Improved Developer Productivity In JDK8

Improved Developer
Productivity With Java SE 8
Simon Ritter
Head of Java Technology Evangelism
Oracle Corp
Twitter: @speakjava
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.

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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)

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(@immutable List data) {…}

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

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

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

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)

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

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

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

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

Lambdas And Streams

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.score;
}
}
• Our code controls iteration
• Inherently serial: iterate from
beginning to end
• Not thread-safe
• Business logic is stateful
• Mutable accumulator variable

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
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();

Internal Iteration With Lambdas
List<Student> students = ...
double highestScore = students
.filter(Student s -> s.getGradYear() == 2011)
.map(Student s -> s.getScore())
.max();
• More readable
• More abstract
• Less error-prone
NOTE: This is not JDK8 code

Lambda Expressions
Some Details
• 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
double highestScore = students.
filter(Student s -> s.getGradYear() == 2011).
map(Student s -> s.getScore())
max();
What
How

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(); }

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);
}

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
– It is not associated with a class
– Therefore there can be no ‘this’ for the Lambda

Referencing Instance Variables
Which are not final, or effectively final
class DataProcessor {
private int currentValue;
public void process() {
DataSet myData = myFactory.getDataSet();
dataSet.forEach(d -> d.use(currentValue++));
}
}

Referencing Instance Variables
The compiler helps us out
class DataProcessor {
private int currentValue;
public void process() {
DataSet myData = myFactory.getDataSet();
dataSet.forEach(d -> d.use(this.currentValue++));
}
}
‘this’ (which is effectively final)
inserted by the compiler

static T void sort(List<T> l, Comparator<? super T> c);
List<String> list = getList();
Collections.sort(list, (String x, String y) -> x.length() - y.length());
Collections.sort(list, (x, y) -> x.length() - y.length());
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

Method And Constructor References
• Method references let us reuse a method as a lambda expression
FileFilter x = File f -> f.canRead();
FileFilter x = File::canRead;
• Same syntax works for constructors
Factory<List<String>> f = ArrayList<String>::new;

Library Evolution

Library Evolution Goal
• Requirement: aggregate operations on collections
–New methods required on Collections to facilitate this
int heaviestBlueBlock = blocks
.filter(b -> b.getColor() == BLUE)
.map(Block::getWeight)
.reduce(0, Integer::max);
• This is problematic
– Can’t add new methods to interfaces without modifying all implementations
– Can’t necessarily find or control all implementations

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());
}
}

Virtual Extension Methods
Stop right there!
• 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

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
• @FunctionalInterface annotation
– Helps ensure the functional interface contract is honoured
– Compiler error if not a SAM

Lambdas In Full Flow:
Streams

Source
Stream Overview
Pipeline
• 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
int total = transactions.stream()
.filter(t -> t.getBuyer().getCity().equals(“London”))
.mapToInt(Transaction::getPrice)
.sum();
Intermediate operation
Terminal operation

Stream Sources
Many Ways To Create
• 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

Stream Terminal Operations
• The pipeline is only evaluated when the terminal operation is called
– All operations can execute sequentially or in parallel
– Intermediate operations can be merged
• Avoiding multiple redundant passes on data
• Short-circuit operations (e.g. findFirst)
• Lazy evaluation
– Stream characteristics help identify optimisations
• DISTINT stream passed to distinct() is a no-op

Helping To Eliminate the NullPointerException
Optional Class
• Terminal operations like min(), max(), etc do not return a direct result
• Suppose the input Stream is empty?
• Optional<T>
– Container for an object reference (null, or real object)
– Think of it like a Stream of 0 or 1 elements
– use get(), ifPresent() and orElse() to access the stored reference
– Can use in more complex ways: filter(), map(), etc

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

Example 1
Convert words in list to upper case (in parallel)
List<String> output = wordList
.parralelStream()
.map(String::toUpperCase)

Example 2
Count lines in a file
• BufferedReader has new method
– Stream<String> lines()
long count = bufferedReader
.lines()
.count();

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

Example 4
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)

Example 5
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())

Example 6: Real World
Infinite stream from thermal sensor
private int double currentTemperature;
...
thermalReader
.lines()
.mapToDouble(s ->
Double.parseDouble(s.substring(0, s.length() - 1)))
.map(t -> ((t – 32) * 5 / 9)
.filter(t -> t != currentTemperature)
.peek(t -> listener.ifPresent(l -> l.temperatureChanged(t)))
.forEach(t -> currentTemperature = t);

Example 6: Real World
Infinite stream from thermal sensor
private int double currentTemperature;
...
thermalReader
.lines()
.mapToDouble(s ->
Double.parseDouble(s.substring(0, s.length() - 1)))
.map(t -> ((t – 32) * 5 / 9)
.filter(t -> t != this.currentTemperature)
.peek(t -> listener.ifPresent(l -> l.temperatureChanged(t)))
.forEach(t -> this.currentTemperature = t);

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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Improved Developer Productivity In JDK8

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