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Program Agenda Lambdas And Streams Primer Delaying ExecuTon Avoiding loops in Streams The Art of ReducTon Conclusions 1 2 3 4 5 Oracle ConfidenTal – Internal/Restricted/Highly Restricted 3
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();
Lambda Expressions • Lambda expressions represent anonymous funcTons – Same structure as a method • typed argument list, return type, set of thrown excepTons, 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
Lambda Expression Types • Single-‐method interfaces are used extensively in Java – DefiniTon: a func%onal interface is an interface with one abstract method – Func%onal interfaces are idenTfied structurally – The type of a lambda expression will be a func%onal interface • Lambda expressions provide implementaTons 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(); }
Local Variable Capture • Lambda expressions can refer to effec%vely final local variables from the surrounding scope – EffecTvely 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); }
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 func%on – It is not associated with a class – Therefore there can be no ‘this’ for the Lambda
Referencing Instance Variables Which are not final, or effecTvely final class DataProcessor { private int currentValue; public void process() { DataSet myData = myFactory.getDataSet(); dataSet.forEach(d -‐> d.use(currentValue++)); } }
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 effecTvely final) inserted by the compiler
Type Inference • The compiler can oeen infer parameter types in a lambda expression § Inferrence based on the target funcTonal interface’s method signature • Fully staTcally 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);
FuncTonal Interface DefiniTon • 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 mulTple inheritance of types to Java – These are, by definiTon, not abstract (they have an implementaTon) • JDK 8 also now allows interfaces to have staTc methods – Again, not abstract • @FunctionalInterface can be used to have the compiler check 13
Is This A FuncTonal Interface? 14 @FunctionalInterface public interface Runnable { public abstract void run(); } Yes. There is only one abstract method
Is This A FuncTonal 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 sTll only one abstract method
Is This A FuncTonal 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
Stream Overview • A stream pipeline consists of three types of things – A source – Zero or more intermediate operaTons – A terminal operaTon • 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 operaTon Terminal operaTon
Stream Sources • From collecTons and arrays – Collection.stream() – Collection.parallelStream() – Arrays.stream(T array) or Stream.of() • StaTc factories – IntStream.range() – Files.walk() • Roll your own – java.util.Spliterator Many Ways To Create
Stream Terminal OperaTons • The pipeline is only evaluated when the terminal operaTon is called – All operaTons can execute sequenTally or in parallel – Intermediate operaTons can be merged • Avoiding mulTple redundant passes on data • Short-‐circuit operaTons (e.g. findFirst) • Lazy evaluaTon – Stream characterisTcs help idenTfy opTmisaTons • DISTINT stream passed to distinct() is a no-‐op
OpTonal Class • Terminal operaTons 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
Performance Impact For Logging • Heisenberg’s uncertainty principle • Semng log level to INFO sTll 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
Supplier<T> • Represents a supplier of results • All relevant logging methods now have a version that takes a Supplier • Pass a descripTon 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 condiTonal acTviTes 23 logger.finest(() -‐> getSomeStatusData());
FuncTonal v. ImperaTve • For funcTonal programming you should not modify state • Java supports closures over values, not closures over variables • But state is really useful… 25
PrinTng And CounTng FuncTonal Interfaces 29 More Func?onal, But Not Pure Func?onal 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 sTll state being modified in the Lambda
PrinTng And CounTng FuncTonal Interfaces 30 Even More Func?onal, But S?ll Not Pure Func?onal int count = functionalParameterMethodMap.get(nameOfClass) .stream() .peek(method -‐> output.println(method)) .mapToInt(m -‐> isNewMethod(nameOfClass, m) ? 1 : 0) .sum(); Strictly speaking prinTng is a side effect, which is not purely funcTonal
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();
Naïve Stream SoluTon • That works, so job done, right? • Not really. Big files will take a long Tme and a lot of resources • Must be a be7er approach 34 String longest = reader.lines(). sort((x, y) -‐> y.length() -‐ x.length()). findFirst(). get();
External IteraTon SoluTon • 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;
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 excepTon 37 List<String> lines = new ArrayList<>(); while ((String s = reader.readLine()) != null) lines.add(s); String longest = findLongestString("", 0, lines);
A Be7er Stream SoluTon • The Stream API uses the well known filter-‐map-‐reduce pa7ern • 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
A Be7er Stream SoluTon • The key is to find the right accumulator – The accumulator takes a parTal result and the next element, and returns a new parTal result – In essence it does the same as our recursive soluTon – But back to front – And without all the stack frames or List overhead 39
A Be7er Stream SoluTon • Use the recursive approach as an accululator for a reducTon 40 String longestLine = reader.lines() .reduce((x, y) -‐> { if (x.length() > y.length()) return x; return y; }) .get();
A Be7er Stream SoluTon • Use the recursive approach as an accululator for a reducTon 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 parTal result, which is the longest string found so far
The Simplest Stream SoluTon • Use a specialised form of max() • One that takes a Comparator as a parameter • comparingInt() is a staTc method on Comparator – Comparator<T> comparingInt(ToIntFunction<? extends T> keyExtractor) 42 reader.lines() .max(comparingInt(String::length)) .get();
Conclusions • Lambdas provide a simple way to parameterise behaviour • The Stream API provides a funcTonal style of programming • Very powerful combinaTon • Does require developers to think differently • Avoid loops, even non-‐obvious ones! 44
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