Lambda Expressions In JDK8: Going Beyond The Basics

Transcription

1 Lambda Expressions In JDK8: Going Beyond The Basics Simon Ri?er Head of Java Technology Evangelism Oracle Corp Copyright 2014, Oracle and/or its affiliates. All rights reserved.

2 Safe Harbor Statement The following is intended to outline our general product direcvon. It is intended for informavon purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or funcvonality, and should not be relied upon in making purchasing decisions. The development, release, and Vming of any features or funcvonality described for Oracle s products remains at the sole discrevon of Oracle. 2

3 Lambdas And Streams Primer

4 Lambda Expressions In JDK8 Simplified Parameterised Behaviour Old style, anonymous inner classes new Thread(new Runnable { public void run() { dosomestuff(); } }).start(); New style, using a Lambda expression new Thread(() - > dosomestuff()).start(); 4

5 Lambda Expressions Some Details Lambda expressions represent anonymous funcvons Same structure as a method typed argument list, return type, set of thrown excepvons, 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). What map(student s - > s.getscore()) max(); How

6 Lambda Expression Types Single- method interfaces are used extensively in Java DefiniVon: a func%onal interface is an interface with one abstract method Func%onal interfaces are idenvfied structurally The type of a lambda expression will be a func%onal interface Lambda expressions provide implementavons 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(); }

7 Local Variable Capture Lambda expressions can refer to effec%vely final local variables from the surrounding scope EffecVvely 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); }

8 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 func%on It is not associated with a class Therefore there can be no this for the Lambda

9 Referencing Instance Variables Which are not final, or effecvvely final class DataProcessor { private int currentvalue; } public void process() { DataSet mydata = myfactory.getdataset(); dataset.foreach(d - > d.use(currentvalue++)); }

10 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 effecvvely final) inserted by the compiler

11 Type Inference The compiler can oden infer parameter types in a lambda expression Inferrence based on the target funcvonal interface s method signature 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()); Fully stavcally typed (no dynamic typing sneaking in) More typing with less typing

12 FuncVonal Interface DefiniVon 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 mulvple inheritance of types to Java These are, by definivon, not abstract (they have an implementavon) JDK 8 also now allows interfaces to have stavc methods Again, not can be used to have the compiler check 12

13 Is This A FuncVonal public interface Runnable { public abstract void run(); } Yes. There is only one abstract method 13

14 Is This A FuncVonal public 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 svll only one abstract method 14

15 Is This A FuncVonal public interface Comparator { // Static and default methods elided int compare(t o1, T o2); boolean equals(object obj); } Therefore only one abstract method The equals(object) method is implicit from the Object class 15

16 Stream Overview Pipeline A stream pipeline consists of three types of things A source Zero or more intermediate operavons A terminal operavon Producing a result or a side- effect int total = transactions.stream().filter(t - > t.getbuyer().getcity().equals( London )).maptoint(transaction::getprice).sum(); Terminal operavon Source Intermediate operavon

17 Stream Sources Many Ways To Create From collecvons and arrays Collection.stream() Collection.parallelStream() Arrays.stream(T array) or Stream.of() StaVc factories IntStream.range() Files.walk() Roll your own java.util.spliterator

18 OpVonal Class Helping To Eliminate the NullPointerException Terminal operavons 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);

19 Lambda Expressions And Delayed ExecuVon

20 Performance Impact For Logging Heisenberg s uncertainty principle Always executed logger.finest(getsomestatusdata()); Selng log level to INFO svll has a performance impact Since Logger determines whether to log the message the parameter must be evaluated even when not used 20

21 Supplier<T> Represents a supplier of results All relevant logging methods now have a version that takes a Supplier logger.finest(() - > getsomestatusdata()); Pass a descripvon of how to create the log message Not the message If the Logger doesn t need the value it doesn t invoke the Lambda Can be used for other condivonal acvvives 21

22 Avoiding Loops In Streams

23 FuncVonal v. ImperaVve For funcvonal programming you should not modify state Java supports closures over values, not closures over variables But state is really useful 23

24 CounVng Methods That Return Streams S4ll Thinking Impera4vely Set<String> sourcekeyset = streamreturningmethodmap.keyset(); LongAdder sourcecount = new LongAdder(); sourcekeyset.stream().foreach(c - > sourcecount.add(streamreturningmethodmap.get(c).size())); Of course, could have just used sourcekeyset.size() 24

25 CounVng Methods That Return Streams Func4onal Way sourcekeyset.stream().maptoint(c - > streamreturningmethodmap.get(c).size()).sum(); 25

26 PrinVng And CounVng FuncVonal Interfaces S4ll Thinking Impera4vely LongAdder newmethodcount = new LongAdder(); functionalparametermethodmap.get(c).stream().foreach(m - > { output.println(m); if (isnewmethod(c, m)) newmethodcount.increment(); }); return newmethodcount.intvalue(); 26

27 PrinVng And CounVng FuncVonal Interfaces More Func4onal, But Not Pure Func4onal 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 svll state being modified in the Lambda 27

28 PrinVng And CounVng FuncVonal Interfaces Even More Func4onal, But S4ll Not Pure Func4onal int count = functionalparametermethodmap.get(nameofclass).stream().peek(method - > output.println(method)).maptoint(m - > isnewmethod(nameofclass, m)? 1 : 0).sum(); Strictly speaking prinvng is a side effect, which is not purely funcvonal 28

29 The Art Of ReducVon (Or The Need tothink Differently)

30 A Simple Problem Find the length of the longest line in a file Hint: BufferedReader has a new method, lines(), that returns a Stream BufferedReader reader =... reader.lines().maptoint(string::length).max().getasint(); 30

31 Another Simple Problem Find the length of the longest line in a file 31

32 Naïve Stream SoluVon String longest = reader.lines(). sort((x, y) - > y.length() - x.length()). findfirst(). get(); That works, so job done, right? Not really. Big files will take a long Vme and a lot of resources Must be a be?er approach 32

33 External IteraVon SoluVon String longest = ""; while ((String s = reader.readline())!= null) if (s.length() > longest.length()) longest = s; Simple, but inherently serial Not thread safe due to mutable state 33

34 Recursive Approach: The Method String findlongeststring(string s, int index, List<String> l) { 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; 34

35 Recursive Approach: Solving The Problem List<String> lines = new ArrayList<>(); while ((String s = reader.readline())!= null) lines.add(s); String longest = findlongeststring("", 0, lines); No explicit loop, no mutable state, so we re all good now, right? Unfortunately not - larger data sets will generate an OOM excepvon 35

36 A Be?er Stream SoluVon The Stream API uses the well known filter- map- reduce pa?ern For this problem we do not need to filter or map, just reduce Optional<T> reduce(binaryoperator<t> accumulator) BinaryOperator is a subclass of BiFunction All types are the same R apply(t t, U u) or T apply(t x, T y) 36

37 A Be?er Stream SoluVon The key is to find the right accumulator The accumulator takes a parval result and the next element, and returns a new parval result In essence it does the same as our recursive soluvon But back to front And without all the stack frames or List overhead IteraVon is internal 37

38 A Be?er Stream SoluVon Use the recursive approach as an accululator for a reducvon String longestline = reader.lines().reduce((x, y) - > { if (x.length() > y.length()) return x; return y; }).get(); 38

39 A Be?er Stream SoluVon Use the recursive approach as an accululator for a reducvon 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 providing the parval result, which is the longest string found so far 39

40 The Simplest Stream SoluVon Use a specialised form of max() One that takes a Comparator as a parameter reader.lines().max(comparingint(string::length)).get(); comparingint() is a stavc method on Comparator Comparator<T> comparingint(tointfunction<? extends T> keyextractor) 40

41 Conclusions

42 Conclusions Lambdas provide a simple way to parameterise behaviour The Stream API provides a funcvonal style of programming Very powerful combinavon Does require developers to think differently Avoid loops, even non- obvious ones! 42

43 Simon Ri?er Oracle CorporarVon Copyright 2014, Oracle and/or its affiliates. All rights reserved.