JSR 335 / java 8 - update reference
- 1. <Insert Picture Here> JSR-335 Update Lambda expressions for the Java Language Brian Goetz Java Language Architect Oracle Corporation
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- 3. JSR-335 • JSR 335 is a coordinated co-evolution of the Java platform • Language – lambda expressions (closures), interface evolution, better type inference • Libraries – Bulk parallel operations on collections • VM – support for default methods and lambda conversion • Major step forward for the Java programming model • More parallel-friendly • Enable delivery of more powerful libraries • Enable developers to write more concise, less error-prone code
- 4. Closures for Java – a long and winding road • 1997 – Java 1.1 added inner classes – a weak form of closures • Too bulky, complex name resolution rules, many limitations • In 2006-2008, a vigorous community debate about closures • Multiple proposals, including BGGA and CICE • Each had a different orientation • BGGA – creating control abstraction in libraries • CICE – reducing syntactic overhead of inner classes • Things ran aground at this point… • Little evolution from Java SE 5 (2004) until now • Project Coin (Small Language Changes) in Java SE 7
- 5. Closures for Java – a long and winding road
- 6. Closures for Java – a long and winding road • Dec 2009 – OpenJDK Project Lambda formed • November 2010 – JSR-335 filed • Current status • EDR #3 filed • Prototype RI (source and binary) available on OpenJDK • Component of Java SE 8 • JSR-335 = Lambda Expressions + Interface Evolution + Bulk Collection Operations
- 7. Evolving a major language • Key evolutionary forces • Adapting to change • Everything changes: hardware, attitudes, fashions, problems, demographics • Righting what’s wrong • Inconsistencies, holes, poor user experience • Maintaining compatibility • Low tolerance for change that will break anything • Preserving the core • Can’t alienate user base in quest for “something better” • Easy to focus on cool new stuff, but there’s lots of cool old stuff too
- 8. Adapting to Change • In 1995, most mainstream languages did not support closures • Perceived to be “too hard” for ordinary developers • Today, Java is just about the last holdout that doesn’t • C++ added them recently • C# added them in 3.0 • New languages being designed today all do
- 9. Adapting to Change • Language design is influenced by the dominant hardware • Which changes over time • In 1995, pervasive sequentiality infected programming language design • For loops are sequential • Why wouldn’t they be? Why invite nondeterminism? • Determinism is convenient – when free • Similarly, Iterator/Iterable is sequential • Pervasive mutability • Mutability is convenient – when free • Object creation was expensive and mutation cheap • In today’s world, these are just the wrong defaults! • Can’t just outlaw for loops and mutability • Instead, gently encourage something better
- 10. Problem – External Iteration • “Take the red blocks and colors them blue” • Typical solution with foreach loop • Loop is inherently sequential • Wasn’t a big problem 20 years ago, but times change • Client has to manage iteration • Conflates “what” with “how” • This is called external iteration • Hides complex interaction between library and client for (Shape s : shapes) { if (s.getColor() == RED) s.setColor(BLUE); }
- 11. Internal Iteration • Re-written to use lambda and Collection.forEach • Not just a syntactic change! • Now the library is in control • Internal iteration – More what, less how • Client passes behavior into the API as data • Library can use parallelism, out-of-order, laziness • Also enable more powerful, expressive APIs • Greater power to abstract over behavior shapes.forEach(s -> { if (s.getColor() == RED) s.setColor(BLUE); })
- 12. Lambda Expressions • A lambda expression is an anonymous method • Has an argument list, a return type, and a body (Object o) -> o.toString() • Can refer to values from the enclosing lexical scope (Person p) -> p.getName().equals(name) • Compiler can often infer parameter types from context p -> p.getName().equals(name) • A method reference is a reference to an existing method Object::toString • All of these forms allow you to treat code as data • Behavior can be stored in variables and passed to methods
- 13. What is the type of a lambda? • Most languages with lambdas have some notion of a function type • Java language has no concept of function type • JVM has no native (unerased) representation of function type in VM type signatures • Adding function types would create many questions • How do we represent functions in VM type signatures? • How do we create instances of function types? • Want to avoid significant VM changes • Obvious tool for representing function types is generics • But then function types would be … erased
- 14. Functional Interfaces • Historically used single-method interfaces to model functions • Runnable, Comparator, ActionListener • Let’s just give these a name: functional interfaces • And add some new ones like Predicate<T>, Block<T> • A lambda expression evaluates to an instance of a functional interface Predicate<String> isEmpty = s -> s.isEmpty(); Predicate<String> isEmpty = String::isEmpty; Runnable r = () -> { System.out.println(“Boo!”) };
- 15. Functional Interfaces • “Just add function types” was obvious … and wrong • Would have introduced complexity and corner cases • Would have bifurcated libraries into “old” and “new” styles • Would have created interoperability challenges • Preserve the Core • Stodgy old approach may be better than shiny new one • Bonus: existing libraries are now forward-compatible to lambdas • Libraries that never imagined lambdas still work with them! • Maintains significant investment in existing libraries • Fewer new concepts
- 16. Problem – Interface Evolution • Example used a new Collection method – forEach() • I thought you couldn’t add new methods to interfaces? • Interfaces are a double-edged sword • Cannot compatibly evolve them unless you control all implementations • Reality: APIs age • As we add cool new language features, existing APIs look even older! • Lots of bad options for dealing with aging APIs • Let the API stagnate • Replace it in entirety (every few years!) • Nail bags on the side (e.g., Collections.sort())
- 17. Interface Evolution • Libraries need to evolve, or they stagnate • Need a mechanism for compatibly evolving APIs • New feature: default methods • Virtual interface method with default implementation • “default” is the dual of “abstract” • Three simple rules for resolving inheritance conflicts • Superclasses win over superinterfaces • More specific interfaces win over less specific • After that, concrete classes must override interface Collection<T> { default void forEach(Block<T> action) { for (T t : this) action.apply(t); } }
- 18. Default Methods • Similar to, but different from, C# extension methods • Java’s default methods are virtual and declaration-site • Core principle: API owners should control their APIs • Primary goal is API evolution • Inheritance rules directed at this primary goal • But very useful as an inheritance mechanism on its own! • Wait, is this multiple inheritance in Java? • Java always had multiple inheritance of types • This adds multiple inheritance of behavior • But not of state, where most of the trouble comes from
- 19. It’s All About The Libraries • Generally, we prefer to evolve the programming model through libraries • Time to market – can evolve libraries faster than language • Decentralized – more library developers than language developers • Risk – easier to change libraries, more practical to experiment • Impact – language changes require coordinated changes to multiple compilers, IDEs, and other tools • Sometimes we reach the limits of what is practical to express in libraries, and need a little help from the language • A little help, in the right places, can go a long way!
- 20. Lambdas Enable Better APIs • Lambda expressions enable more powerful APIs • Boundary between client and library is more permeable • Client provides bits of behavior to be mixed into execution (“what”) • Library remains in control of the computation (“how”) • Safer, exposes more opportunities for optimization • Key effect on APIs is: more composability • Leads to better factoring, more regular client code, more reuse • Lambdas in the language → can write better libraries → more readable, less error-prone user code
- 21. Example – Sorting • Default methods can enhance composability • Comparator.reverse(), Comparator.compose() • Default methods offer a “right place” to put certain code interface Comparator<T> { int compare(T o1, T o2); default Comparator<T> reverse() { return (o1, o2) -> –(compare(o1, o2)); } default Comparator<T> compose(Comparator<T> other) { return (o1, o2) -> { int cmp = compare(o1, o2); return (cmp != 0) ? cmp : other.compare(o1, o2); } } } Comparator<Person> byFirst = ... Comparator<Person> byLast = ... Comparator<Person> byFirstLast = byFirst.compose(byLast); Comparator<Person> byLastDescending = byLast.reverse();
- 22. Example – Sorting • If we want to sort a List today, we write a Comparator • Many layers of nastiness here! • Conflates extraction of sort key with ordering of that key • Collections class required for helper methods • Syntactically verbose • Could replace with a lambda, but only gets us so far • Better to untangle the intertwined aspects • Fewer opportunities for reuse Collections.sort(people, new Comparator<Person>() { public int compare(Person x, Person y) { return x.getLastName().compareTo(y.getLastName()); } });
- 23. Example – Sorting • Lambdas encourage finer-grained APIs • We add a method that takes a “key extractor” and returns Comparator • The comparing() method is one built for lambdas • Higher-order function • Eliminates redundancy, boilerplate Comparator<Person> byLastName = Comparators.comparing(p -> p.getLastName()); Class Comparators { public static<T, U extends Comparable<? super U>> Comparator<T> comparing(Mapper<T, U> m) { return (x, y) -> m.map(x).compareTo(m.map(y)); } }
- 24. Example – Sorting Comparator<Person> byLastName = Comparators.comparing(p -> p.getLastName()); Collections.sort(people, byLastName); Collections.sort(people, comparing(p -> p.getLastName()); people.sort(comparing(p -> p.getLastName()); people.sort(comparing(Person::getLastName)); people.sort(comparing(Person::getLastName).reverse()); people.sort(comparing(Person::getLastName) .compose(comparing(Person::getFirstName)));
- 25. Bulk operations on Collections • Compute sum of weights of blue shapes • Compose compound operations from basic building blocks • Each stage does one thing • Client code reads more like the problem statement • Structure of client code is less brittle • Less extraneous “noise” from intermediate results • No “garbage variables” • Library can use parallelism, out-of-order, laziness for performance int sumOfWeight = shapes.stream() .filter(s -> s.getColor() == BLUE) .map(s -> s.getWeight()) .sum(); int sumOfWeight = shapes.stream() .filter(s -> s.getColor() == BLUE) .map(Shape::getWeight) .sum();
- 26. Streams • To add bulk operations, we create a new abstraction, Stream (in package java.util.stream) • Key new library abstraction for JSR-335 • Represents a stream of values • Not a data structure – doesn’t store the values • Source can be a Collection, array, generating function, IO • Encourages a “fluent” usage style • Supports operations like filter(), map(), reduce() • Retrofit stream() method on Collection • As well as: Reader.lines(), Random.ints(), String.chars(), etc • Easy to adapt any aggregate to be a Stream source
- 27. Streams • What does this code do? Set<Group> groups = new HashSet<>(); for (Person p : people) { if (p.getAge() >= 65) groups.add(p.getGroup()); } List<Group> sorted = new ArrayList<>(groups); Collections.sort(sorted, new Comparator<Group>() { public int compare(Group a, Group b) { return Integer.compare(a.getSize(), b.getSize()) } }); for (Group g : sorted) System.out.println(g.getName()); people.stream() .filter(p -> p.getAge() > 65) .map(p -> p.getGroup()) .removeDuplicates() .sorted(comparing(g -> g.getSize()) .forEach(g -> System.out.println(g.getName());
- 28. Parallelism • Goal: easy-to-use parallel libraries for Java • Libraries can hide a host of complex concerns (task scheduling, thread management, load balancing) • Goal: reduce conceptual and syntactic gap between serial and parallel expressions of the same computation • Right now, the serial code and the parallel code for a given computation don’t look anything like each other • Fork-join (added in Java SE 7) is a good start, but not enough • Goal: parallelism should be explicit, but unobtrusive
- 29. Fork/Join Parallelism • JDK7 added general-purpose Fork/Join framework • Powerful and efficient, but not so easy to program to • Based on recursive decomposition • Divide problem into subproblems, solve in parallel, combine results • Keep dividing until small enough to solve sequentially • Tends to be efficient across a wide range of processor counts • Generates reasonable load balancing with no central coordination
- 30. Parallel Sum with Fork/Join class SumProblem { final List<Shape> shapes; final int size; SumProblem(List<Shape> ls) { this.shapes = ls; size = ls.size(); } public int solveSequentially() { int sum = 0; for (Shape s : shapes) { if (s.getColor() == BLUE) sum += s.getWeight(); } return sum; } public SumProblem subproblem(int start, int end) { return new SumProblem(shapes.subList(start, end)); } } ForkJoinExecutor pool = new ForkJoinPool(nThreads); SumProblem finder = new SumProblem(problem); pool.invoke(finder); class SumFinder extends RecursiveAction { private final SumProblem problem; int sum; protected void compute() { if (problem.size < THRESHOLD) sum = problem.solveSequentially(); else { int m = problem.size / 2; SumFinder left, right; left = new SumFinder(problem.subproblem(0, m)) right = new SumFinder(problem.subproblem(m, problem.size)); forkJoin(left, right); sum = left.sum + right.sum; } } }
- 31. Parallel Sum with Streams • Explicit but unobtrusive parallelism • All three operations fused into a single parallel pass • Works with ordinary, non-thread-safe collections • Extensible mechanism to work with any bulk container int sumOfWeight = shapes.stream() .filter(s -> s.getColor() == BLUE) .map(s -> s.getWeight()) .sum(); int sumOfWeight = shapes.stream() .filter(s -> s.getColor() == BLUE) .map(Shape::getWeight) .sum(); int sumOfWeight = shapes.parallelStream() .filter(s -> s.getColor() == BLUE) .map(Shape::getWeight) .sum();
- 32. So … Why Lambda? • It’s about time! • Java is the lone holdout among mainstream OO languages at this point to not have closures • Adding closures to Java is no longer a radical idea • Provide libraries a path to multicore • Parallel-friendly APIs need internal iteration • Internal iteration needs a concise code-as-data mechanism • Empower library developers • More powerful, flexible libraries • Higher degree of cooperation between libraries and client code • Encourage better idioms • Gentle push towards a more functional style of programming
- 33. <Insert Picture Here> JSR-335 Update Lambda expressions for the Java Language Brian Goetz Java Language Architect Oracle Corporation