Lambdas and Streams in Java SE 8: Making Bulk Operations simple - Simon Ritter

Lambdas & Streams In JDK8 Making Bulk Opera/ons Simple

Simon Ri6er Head of Java Technology Evangelism Oracle Corp. Twi6er: @speakjava

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


Safe Harbor Statement The following is intended to outline our general product direcTon. It is intended for informaTon purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or funcTonality, and should not be relied upon in making purchasing decisions. The development, release, and Tming of any features or funcTonality described for Oracle’s products remains at the sole discreTon of Oracle.

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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 IteraTon

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 iteraTon •  Inherently serial: iterate from

beginning to end •  Not thread-‐safe

•  Business logic is stateful •  Mutable accumulator variable


Internal IteraTon With Inner Classes

•  IteraTon 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

– SyntacTcally ugly

More FuncTonal 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 IteraTon 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 funcTons – Same structure as a method

•  typed argument list, return type, set of thrown excepTons, 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 – DefiniTon: a func2onal interface is an interface with one abstract method – Func2onal interfaces are idenTfied structurally – The type of a lambda expression will be a func2onal interface

•  Lambda expressions provide implementaTons 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

•  EffecTvely 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 effecTvely 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 effecTvely final) inserted by the compiler


Type Inference

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

•  Fully staTcally 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 EvoluTon


Library EvoluTon Goal

• Requirement: aggregate operaTons on collecTons – New methods required on CollecTons to facilitate this

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

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


SoluTon: Default Methods

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

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

•  ImplementaTon classes can provide a be6er version, or not

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


Virtual Extension Methods

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

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

•  StaTc methods also allowed in interfaces in Java SE 8

Stop right there!


FuncTonal Interface DefiniTon

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

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

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


Lambdas In Full Flow: Streams


Aggregate OperaTons

• Most business logic is about aggregate operaTons – “Most profitable product by region” – “Group transacTons by currency”

• As we have seen, up to now, Java uses external iteraTon – Inherently serial – FrustraTngly imperaTve

•  Java SE 8’s answer: The Stream API – With help from Lambdas


Stream Overview

• AbstracTon for specifying aggregate computaTons – Not a data structure – Can be infinite

•  Simplifies the descripTon of aggregate computaTons – Exposes opportuniTes for opTmisaTon – 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 operaTons – A terminal operaTon

•  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 operaTon Terminal operaTon


Stream Sources

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

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

• Roll your own – java.util.Spliterator

Many Ways To Create


Stream Sources Provide

• Access to stream elements • DecomposiTon (for parallel operaTons)

– Fork-‐join framework

•  Stream characterisTcs – ORDERED – SORTED – DISTINCT – SIZED – NONNULL – IMMUTABLE – CONCURRENT


Stream Terminal OperaTons

•  The pipeline is only evaluated when the terminal operaTon is called – All operaTons can execute sequenTally or in parallel – Intermediate operaTons can be merged

•  Avoiding mulTple redundant passes on data •  Short-‐circuit operaTons (e.g. findFirst) •  Lazy evaluaTon

– Stream characterisTcs help idenTfy opTmisaTons •  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


Optional<T> Reducing NullPointerException Occurrences String direction = gpsData.getPosition().getLatitude().getDirection(); String direction = “UNKNOWN”; if (gpsData != null) { Position p = gpsData.getPosition(); if (p != null) { Latitude latitude = p.getLatitude(); if (latitude != null) direction = latitude.getDirection(); } }

String direction = gpsData.getPosition().getLatitude().getDirection();


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());


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 exisTng libraries are required

• Require a mechanism for interface evoluTon – SoluTon: virtual extension methods

• Bulk operaTons on CollecTons – Much simpler with Lambdas

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

Simon Ri6er Oracle CorporarTon Twi6er: @speakjava


Presentations & Public Speaking

Lambdas and Streams in Java SE 8: Making Bulk Operations simple - Simon Ritter