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JDK8 Lambdas and Streams:
Changing The Way You Think
When Developing Java
Simon Ritter
Head of Java Technology Evangelism
...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Safe Harbor Statement
The following is intended to ou...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Lambda & Streams
Primer
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Lambda Expressions In JDK8
• Old style, anonymous inn...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Lambda Expressions
• Lambda expressions represent ano...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Lambda Expression Types
• Single-method interfaces ar...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Local Variable Capture
• Lambda expressions can refer...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
What Does ‘this’ Mean In A Lambda
• ‘this’ refers to ...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Referencing Instance Variables
Which are not final, o...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Referencing Instance Variables
The compiler helps us ...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Type Inference
• The compiler can often infer paramet...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Method References
• Method references let us reuse a ...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Method References
13
Rules For Construction
Lambda
Me...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Constructor References
• Same concept as a method ref...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Useful Methods That Can Use Lambdas
• Iterable.forEac...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Useful Methods That Can Use Lambdas
• Collection.remo...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Useful Methods That Can Use Lambdas
• List.replaceAll...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Useful Methods That Can Use Lambdas
• List.sort(Compa...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Useful Methods That Can Use Lambdas
• Logger.finest(S...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Functional Interface Definition
• An interface
• Must...
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Is This A Functional Interface?
21
@FunctionalInterfa...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Is This A Functional Interface?
22
@FunctionalInterfa...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Is This A Functional Interface?
23
@FunctionalInterfa...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Stream Overview
• A stream pipeline consists of three...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Stream Sources
• From collections and arrays
– Collec...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Maps and FlatMaps
Map Values in a Stream
Map
FlatMap
...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
The Optional Class
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Optional<T>
Reducing NullPointerException Occurrences...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Optional Class
• Terminal operations like min(), max(...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Optional ifPresent()
Do something when set
if (x != n...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Optional filter()
Reject certain values of the Option...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Optional map()
Transform value if present
if (x != nu...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Optional flatMap()
Going deeper
public String findSim...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Update Our GPS Code
class GPSData {
public Optional<P...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Update Our GPS Code
String direction = Optional.ofNul...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Advanced Lambdas And Streams
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Streams and Concurrency
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Serial And Parallel Streams
• Collection Stream sourc...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Parallel Streams
• Implemented underneath using the f...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
When To Use Parallel Streams
• Data set size is impor...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
When To Use Parallel Streams
• N = size of the data s...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Streams: Pitfalls For The Unwary
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Functional v. Imperative
• For functional programming...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Counting Methods That Return Streams
44
Still Thinkin...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Counting Methods That Return Streams
45
Functional Wa...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Printing And Counting Functional Interfaces
46
Still ...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Printing And Counting Functional Interfaces
47
More F...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Printing And Counting Functional Interfaces
48
Even M...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
The Art Of Reduction
(Or The Need to Think Differentl...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
A Simple Problem
• Find the length of the longest lin...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Another Simple Problem
• Find the length of the longe...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Naïve Stream Solution
• That works, so job done, righ...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
External Iteration Solution
• Simple, but inherently ...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Recursive Approach: The Method
54
String findLongestS...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Recursive Approach: Solving The Problem
• No explicit...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
A Better Stream Solution
• The Stream API uses the we...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
A Better Stream Solution
• Use the recursive approach...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
A Better Stream Solution
• Use the recursive approach...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
The Simplest Stream Solution
• Use a specialised form...
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Conclusions
• Java needs lambda statements
– Signific...
Simon Ritter
Oracle Corporartion
Twitter: @speakjava
Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
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JDK8 Lambdas and Streams: Changing The Way You Think When Developing Java

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

  1. 1. JDK8 Lambdas and Streams: Changing The Way You Think When Developing Java Simon Ritter Head of Java Technology Evangelism Oracle Corp. Twitter: @speakjava Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
  2. 2. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Safe Harbor Statement 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. 2
  3. 3. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Lambda & Streams Primer
  4. 4. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Lambda Expressions In JDK8 • Old style, anonymous inner classes • New style, using a Lambda expression 4 Simplified Parameterised Behaviour new Thread(new Runnable { public void run() { doSomeStuff(); } }).start(); new Thread(() -> doSomeStuff()).start();
  5. 5. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  6. 6. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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(); }
  7. 7. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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); }
  8. 8. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  9. 9. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Referencing Instance Variables Which are not final, or effectively final class DataProcessor { private int currentValue; public void process() { DataSet myData = myFactory.getDataSet(); dataSet.forEach(d -> d.use(currentValue++)); } }
  10. 10. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 effectively final) inserted by the compiler
  11. 11. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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);
  12. 12. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Method References • Method references let us reuse a method as a lambda expression FileFilter x = File f -> f.canRead(); FileFilter x = File::canRead;
  13. 13. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Method References 13 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
  14. 14. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  15. 15. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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);
  16. 16. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Useful Methods That Can Use Lambdas • Collection.removeIf(Predicate p) List<String> myList = ... myList.removeIf(s -> s.length() == 0)
  17. 17. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Useful Methods That Can Use Lambdas • List.replaceAll(UnaryOperator o) List<String> myList = ... myList.replaceAll(s -> s.toUpperCase()); myList.replaceAll(String::toUpper);
  18. 18. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Useful Methods That Can Use Lambdas • List.sort(Comparator c) • Replaces Collections.sort(List l, Comparator c) List<String> myList = ... myList.sort((x, y) -> x.length() – y.length());
  19. 19. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Useful Methods That Can Use Lambdas • Logger.finest(Supplier<String> msgSupplier) • Overloads Logger.finest(String msg) logger.finest(produceComplexMessage()); logger.finest(() -> produceComplexMessage());
  20. 20. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 20
  21. 21. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 21 @FunctionalInterface public interface Runnable { public abstract void run(); } Yes. There is only one abstract method
  22. 22. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 22 @FunctionalInterface 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 still only one abstract method
  23. 23. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 23 @FunctionalInterface public interface Comparator { // default and static methods elided int compare(T o1, T o2); boolean equals(Object obj); } The equals(Object) method is implicit from the Object class Therefore only one abstract method
  24. 24. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  25. 25. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  26. 26. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  27. 27. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. The Optional Class
  28. 28. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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();
  29. 29. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  30. 30. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Optional ifPresent() Do something when set if (x != null) { print(x); } opt.ifPresent(x -> print(x)); opt.ifPresent(this::print);
  31. 31. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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);
  32. 32. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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);
  33. 33. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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);
  34. 34. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Update Our GPS Code class GPSData { public Optional<Position> getPosition() { ... } } class Position { public Optional<Latitude> getLatitude() { ... } } class Latitude { public String getString() { ... } }
  35. 35. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Update Our GPS Code String direction = Optional.ofNullable(gpsData) .flatMap(GPSData::getPosition) .flatMap(Position::getLatitude) .map(Latitude::getDirection) .orElse(“None”);
  36. 36. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Advanced Lambdas And Streams
  37. 37. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Streams and Concurrency
  38. 38. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 38
  39. 39. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 39
  40. 40. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 40 Sadly, no simple answer
  41. 41. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 41 Quantative Considerations
  42. 42. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Streams: Pitfalls For The Unwary
  43. 43. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  44. 44. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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()));
  45. 45. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Counting Methods That Return Streams 45 Functional Way sourceKeySet.stream() .mapToInt(c -> streamReturningMethodMap.get(c).size()) .sum();
  46. 46. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 46 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();
  47. 47. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 47 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
  48. 48. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 48 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
  49. 49. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. The Art Of Reduction (Or The Need to Think Differently)
  50. 50. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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();
  51. 51. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Another Simple Problem • Find the length of the longest line in a file 51
  52. 52. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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();
  53. 53. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. External Iteration Solution • Simple, but inherently serial • Not thread safe due to mutable state 53 String longest = ""; while ((String s = reader.readLine()) != null) if (s.length() > longest.length()) longest = s;
  54. 54. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Recursive Approach: The Method 54 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; }
  55. 55. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 55 List<String> lines = new ArrayList<>(); while ((String s = reader.readLine()) != null) lines.add(s); String longest = findLongestString("", 0, lines);
  56. 56. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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) 56
  57. 57. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • 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();
  58. 58. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • Use the recursive approach as an accululator for a reduction 58 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
  59. 59. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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) 59 reader.lines() .max(comparingInt(String::length)) .get();
  60. 60. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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
  61. 61. Simon Ritter Oracle Corporartion Twitter: @speakjava Copyright © 2014, Oracle and/or its affiliates. All rights reserved.

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