Lambda Lambada

Transcript

1. Lambda Lambada Functional Programming in Java
2. Well, actually ... • We will discuss Lambda Expressions and higher order functions • We will explore the Stream API • But there is more to functional programming, we won't cover • Handling side effects • Immutable datastructures • Monads
3. Agenda • Introduction • Lambdas, Higher Order Functions and Functional Interfaces • Stream API • Hands on Session (Java 8 & IDE required)
4. About us Jens Bendisposto bendisposto@hhu.de @jbendisposto Visit us at rheinjug.de clojure-duesseldorf.de Christian Meter meter@hhu.de @cmeter_
5. Functions are first class citizen* * i.e., they can be pushed around by a cruel developer
6. Lambda Expressions • Parameters must not hide local variables • We can add types (or var) to the parameters • If there is exactly one parameter, we can omit the parenthesis • If the body is a single expression, we can omit return and the curly braces (a,b,c,..) -> { body } Parameterlist
7. Examples • dec = (Integer e) -> { return e - 1; } • square = (e) -> e * e; • inc = e -> e + 1; • div = (n1,n2) -> { 
 if (n2 != 0) 
 return (n1/n2); 
 else 
 throw new IllegalArgumentException("...");
 };
8. Using Lambda Expressions • Can be passed to methods if a functional interface is expected • Single method interface (+ default methods) • Predefined interfaces: java.util.function
9. java.util.function • Consumer a → void, BiConsumer (a,b) → void • Function a → b, BiFunction (a,b) → c • Supplier ( ) → a • Predicate a → boolean BiPredicate (a,b) → boolean • UnaryOperator a → a, BinaryOperator (a,a) → a • Specialized interfaces for primitives (double, int, long)
10. Happy little accidents Bild: The joy of painting
11. Evaluating Lambda Expressions • someFunction.apply(3); • someConsumer.accept("foo"); • somePredicate.test("bar"); • someSupplier.get(); • someRunnable.run(); • ...
 Method name depends on the functional interface
12. Higher Order Function (nothing special: just a function that takes or returns a function)
13. filter ( != ) b r b r y g map ( firstLetter ) b r b r y g reduce ( + ) "brbryg" The usual suspects ...
14. DIY HOF static <A, B> Collection<B> map(Function<A, B> f, Collection<A> c) { Collection<B> result = new ArrayList<>(); for (A a : c) result.add(f.apply(a)); return result; } Function as a parameter static <A> Supplier<A> constantly(A v) { return () -> v; } Function as a return value
15. reduce static <A, E> A reduce(A init, BiFunction<A, E, A> rf, Collection<E> c) { A a = init; for (E e : c) a = rf.apply(a,e); return a; } reduce function (a,e) → a e1 a0 e2 a1 ... e3 a2 a
16. Anonymous inner classes? • We know how to pass functions around almost since the dawn of Java • So, are Lambda Expressions just syntactic sugar? new Thread(new Runnable() { public void run() { // do some stuff } }).start(); new Thread(() -> {/* do some stuff */});
17. Favor lambda expressions over anonymous inner classes
18. Don't overuse lambdas Lambda expressions are great for simple small functions Function<Integer, Boolean> perfectNumber = n -> { int sum = 0; for (int i = 1; i < n; i++) { if (n % i == 0) { sum += i; } } return n == sum; };
19. private boolean perfect(int n) { int sum = 0; for (int i = 1; i < n; i++) { if (n % i == 0) { sum += i; } } return n == sum; } n -> perfect(n); this::perfect You can have names!
20. Extract code into methods
23. Processing Collections • Before Java 8: Loops • Since Java 8: Streams
24. What's wrong with the good old for loop that I used for 20 years?
25. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
26. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 100 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
27. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X 100 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
28. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X 100 144 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
29. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X 100 144 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
30. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X 100 144 196 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
31. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X X 100 144 196 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
32. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XX 100 144 196 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
33. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX 100 144 196 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
34. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X 100 144 196 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
35. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X 100 144 196 400 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
36. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X 100 144 196 400 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
37. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X X 100 144 196 400 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
38. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X X 100 144 196 400 840 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8
39. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X X 100 144 196 400 840 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8 X X XXX X 100 144 196 400
40. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X X 100 144 196 400 840 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8 X X XXX X X 100 144 196 400
41. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X X 100 144 196 400 840 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8 X X XXX X X 100 144 196 400 144
42. Given a collection ns of numbers: • Find the sum • of the squares • of the first k numbers • of the even numbers • greater than 5 • If there are less than k matching numbers take all of them Inspired by Venkat Subramaniam's talk given at US Devoxx ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 4 X X XXX X X X 100 144 196 400 840 ns = [10,11,12,17,14,2,3,4,5,20,27,12], k = 8 X X XXX X X 100 144 196 400 984 144
43. Solution A Is this code correct? public static int compute(List<Integer> ns, int k) { int result = 0; int found = 0; for (int i = 0; (i <= ns.size() && found < k); i++) { int v = ns.get(i); if (v > 5 && v % 2 == 0) { result += v * v; found++; } } return result; }
44. Solution A Is this code correct? public static int compute(List<Integer> ns, int k) { int result = 0; int found = 0; for (int i = 0; (i <= ns.size() && found < k); i++) { int v = ns.get(i); if (v > 5 && v % 2 == 0) { result += v * v; found++; } } return result; }
45. Solution B How about this code? public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); }
46. Solution B How about this code? public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); }
47. So ... what is the problem with for loops?
48. http://www.sideshowcollectors.com/forums/marvel-action-figures/176882-spectulation- hot-toys-mms-spider-homecoming-vulture-collectible-figure-20.html
49. http://www.sideshowcollectors.com/forums/marvel-action-figures/176882-spectulation- hot-toys-mms-spider-homecoming-vulture-collectible-figure-20.html
50. Bild: https://www.flickr.com/photos/mr_t_in_dc/
51. It's simpler! • Reasoning about imperative code is hard! • Brain has to simulate the computer • Iteration logic mixed* with domain logic • Reasoning about functional code is easier • Single pass • Iteration logic is abstracted away * or as a Clojure programmer would say: complected
52. Streams • Generate a stream (e.g. from a collection) • Apply some intermediate operations (returning a stream)
 Examples: map, filter, distinct, sorted, ... • Apply a terminal Operation (reduces the stream) • Note • Operations are combined into a single operation • Evaluation is driven by the terminal operation - without a terminal operation it is a no op (lazy evaluation)
53. Streams Source Collection Processing Pipeline Result stream() collect(..) reduce(..) ns .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0,(x,y)->x+y)
54. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); }
55. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); } unboxing
56. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); } unboxing unboxing
57. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); } unboxing unboxing unboxing + boxing
58. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); } unboxing unboxing unboxing + boxing unboxing + boxing
59. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); } unboxing unboxing unboxing + boxing unboxing + boxing A lot of unboxing and boxing is going on Faster: Use specialized primitive streams
60. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .mapToInt(x -> x * x) .limit(k) .sum(); }
61. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .mapToInt(x -> x * x) .limit(k) .sum(); } IntStream instead of Stream<Integer>
62. Performance public static int compute(List<Integer> ns, int k) { return ns.stream() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .mapToInt(x -> x * x) .limit(k) .sum(); } IntStream instead of Stream<Integer> Specialized reduce methods
63. collect • reduce - immutable reduction • Initial value of Type A • reducing function BiFunction<A,E,A> • joining function: BinaryOperator<A> • collect - mutable reduction • Supplier<A> for initial value • BiConsumer<A,E> • BiConsumer<A,A>
64. Collectors • collect can take a java.util.stream.Collector (encapsulates the functions) • see java.util.stream.Collectors for predefined Collectors • toList() / toSet() • joining(...) • groupingBy(...) • ...
65. Parallel Streams public static int compute(List<Integer> ns, int k) { return ns.stream() .parallel() .filter(x -> x % 2 == 0) .filter(x -> x > 5) .map(x -> x * x) .limit(k) .reduce(0, (x,y)->x+y); } Uses fork join framework (Java 7)
66. Parallel stream fork fork reduce reduce join join
67. Constraints • Reduce (accumulator) function must be associative • Initial value must be the (left) identity for the reduce function • Combine function must be associative
68. Test with parallel streams