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Functional programming with_jdk8-s_ritter

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A two-hour session on Lambdas and Streams delivered at Progscon '15 in London on 17 April, 2015.

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Functional programming with_jdk8-s_ritter

  1. 1. Functional Programming With JDK8 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 Expressions
  4. 4. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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 LambdaConcurrency in Java Phasers, etc (jsr166)
  5. 5. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. The Problem: External Iteration 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 iteration • Inherently serial: iterate from beginning to end • Not thread-safe • Business logic is stateful • Mutable accumulator variable
  6. 6. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Internal Iteration With Inner Classes • Iteration 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 – Syntactically ugly More Functional 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();
  7. 7. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Internal Iteration 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
  8. 8. 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
  9. 9. 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(); }
  10. 10. 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); }
  11. 11. 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
  12. 12. 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++)); } }
  13. 13. 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
  14. 14. 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);
  15. 15. 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;
  16. 16. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Method References • Format: target_reference::method_name • Three kinds of method reference – Static method (e.g. Integer::parseInt) – Instance method of an arbitary type (e.g. String::length) – Instance method of an existing object 16 More Detail
  17. 17. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Method References 17 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
  18. 18. 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
  19. 19. 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);
  20. 20. 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)
  21. 21. 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);
  22. 22. 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());
  23. 23. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Library Evolution
  24. 24. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Library Evolution Goal • Requirement: aggregate operations on collections –New methods required on Collections to facilitate this • This is problematic – Can’t add new methods to interfaces without modifying all implementations – Can’t necessarily find or control all implementations int heaviestBlueBlock = blocks .filter(b -> b.getColor() == BLUE) .map(Block::getWeight) .reduce(0, Integer::max);
  25. 25. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Solution: Default Methods • Specified in the interface • From the caller’s perspective, just an ordinary interface method • Provides a default implementation • Default only used when implementation classes do not provide a body for the extension method • Implementation classes can provide a better version, or not interface Collection<E> { default Stream<E> stream() { return StreamSupport.stream(spliterator()); } }
  26. 26. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Virtual Extension Methods • Err, isn’t this implementing multiple inheritance for Java? • Yes, but Java already has multiple inheritance of types • This adds multiple inheritance of behavior too • But not state, which is where most of the trouble is • Can still be a source of complexity • Class implements two interfaces, both of which have default methods • Same signature • How does the compiler differentiate? • Static methods also allowed in interfaces in Java SE 8 Stop right there!
  27. 27. 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 27
  28. 28. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 28 @FunctionalInterface public interface Runnable { public abstract void run(); } Yes. There is only one abstract method
  29. 29. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 29 @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
  30. 30. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 30 @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
  31. 31. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Lambdas In Full Flow: Streams
  32. 32. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Stream Overview • Abstraction for specifying aggregate computations – Not a data structure – Can be infinite • Simplifies the description of aggregate computations – Exposes opportunities for optimisation – Fusing, laziness and parallelism At The High Level
  33. 33. 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
  34. 34. 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
  35. 35. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Stream Terminal Operations • The pipeline is only evaluated when the terminal operation is called – All operations can execute sequentially or in parallel – Intermediate operations can be merged • Avoiding multiple redundant passes on data • Short-circuit operations (e.g. findFirst) • Lazy evaluation – Stream characteristics help identify optimisations • DISTINT stream passed to distinct() is a no-op
  36. 36. 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
  37. 37. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. The Optional Class
  38. 38. 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();
  39. 39. 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
  40. 40. 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);
  41. 41. 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);
  42. 42. 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);
  43. 43. 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);
  44. 44. 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() { ... } }
  45. 45. 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”);
  46. 46. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Stream Examples
  47. 47. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 1 Convert words in list to upper case List<String> output = wordList .stream() .map(String::toUpperCase) .collect(Collectors.toList());
  48. 48. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 1 Convert words in list to upper case (in parallel) List<String> output = wordList .parallelStream() .map(String::toUpperCase) .collect(Collectors.toList());
  49. 49. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 2 • BufferedReader has new method – Stream<String> lines() Count lines in a file long count = bufferedReader .lines() .count();
  50. 50. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 3 Join lines 3-4 into a single string String output = bufferedReader .lines() .skip(2) .limit(2) .collect(Collectors.joining());
  51. 51. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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());
  52. 52. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. 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());
  53. 53. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 6 Create a map: Keys are word length, values are lists of words of that length Map<Integer, List<String>> result = reader.lines() .flatMap(line -> Stream.of(line.split(REGEXP))) .collect(groupingBy(String::length));
  54. 54. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 7 Create a map: Keys are word length, values are how many words of that length Map<Integer, Long> result = reader.lines() .flatMap(line -> Stream.of(line.split(REGEXP))) .collect(groupingBy(String::length, counting())); Downstream collector to return count of elements
  55. 55. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 8: 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);
  56. 56. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Example 8: 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);
  57. 57. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Advanced Lambdas And Streams
  58. 58. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Exception Handling In Lambdas
  59. 59. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Exception Handling In Lambda Expressions • Compiler error: uncaught exception in filter() Can Be Problematic public boolean isValid(Object o) throws IOException { ... } public List<String> processList(List<String> list) throws IOException { return list.stream() .filter(w -> isValid(w)) .collect(Collectors.toList()); }
  60. 60. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Exception Handling In Lambda Expressions 60 Catch the Exception In The Lambda public double processList(List list) { return list.stream() .filter(w -> { try { return isValid(w) } catch (IOException ioe) { return false; } }) .collect(Collectors.toList()); }
  61. 61. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Exception Handling In Lambda Expressions • Which is great, except you can't use it with Streams 61 Define A New Functional Interface @FunctionalInterface public interface PredicateWithException { public boolean test(String s) throws IOException; }
  62. 62. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Debugging
  63. 63. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Logging Mid-Stream The peek() method List<String> output = reader .lines() .flatMap(line -> Stream.of(line.split(REGEXP))) .filter(word -> word.length() > 5) .peek(s -> System.out.println(“Word = “ + s)) .collect(Collectors.toList());
  64. 64. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Using peek() To Set A Breakpoint List<String> output = reader .lines() .flatMap(line -> Stream.of(line.split(REGEXP))) .filter(word -> word.length() > 5) .peek(s -> s) .collect(Collectors.toList()); Set breakpoint on peek Some debuggers don’t like empty bodies
  65. 65. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Streams and Concurrency
  66. 66. 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 66
  67. 67. 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 67
  68. 68. 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 68 Sadly, no simple answer
  69. 69. 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 69 Quantative Considerations
  70. 70. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Streams: Pitfalls For The Unwary
  71. 71. 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… 71
  72. 72. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Counting Methods That Return Streams 72 Still Thinking Imperatively Set<String> sourceKeySet = streamReturningMethodMap.keySet(); LongAdder sourceCount = new LongAdder(); sourceKeySet.stream() .forEach(c -> sourceCount.add(streamReturningMethodMap.get(c).size()));
  73. 73. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Counting Methods That Return Streams 73 Functional Way sourceKeySet.stream() .mapToInt(c -> streamReturningMethodMap.get(c).size()) .sum();
  74. 74. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 74 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();
  75. 75. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 75 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
  76. 76. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 76 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
  77. 77. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. The Art Of Reduction (Or The Need to Think Differently)
  78. 78. 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 78 BufferedReader reader = ... reader.lines() .mapToInt(String::length) .max() .getAsInt();
  79. 79. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Another Simple Problem • Find the length of the longest line in a file 79
  80. 80. 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 80 String longest = reader.lines(). sort((x, y) -> y.length() - x.length()). findFirst(). get();
  81. 81. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. External Iteration Solution • Simple, but inherently serial • Not thread safe due to mutable state 81 String longest = ""; while ((String s = reader.readLine()) != null) if (s.length() > longest.length()) longest = s;
  82. 82. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Recursive Approach: The Method 82 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; }
  83. 83. 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 83 List<String> lines = new ArrayList<>(); while ((String s = reader.readLine()) != null) lines.add(s); String longest = findLongestString("", 0, lines);
  84. 84. 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) 84
  85. 85. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • Use the recursive approach as an accululator for a reduction 85 String longestLine = reader.lines() .reduce((x, y) -> { if (x.length() > y.length()) return x; return y; }) .get();
  86. 86. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • Use the recursive approach as an accululator for a reduction 86 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
  87. 87. 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) 87 reader.lines() .max(comparingInt(String::length)) .get();
  88. 88. 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
  89. 89. Simon Ritter Oracle Corporartion Twitter: @speakjava Copyright © 2014, Oracle and/or its affiliates. All rights reserved.

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