Java Workshop Lambda Expressions

Transcription

1 Java Workshop Lambda Expressions AP Java Workshop 2015 Hanno Hüther and Martin Stein

2 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

3 Lambda expressions in Java originate from the mathematical system of lambda calculus The lambda calculus provides a formal language and rules for working with computable functions. In expressions, the letter λ (lambda) marks a bound variable, e.g. λx. x + 2. In other notations, the same function could be written as f x = x + 2 or x x + 2. Functional programming languagues, like Lisp and Haskell, are based on the lambda calculus. Recently, many imperative programming languages were extended with features from the lambda calculus (e.g. Python, C++) and now Java follows.

4 You can think of a lambda expression as a method without a name Before lambdas, to pass methods as arguments, one needed to encapsulate them in objects, e.g. by using the command or visitor patterns. Now, lambdas can be used to describe functionality in a way that it can be passed around and stored: (int x) -> { return x+1; } Simply put, the syntax is as follows: (arguments) -> { body }

5 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

6 The need for an easier means of expressing parallelization led to Project Lambda After a long discussion, there was some stagnation on functional programming features to be included in Java until parallelization experts took the lead. Discussion started on functional programming features in Java Three proposals competed in the "closure debate", but a consensus was not achieved "Project Lambda" working group founded with motivation to make expression of parallel execution more feasible Java language specification draft for lambda expressions JSR 335: Lambda Expressions for the JavaTM Programming Language Public Review Java 8 released

7 Disregarding the details, parallelization and code quality seem to be the main motivations Fine-grained parallelization on multi-core hardware, especially bulk-operation in the collection framework List<Integer> someintegerssquared = someintegers.parallelstream().map(x -> x * x).collect(collectors.tolist()); More fluent / precise / clean code, more powerful APIs button.addactionlistener( e -> System.out.println( click )); What do you think?

8 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

9 Exercise 1: Get acquainted to the syntax of lambdas by refactoring non-lambda code Have a look at the LambdaRefactorings.java class. Refactor the following methods to use Lambdas: (1.1) executerunnableinbackgroundthread() Hint: An empty arguments list is expressed by () ->... (1.2) attachactionlistenertobutton(jbutton) Hint: You need one argument to satisfy actionperformed(actionevent) s signature (1.3) printallnamesandages(map<string, LocalDate>) Hint: The Map interface now has a foreach method

10 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

11 Method and constructor references can be used as a syntactic shortcut for lambdas Consider the following lambda expression: File[] files = mydir.listfiles((file f) -> { return f.isfile(); }); One can also express this more concisely: File[] files = mydir.listfiles( File::isFile ); Method Reference Type Syntax Example static ClassName::StaticMethodName String::valueOf constructor ClassName::new ArrayList::new specific object instance objectreference::methodname x::tostring arbitrary object of a given type ClassName::InstanceMethodName Object::toString Example taken from: Table taken from:

12 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

13 Lambdas are represented as objects in Java whose type is determinded by the context Internally, lambda expressions are represented as objects, i.e. they have an address and a type. The type, though, is not expressed explicitly (which is why we can write lambdas like x -> x+1), but determined from the context that the expression is used in. Consequently, the same lambda expression can have different types: public interface FileFilter { boolean accept(file); } FileFilter filter = (File f) -> { return f.isfile(); }; public interface Predicate <T> { boolean test (T t); } Predicate<File> predicate = (File f) -> { return f.isfile(); };

14 Instead of creating a new function type for lambdas, Functional Interfaces are utilized Interfaces that have only a single method (specifying parameter and return types as well as thrown exceptions) like Runnable, Callable, Comparable, Iterable received a new meaning: They are now Functional Interfaces, which are used as the target type for lambdas. In the package java.util.function, there are quite a lot of additional predefined Functional Interfaces, e.g.: public interface BiPredicate<T, U> { boolean test(t t, U u); } public interface BiFunction<T, U, R> { R apply(t t, U u); } e.g.: (x, y) -> x > y e.g.: (x, y) -> x + y

15 A λ expression can only be used in contexts where the target type can be inferred The most important usage contexts are*: Variable declaration or assignment, e.g.: BiPredicate<String,String> b = (s,t) -> s.equals(t); As a method or constructor argument, e.g.: button.addactionlistener( e -> System.out.println( click )); As the value of a return statement, e.g.: public static BiFunction<Integer, Integer, Integer> add() { return (a, b) -> a + b; } * Also: In array initializers, lambda bodies (nested lambdas), ternary conditional expressions, cast expressions. For examples, see e.g.

16 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

17 A lambda expression can bind final or effectively final variables from its context Lambda expressions can access variables of the enclosing scope. But if they, do the compiler checks that these are assigned only once. This is necessary to preserve the following characteristics of local variables: Their lifetime ends with exiting the enclosing method. They are invisible to other threads / immune to race conditions. void method() { int cnt = 16; Runnable r = () -> { System.out.println("count: " + cnt); }; Thread t = new Thread(r); t.start(); cnt++; // Causes error: cnt must be final or effectively final } Example taken from

18 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

19 Excercise 2: Learn more about working with λs through some commonly done mistakes Have a look at the LambdaPitfalls.java class. Try to correct the mistakes that have been made. (2.1) addone() Mistake: Trying to modify a bound variable. (2.2) printfilesindirectory(string path) Mistake: Not paying attention to target types. (2.3) isfileempty(path) Mistake: Lacking a checked exception in the signature. Exercise 2.3 taken from

20 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

21 There are some excellent tutorials available. Going through them takes some time, though Relatively quick introduction (~40 pages): Angelika Langer and Klaus Kreft Lambda Expressions in Java Tutorial Same authors, very thorough (~160 pages): Angelika Langer and Klaus Kreft Lambda Expressions in Java Reference LambdaReference.pre-release.pdf FAQ, easy to navigate and browse: Maurice Naftalin Lambda FAQ

22 Sources Aho, Alfred V. Design and Implementation of Lambdas in Java 8: Friesen, Jeff. Java SE 8 s New Language Features, Part 1 - Interface Default/Static Methods and Lambda Expressions: Kreft, Klaus and Langer, Angelika. Java 8 - Lambda-Ausdrücke und Methoden-Referenzen: Kreft, Klaus and Langer, Angelika. Understanding the closures debate: Langer, Angelika and Kreft, Klaus. The big update: Javamagazin Langer, Angelika and Kreft, Klaus. Lambda Expressions in Java - Reference: Langer, Angelika and Kreft, Klaus. Lambda Expressions in Java - Tutorial: Reinhold, Mark. Closures for Java: Oracle. The Java Tutorials - Lambda Expressions: Christian Ullenboom. Ausnahmen in Lambda-Ausdrücken: Wikipedia. Lambda calculus:

23 Agenda 1. Origin and syntax 2. History and motivation 3. Exercise 1: Refactoring to lambdas 4. Method references 5. Functional interfaces 6. Variable binding and scoping 7. Exercise 2: Avoiding pitfalls 8. Further reading and sources 9. Exercise solutions

24 Sample solutions for exercise 1 LambdaRefactorings.java private static void executerunnableinbackgroundthread() { new Thread(() -> System.out.println("GSI" + "\n" + "---")).start(); } private static void attachactionlistenertobutton(jbutton button) { button.addactionlistener(ae -> System.out.println("Click" + "\n" + "-----")); } private static void printallnamesandages(map<string, LocalDate> names2datesofbirth) { names2datesofbirth.foreach((name, dateofbirth) -> { int age = Period.between(dateOfBirth, LocalDate.now()).getYears(); System.out.println(name + " is " + age + " years old. "); }); }

25 Sample solutions for exercise 2 (1/2) LambdaPitfalls.java private static int addone(int value) { new Thread(() -> System.out.println("Value was: " + value)).start(); int newvalue = value + 1; new Thread(() -> System.out.println("Value is now: " + newvalue)).start(); return newvalue; private interface MyPredicate<T> { boolean test(t t) throws IOException; } private static void isfileempty(path path) throws IOException { MyPredicate<Path> isemptyfile = p -> Files.size(p) == 0; System.out.println("isFileEmpty('" + path + "'): " + isemptyfile.test(path)); }

26 Sample solutions for exercise 2 (2/2) LambdaPitfalls.java private static void printfilesindirectory(string path) { File directory = new File(path); List<File> files = new ArrayList<>(); files.addall(arrays.aslist(directory.listfiles( (File file) -> file.getname().endswith(".txt")))); FileFilter mypred = (File file) -> file.getname().endswith(".html"); files.addall(arrays.aslist(directory.listfiles(mypred))); } System.out.println(files);