lamda expressions tutorial

8/12/2019 Lamda Expressions Tutorial

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Copyright 2012, Oracle and/or its affiliates. All rights reserved.1

Virtual developer Day Java2014

Lambda Expressions TutorialSimon Ritter

Head of Java Technology Evangelism

Twitter: @speakjava


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2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2

1.4 5.0 6 7

java.lang.Thread

java.util.concurrent(jsr166)

Fork/Join Framework(jsr166y)

PConcurrency in Java

Phasers, etc(jsr166)


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Lambdas In Java


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The Problem: External Iteration

List students = ...

double highestScore = 0.0;

for (Student s : students) {

if (s.gradYear == 2011) {

if (s.score > highestScore) {

highestScore = s.score;

}

}

}

Client controls it Inherently serial

beginning to en

Not thread-safebusiness logic is

(mutable accum

variable)


5/33Copyright 2012, Oracle and/or its affiliates. All rights reserved.5

Internal Iteration With Inner Classes

! Iteration, filtering anaccumulation are h

library

! Not inherently seriamaybe done in par

! Traversal maybe done pass, rather tha

! Thread safe clienstateless

! High barrier to use Syntactically ugly

More Functional, Fluent

List students = ...

double highestScore = students.

filter(new Predicate() {

public boolean op(Student s) {

return s.getGradYear() == 2011;

}

}).

map(new Mapper() {

public Double extract(Student s) {

return s.getScore();

}

}).

max();


6/33Copyright 2012, Oracle and/or its affiliates. All rights reserved.6

Internal Iteration With Lambdas

SomeList students = ...

double highestScore = students.filter(Student s -> s.getGradYear() == 2011

map(Student s -> s.getScore()).

max(); More readab More abstra Less error-p No reliance Easier to ma


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Lambda Expressions

! Lambda expressions represent anonymous functions Like a method, has a typed argument list, a return type, a set

exceptions, and a body

Not associated with a class! We now have parameterised behaviour, not just values

Some Details

double highestScore = students.

filter(Students -> 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 interfaceis an interface with one abstra

! Not necessarily only one method Functional interfacesare identified structurally The typeof a lambda expression will be a functional interfaceinterface Comparator { boolean compare(T x, T y); }

interface FileFilter { boolean accept(File x); }

interface Runnable { void run(); }

interface ActionListener { void actionPerformed(); }

interface Callable { T call(); }


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Local Variable Capture

Lambda expressions can refer to effectively final local varithe enclosing scope Effectively final means that the variable meets the requirem

variables (i.e., assigned once), even if not explicitly declared

This is a form of type inferencevoid expire(File root, longbefore) {

root.listFiles(File p -> p.lastModified()


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Lexical Scoping

The meaning of names are the same inside the lambda as A this reference refers to the enclosing object, not the lam Think of this as a final predefined local Remember the type of a Lambda is a functionalinterface

class SessionManager {long before = ...;

void expire(File root) {

// refers to this.before, just like outside the lambdaroot.listFiles(File p -> checkExpiry(p.lastModified(), this.before

}

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


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Type Inferrence

! The compiler can often infer parameter types in a lambda e! Inferrence based on the target functional interfaces method! Fully statically typed (no dynamic typing sneaking in)

More typing with less typing

List ls = getList();Collections.sort(ls, (String x, String y) -> x.length() - y.le

Collections.sort(ls, (x, y) -> x.length() - y.length());

static T void sort(List l, Comparator


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Method References

Method references let us reuse a method as a lambda exp

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

FileFilter x = File::canRead;


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Constructor References

! Same concept as a method reference For the constructor

Factory f =ArrayList::new;

Factory f = () -> return newArrayList

Equivalent to


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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 Cant add new methods to interfaces without modifying all implem Cant 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: Extension Methods

Specified in the interface From the callers perspective, just an ordinary interface m Provides a default implementation

Default is only used when implementation classes do not provfor the extension method

Implementation classes can provide a better version, or not

AKA Defender Methods

interface Collection {

defaultStream stream() {

return StreamSupport.stream(spliterator());

}

}


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Virtual Extension Methods

Err, isnt this implementing multiple inheritance for Java?

Yes, but Java already has multiple inheritance of types This adds multiple inheritance of behaviortoo But not state, which is where most of the trouble is Can still be a source of complexity due to separate compilatio

dynamic linking Class implements two interfaces, both of which have defau Same signature How does the compiler differentiate?

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 m Represents a single function contract Doesnt mean it only has one method

!Abstract classes may be considered later!@FunctionalInterfaceannotation

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

Intermedia

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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Stream Sources Provide

!Access to stream elements! Decomposition (for parallel operations)

Fork-join framework! Stream characteristics

ORDERED DISTINCT SORTED SIZED SUBSIZEDNONNULL IMMUTABLE CONCURRENT


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Stream Terminal Operations

!Invoking a terminal operation executes the pipeline

All operations can execute sequentially or in parallel! Terminal operations can take advantage of pipeline charact

toArray()can avoid copying for SIZEDpipelines by allocatadvance


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mapand flatMapMap Values in a Stream

!map(one to one)

Each element of the input stream is mapped to an element in 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 outp


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Example 1Convert words in list to upper case

List output = wordList.

stream().

map(String::toUpperCase).

collect(Collectors.toList());


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Example 2Find words in list with even length

List output = wordList.

stream().

filter(w -> (w.length() & 1 == 0).



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Example 3

!BufferedReader has new method

Stream lines()

Count lines in a file

long count = bufferedReader.

lines().

count();


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Example 4Join lines 3-4 into a single string

String output = bufferedReader.

lines().

skip(2).

limit(2).

collect(Collectors.joining());


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Example 5Find the length of the longest line in a file

int longest = reader.

lines().

mapToInt(String::length).

max().

getAsInt(); Returns an Optiona


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Example 6Collect all words in a file into a list

List output = reader.

lines().

flatMap(line -> Stream.of(line.split(REG

filter(word -> word.length() > 0).



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Example 7List of words lowercased, in aphabetical order

List output = reader.

lines().

flatMap(line -> Stream.of(line.split(REG

filter(word -> word.length() > 0).

map(String::toLowerCase).sorted().



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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


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