JDK8 Lambdas and Streams: Changing The Way You Think When Developing Java

JDK8 Lambdas and Streams:Changing The Way You Think When Developing Java

Simon RitterHead of Java Technology EvangelismOracle 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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Lambda & StreamsPrimer


Lambda Expressions In JDK8

• Old style, anonymous inner classes

• New style, using a Lambda expression

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Simplified Parameterised Behaviour

new Thread(new Runnable {public void run() {

doSomeStuff();}

}).start();

new Thread(() -> doSomeStuff()).start();


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


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 surrounding 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 In A Lambda

• ‘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 VariablesWhich 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 VariablesThe 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


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);


Method References

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

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

FileFilter x = File::canRead;


Method References

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Rules For Construction

Lambda

Method Ref

Lambda

Method Ref

Lambda

Method Ref

(args) -> ClassName.staticMethod(args)

(arg0, rest) -> arg0.instanceMethod(rest)

(args) -> expr.instanceMethod(args)

ClassName::staticMethod

ClassName::instanceMethod

expr::instanceMethod

instanceOf


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


Useful Methods That Can Use Lambdas

• Iterable.forEach(Consumer c)

List<String> myList = ...

myList.forEach(s -> System.out.println(s));

myList.forEach(System.out::println);



• Collection.removeIf(Predicate p)


myList.removeIf(s -> s.length() == 0)



• List.replaceAll(UnaryOperator o)


myList.replaceAll(s -> s.toUpperCase());

myList.replaceAll(String::toUpper);



• List.sort(Comparator c)

• Replaces Collections.sort(List l, Comparator c)


myList.sort((x, y) -> x.length() – y.length());



• Logger.finest(Supplier<String> msgSupplier)

• Overloads Logger.finest(String msg)

logger.finest(produceComplexMessage());

logger.finest(() -> produceComplexMessage());


Functional Interface Definition

• An interface

• Must have only one abstract method

– In JDK 7 this would mean only one method (like ActionListener)

• JDK 8 introduced default methods

– Adding multiple inheritance of types to Java

– These are, by definition, not abstract (they have an implementation)

• JDK 8 also now allows interfaces to have static methods

– Again, not abstract

• @FunctionalInterface can be used to have the compiler check

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Is This A Functional Interface?

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@FunctionalInterfacepublic interface Runnable {

public abstract void run();}

Yes. There is only one abstract method



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@FunctionalInterfacepublic interface Predicate<T> {

default Predicate<T> and(Predicate<? super T> p) {…};default Predicate<T> negate() {…};default Predicate<T> or(Predicate<? super T> p) {…};static <T> Predicate<T> isEqual(Object target) {…};boolean test(T t);

}

Yes. There is still only one abstract method



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@FunctionalInterfacepublic interface Comparator {

// default and static methods elidedint compare(T o1, T o2);boolean equals(Object obj);

}

The equals(Object) method is implicit from the Object class

Therefore only oneabstract method


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

Source

Intermediate operation

Terminal operation


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


Maps and FlatMapsMap Values in a Stream

Map

FlatMap

Input Stream

Input Stream

1-to-1 mapping

1-to-many mapping

Output Stream

Output Stream


The Optional Class


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 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

– gpsMaybe.filter(r -> r.lastReading() < 2).ifPresent(GPSData::display);

Helping To Eliminate the NullPointerException


Optional ifPresent()Do something when set

if (x != null) {print(x);

}

opt.ifPresent(x -> print(x));opt.ifPresent(this::print);


Optional filter()Reject certain values of the Optional

if (x != null && x.contains("a")) {print(x);

}

opt.filter(x -> x.contains("a")).ifPresent(this::print);


Optional map()Transform value if present

if (x != null) {String t = x.trim();if (t.length() > 1)

print(t);}

opt.map(String::trim).filter(t -> t.length() > 1).ifPresent(this::print);


Optional flatMap()Going deeper

public String findSimilar(String s)

Optional<String> tryFindSimilar(String s)

Optional<Optional<String>> bad = opt.map(this::tryFindSimilar);Optional<String> similar = opt.flatMap(this::tryFindSimilar);


Update Our GPS Code

class GPSData {public Optional<Position> getPosition() { ... }

}

class Position {public Optional<Latitude> getLatitude() { ... }

}

class Latitude {public String getString() { ... }

}


Update Our GPS Code

String direction = Optional.ofNullable(gpsData).flatMap(GPSData::getPosition).flatMap(Position::getLatitude).map(Latitude::getDirection).orElse(“None”);


Advanced Lambdas And Streams


Streams and Concurrency


Serial And Parallel Streams

• Collection Stream sources

– stream()

– parallelStream()

• Stream can be made parallel or sequential at any point

– parallel()

– sequential()

• The last call wins

–Whole stream is either sequential or parallel

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Parallel Streams

• Implemented underneath using the fork-join framework

• Will default to as many threads for the pool as the OS reports processors

–Which may not be what you want

System.setProperty("java.util.concurrent.ForkJoinPool.common.parallelism", "32767");

• Remember, parallel streams always need more work to process

– But they might finish it more quickly

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When To Use Parallel Streams

• Data set size is important, as is the type of data structure

– ArrayList: GOOD

– HashSet, TreeSet: OK

– LinkedList: BAD

• Operations are also important

– Certain operations decompose to parallel tasks better than others

– filter() and map() are excellent

– sorted() and distinct() do not decompose well

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Sadly, no simple answer


When To Use Parallel Streams

• N = size of the data set

• Q = Cost per element through the Stream pipeline

• N x Q = Total cost of pipeline operations

• The bigger N x Q is the better a parallel stream will perform

• It is easier to know N than Q, but Q can be estimated

• If in doubt, profile

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Quantative Considerations


Streams: Pitfalls For The Unwary


Functional v. Imperative

• For functional programming you should not modify state

• Java supports closures over values, not closures over variables

• But state is really useful…

43


Counting Methods That Return Streams

44

Still Thinking Imperatively

Set<String> sourceKeySet = streamReturningMethodMap.keySet();

LongAdder sourceCount = new LongAdder();

sourceKeySet.stream().forEach(c ->

sourceCount.add(streamReturningMethodMap.get(c).size()));


Counting Methods That Return Streams

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Functional Way

sourceKeySet.stream().mapToInt(c -> streamReturningMethodMap.get(c).size()).sum();


Printing And Counting Functional Interfaces

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Still Thinking Imperatively

LongAdder newMethodCount = new LongAdder();

functionalParameterMethodMap.get(c).stream().forEach(m -> {

output.println(m);

if (isNewMethod(c, m)) newMethodCount.increment();

});

return newMethodCount.intValue();



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More Functional, But Not Pure Functional

int count = functionalParameterMethodMap.get(c).stream().mapToInt(m -> {

int newMethod = 0;output.println(m);

if (isNewMethod(c, m)) newMethod = 1;

return newMethod}).sum();

There is still state being modified in the Lambda



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Even More Functional, But Still Not Pure Functional

int count = functionalParameterMethodMap.get(nameOfClass).stream().peek(method -> output.println(method)).mapToInt(m -> isNewMethod(nameOfClass, m) ? 1 : 0) .sum();

Strictly speaking printing is a side effect, which is not purely functional


The Art Of Reduction(Or The Need to Think Differently)


A Simple Problem

• Find the length of the longest line in a file

• Hint: BufferedReader has a new method, lines(), that returns a Stream

50

BufferedReader reader = ...

reader.lines().mapToInt(String::length).max().getAsInt();


Another Simple Problem

• Find the length of the longest line in a file

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Naïve Stream Solution

• That works, so job done, right?

• Not really. Big files will take a long time and a lot of resources

• Must be a better approach

52

String longest = reader.lines().sort((x, y) -> y.length() - x.length()).findFirst().get();


External Iteration Solution

• Simple, but inherently serial

• Not thread safe due to mutable state

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String longest = "";

while ((String s = reader.readLine()) != null)if (s.length() > longest.length())

longest = s;


Recursive Approach: The Method

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String findLongestString(String s, int index, List<String> l) {if (index >= l.size())return s;

if (index == l.size() - 1) {if (s.length() > l.get(index).length())return s;

return l.get(index);}

String s2 = findLongestString(l.get(start), index + 1, l);

if (s.length() > s2.length())return s;

return s2;}


Recursive Approach: Solving The Problem

• No explicit loop, no mutable state, we’re all good now, right?

• Unfortunately not - larger data sets will generate an OOM exception

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List<String> lines = new ArrayList<>();

while ((String s = reader.readLine()) != null)lines.add(s);

String longest = findLongestString("", 0, lines);


A Better Stream Solution

• The Stream API uses the well known filter-map-reduce pattern

• For this problem we do not need to filter or map, just reduce

Optional<T> reduce(BinaryOperator<T> accumulator)

• The key is to find the right accumulator

– The accumulator takes a partial result and the next element, and returns a new partial result

– In essence it does the same as our recursive solution

–Without all the stack frames

• BinaryOperator is a subclass of BiFunction, but all types are the same

• R apply(T t, U u) or T apply(T x, T y)

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• Use the recursive approach as an accululator for a reduction

57

String longestLine = reader.lines().reduce((x, y) -> {

if (x.length() > y.length())return x;

return y;}).get();



• Use the recursive approach as an accululator for a reduction

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String longestLine = reader.lines().reduce((x, y) -> {

if (x.length() > y.length())return x;

return y;}).get();

x in effect maintains state for us, by always holding the longest string found so far


The Simplest Stream Solution

• Use a specialised form of max()

• One that takes a Comparator as a parameter

• comparingInt() is a static method on Comparator– Comparator<T> comparingInt(ToIntFunction<? extends T> keyExtractor)

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reader.lines().max(comparingInt(String::length)).get();


Conclusions

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

Simon RitterOracle Corporartion

Twitter: @speakjava


Software

JDK8 Lambdas and Streams: Changing The Way You Think When Developing Java