used when implementation classes do not provide a body for
the extension method
• Implementation classes can provide a better version, or not
interface Collection<E> {
default Stream<E> stream() {
return StreamSupport.stream(spliterator());
}
}

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Virtual Extension Methods
• Err, isn’t this implementing multiple inheritance for Java?
• Yes, but Java already has multiple inheritance of types
• This adds multiple inheritance of behavior too
• But not state, which is where most of the trouble is
• Can still be a source of complexity
• Class implements two interfaces, both of which have default methods
• Same signature
• How does the compiler differentiate?
• Static methods also allowed in interfaces in Java SE 8
Stop right there!

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Functional Interface Definition
! Single Abstract Method (SAM) type
! A functional interface is an interface that has one abstract method
– Represents a single function contract
– Doesn’t mean it only has one method
! Interfaces can now have static methods
! @FunctionalInterface annotation
– Helps ensure the functional interface contract is honoured
– Compiler error if not a SAM

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Lambdas In Full Flow:
Streams

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Stream Overview
! A stream pipeline consists of three types of things
– A source
– Zero or more intermediate operations
– A terminal operation
! Producing a result or a side-effect
Pipeline
int sum = transactions. stream().
filter(t -> t.getBuyer().getCity().equals(“London”)).
mapToInt(Transaction::getPrice).
sum();
Source
Intermediate operation
Terminal operation

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Stream Sources
! From collections and arrays
– Collection.stream()
– Collection.parallelStream()
– Arrays.stream(T array) or Stream.of()
! Static factories
– IntStream.range()
– Files.walk()
! Roll your own
– java.util.Spliterator
Many Ways To Create

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map and flatMap
Map Values in a Stream
! map (one to one)
– Each element of the input stream is mapped to an element in the output
stream
! flatMap (one to many)
– Each element of the input stream is mapped to a new stream
– Streams from all elements are concatenated to form one output stream

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Optional<T>
! Indicates that reference may, or may not have a value
– Makes developer responsible for checking
– A bit like a stream that can only have zero or one elements
Reducing NullPointerException Occurrences

Optional<GPSData> maybeGPS = Optional.ofNullable(gpsData);

maybeGPS.ifPresent(GPSData::printPosition);

GPSData gps = maybeGPS.orElse(new GPSData());

maybeGPS.filter(g -> g.lastRead() < 2).ifPresent(GPSData.display());

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Example 1
Convert words in list to upper case
List<String> output = wordList.
stream().
map(String::toUpperCase).
collect(Collectors.toList());

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Example 1
Convert words in list to upper case (in parallel)
List<String> output = wordList.
parallelStream().
map(String::toUpperCase).
collect(Collectors.toList());

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Example 2
Find words in list with even length
List<String> output = wordList.
parallelStream().
filter(w -> (w.length() & 1) == 0).
collect(Collectors.toList());

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Example 3
! BufferedReader has new method
– Stream<String> lines()

Count lines in a file
long count = bufferedReader.
lines().
count();

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Example 4
Join lines 3-4 into a single string
String output = bufferedReader.
lines().
skip(2).
limit(2).
collect(Collectors.joining() );

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Example 5
Collect all words in a file into a list
List<String> output = reader.
lines().
flatMap(line -> Stream.of(line.split(REGEXP))).
filter(word -> word.length() > 0).
collect(Collectors.toList());

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Example 6
List of unique words in lowercase, sorted by length
List<String> output = reader.
lines().
flatMap(line -> Stream.of(line.split(REGEXP))).
filter(word -> word.length() > 0).
map(String::toLowerCase).
distinct().
sorted((x, y) -> x.length() - y.length()).
collect(Collectors.toList());

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Example 7: Real World
Infinite stream from thermal sensor
thermalReader
.lines()
.mapToDouble(s ->
Double.parseDouble(s.substring(0, s.length() - )))
.map(t -> ((t – 32) / 5 / 9)
.filter(t -> t != currentTemperature)
.peek(t -> listener.ifPresent(l -> l.temperatureChanged(t)))
.forEach(t -> currentTemperature = t);

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Conclusions
! Java SE 8 is a significant change to Java
– New features in language, libraries and VM
! Java needs lambda statements
– Significant improvements in existing libraries are required
! Require a mechanism for interface evolution
– Solution: virtual extension methods
! Bulk operations on Collections
– Much simpler with Lambdas
! Java will continue to evolve to meet developer's needs

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