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Lambdas : Beyond The Basics

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Presentation from Riga Dev Day talking about Lambda expressions and the Stream API in JDK8. This session, as the title suggests, goes beyond the basics and discusses some of the different ways you need to think to use a functional style of programming in Java.

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Lambdas : Beyond The Basics

  1. 1. Lambda Expressions In JDK8 Going Beyond The Basics 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. Program Agenda Lambdas And Streams Primer Delaying Execution Avoiding loops in Streams The Art of Reduction Conclusions 1 2 3 4 5 Oracle Confidential – Internal/Restricted/Highly Restricted 3
  4. 4. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Lambdas And Streams Primer
  5. 5. 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 5 Simplified Parameterised Behaviour new Thread(new Runnable { public void run() { doSomeStuff(); } }).start(); new Thread(() -> doSomeStuff()).start();
  6. 6. 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
  7. 7. 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(); }
  8. 8. 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); }
  9. 9. 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
  10. 10. 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++)); } }
  11. 11. 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
  12. 12. 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);
  13. 13. 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 13
  14. 14. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 14 @FunctionalInterface public interface Runnable { public abstract void run(); } Yes. There is only one abstract method
  15. 15. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 15 @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
  16. 16. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Is This A Functional Interface? 16 @FunctionalInterface public interface Comparator { // Static and default 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
  17. 17. 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
  18. 18. 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
  19. 19. 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
  20. 20. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Lambda Expressions and Delayed Execution
  21. 21. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Performance Impact For Logging • Heisenberg’s uncertainty principle • Setting log level to INFO still has a performance impact • Since Logger determines whether to log the message the parameter must be evaluated even when not used 22 logger.finest(getSomeStatusData()); Always executed
  22. 22. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Supplier<T> • Represents a supplier of results • All relevant logging methods now have a version that takes a Supplier • Pass a description 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 conditional activities 23 logger.finest(() -> getSomeStatusData());
  23. 23. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Avoiding Loops In Streams
  24. 24. 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… 25
  25. 25. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Counting Methods That Return Streams 26 Still Thinking Impaeratively Way Set<String> sourceKeySet = streamReturningMethodMap.keySet(); LongAdder sourceCount = new LongAdder(); sourceKeySet.stream() .forEach(c -> sourceCount.add(streamReturningMethodMap.get(c).size()));
  26. 26. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Counting Methods That Return Streams 27 Functional Way sourceKeySet.stream() .mapToInt(c -> streamReturningMethodMap.get(c).size()) .sum();
  27. 27. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 28 Still Thinking Imperatively Way LongAdder newMethodCount = new LongAdder(); functionalParameterMethodMap.get(c).stream() .forEach(m -> { output.println(m); if (isNewMethod(c, m)) newMethodCount.increment(); }); return newMethodCount.intValue();
  28. 28. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 29 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
  29. 29. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Printing And Counting Functional Interfaces 30 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
  30. 30. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. The Art Of Reduction (Or The Need to Think Differently)
  31. 31. 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 32 BufferedReader reader = ... reader.lines() .mapToInt(String::length) .max() .getAsInt();
  32. 32. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Another Simple Problem • Find the length of the longest line in a file 33
  33. 33. 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 34 String longest = reader.lines(). sort((x, y) -> y.length() - x.length()). findFirst(). get();
  34. 34. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. External Iteration Solution • Simple, but inherently serial • Not thread safe due to mutable state 35 String longest = ""; while ((String s = reader.readLine()) != null) if (s.length() > longest.length()) longest = s;
  35. 35. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Recursive Approach: The Method 36 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; }
  36. 36. 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 37 List<String> lines = new ArrayList<>(); while ((String s = reader.readLine()) != null) lines.add(s); String longest = findLongestString("", 0, lines);
  37. 37. 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) • 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) 38
  38. 38. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • 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 – But back to front – And without all the stack frames or List overhead 39
  39. 39. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • Use the recursive approach as an accululator for a reduction 40 String longestLine = reader.lines() .reduce((x, y) -> { if (x.length() > y.length()) return x; return y; }) .get();
  40. 40. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. A Better Stream Solution • Use the recursive approach as an accululator for a reduction 41 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 partial result, which is the longest string found so far
  41. 41. 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) 42 reader.lines() .max(comparingInt(String::length)) .get();
  42. 42. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Conclusions
  43. 43. Copyright © 2014, Oracle and/or its affiliates. All rights reserved. Conclusions • Lambdas provide a simple way to parameterise behaviour • The Stream API provides a functional style of programming • Very powerful combination • Does require developers to think differently • Avoid loops, even non-obvious ones! 44
  44. 44. Simon Ritter Oracle Corporartion Twitter: @speakjava Copyright © 2014, Oracle and/or its affiliates. All rights reserved.

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