Lambdas’and’Streams’in’JDK’8’ · Copyright©’2014...

Lambdas and Streams in JDK 8

Simon Ri4er Head of Java Technology Evangelism Oracle Corp Twi4er: @speakjava

Copyright © 2014, Oracle and/or its affiliates. All rights reserved.


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1996 … 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 1.0 5.0 6 7 8 java.lang.Thread

java.util.concurrent (jsr166)

Fork/Join Framework (jsr166y)

Project Lambda Concurrency in Java

Phasers, etc (jsr166)


Lambdas In Java


The Problem: External IteraRon

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 iteraRon •  Inherently serial: iterate from

beginning to end •  Not thread-‐safe

•  Business logic is stateful •  Mutable accumulator variable


Internal IteraRon With Inner Classes

•  IteraRon 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

– SyntacRcally ugly

More FuncRonal 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 IteraRon 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

•  Lambda expressions represent anonymous funcRons – Same structure as a method

•  typed argument list, return type, set of thrown excepRons, 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


Lambda Expression Types

•  Single-‐method interfaces are used extensively in Java – DefiniRon: a func2onal interface is an interface with one abstract method – Func2onal interfaces are idenRfied structurally – The type of a lambda expression will be a func2onal interface

•  Lambda expressions provide implementaRons 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 effec2vely final local variables from the enclosing scope

•  EffecRvely 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 func2on

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


Referencing Instance Variables Which are not final, or effecRvely 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 effecRvely final) inserted by the compiler


Type Inference

•  The compiler can ogen infer parameter types in a lambda expression § Inferrence based on the target funcRonal interface’s method signature

•  Fully staRcally 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);


Method References

•  Method references let us reuse a method as a lambda expression

FileFilter x = File f -‐> f.canRead();

FileFilter x = File::canRead;


Constructor References

•  Same concept as a method reference – For the constructor

Factory<List<String>> f = ArrayList<String>::new;

Factory<List<String>> f = () -‐> return new ArrayList<String>();

Replace with


Library EvoluRon


Library EvoluRon Goal

• Requirement: aggregate operaRons on collecRons – New methods required on CollecRons to facilitate this

• This is problemaRc – Can’t add new methods to interfaces without modifying all implementaRons – Can’t necessarily find or control all implementaRons

int heaviestBlueBlock = blocks .filter(b -‐> b.getColor() == BLUE) .map(Block::getWeight) .reduce(0, Integer::max);


SoluRon: Default Methods

• Specified in the interface • From the caller’s perspecRve, just an ordinary interface method • Provides a default implementaRon

•  Default only used when implementaRon classes do not provide a body for the extension method

•  ImplementaRon classes can provide a be4er version, or not

interface Collection<E> { default Stream<E> stream() { return StreamSupport.stream(spliterator()); } }


Virtual Extension Methods

•  Err, isn’t this implemenRng mulRple inheritance for Java? •  Yes, but Java already has mulRple inheritance of types •  This adds mulRple inheritance of behavior too •  But not state, which is where most of the trouble is •  Can sRll be a source of complexity

•  Class implements two interfaces, both of which have default methods •  Same signature •  How does the compiler differenRate?

•  StaRc methods also allowed in interfaces in Java SE 8

Stop right there!


FuncRonal Interface DefiniRon

•  Single Abstract Method (SAM) type • A funcRonal interface is an interface that has one abstract method

– Represents a single funcRon contract – Doesn’t mean it only has one method

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


Lambdas In Full Flow: Streams


Stream Overview

• AbstracRon for specifying aggregate computaRons – Not a data structure – Can be infinite

•  Simplifies the descripRon of aggregate computaRons – Exposes opportuniRes for opRmisaRon – Fusing, laziness and parallelism

At The High Level


Stream Overview

• A stream pipeline consists of three types of things – A source – Zero or more intermediate operaRons – A terminal operaRon

•  Producing a result or a side-‐effect

Pipeline

int total = transactions.stream() .filter(t -‐> t.getBuyer().getCity().equals(“London”)) .mapToInt(Transaction::getPrice) .sum();

Source

Intermediate operaRon Terminal operaRon


Stream Sources

•  From collecRons and arrays – Collection.stream() – Collection.parallelStream() – Arrays.stream(T array) or Stream.of()

•  StaRc factories – IntStream.range() – Files.walk()

• Roll your own – java.util.Spliterator

Many Ways To Create


Stream Terminal OperaRons

•  The pipeline is only evaluated when the terminal operaRon is called – All operaRons can execute sequenRally or in parallel – Intermediate operaRons can be merged

•  Avoiding mulRple redundant passes on data •  Short-‐circuit operaRons (e.g. findFirst) •  Lazy evaluaRon

– Stream characterisRcs help idenRfy opRmisaRons •  DISTINT stream passed to distinct() is a no-‐op


Maps and FlatMaps Map Values in a Stream

Map

FlatMap

Input Stream

Input Stream

1-‐to-‐1 mapping

1-‐to-‐many mapping

Output Stream

Output Stream


OpRonal Class

•  Terminal operaRons 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

Helping To Eliminate the NullPointerException


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 .parallelStream() .map(String::toUpperCase) .collect(Collectors.toList());


Example 2

• BufferedReader has new method – Stream<String> lines()

Count lines in a file

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) .collect(Collectors.toList());


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()) .collect(Collectors.toList());


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() -‐ ))) .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 needs lambda statements – Significant improvements in exisRng libraries are required

• Require a mechanism for interface evoluRon – SoluRon: virtual extension methods

• Bulk operaRons on CollecRons – Much simpler with Lambdas

•  Java SE 8 evolves the language, libraries, and VM together

Simon Ri4er Oracle CorporarRon Twi4er: @speakjava


Lambdas’and’Streams’in’JDK’8’ · Copyright©’2014...

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Transcript of Lambdas’and’Streams’in’JDK’8’ · Copyright©’2014...