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Lambdas and Functions Library Review

Lambda Expressions Lambda expression is an anonymous function Think of it like a method But not associated with a class Can be used wherever you would use an anonymous inner class Single abstract method type Syntax ([optional-parameters]) -> body Types can be inferred (parameters and return type)

Lambda Examples SomeList<Student> students = ... double highestScore = students.stream(). filter(Student s -> s.getGradYear() == 2011). map(Student s -> s.getScore()). max(); Question: how are we going to get there with real collections?

Method References Method references let us reuse a method as a lambda expression FileFilter x = new FileFilter() { public boolean accept(File f) { return f.canRead(); } }; FileFilter x = (File f) -> f.canRead(); FileFilter x = File::canRead;

The Stream Class java.util.stream Stream<T> A sequence of elements supporting sequential and parallel operations A Stream is opened by calling: Collection.stream() Collection.parallelStream() Many Stream methods return Stream objects Very simple (and logical) method chaining

Stream Basics Using a Stream means having three things A source Something that creates a Stream of objects Zero or more intermediate objects Take a Stream as input, produce a Stream as output Potentially modify the contents of the Stream (but don’t have to) A terminal operation Takes a Stream as input Consumes the Stream, or generates some other type of output

Stream Usage Multiple operations available collect, filter, count, skip, limit, sorted map (and map to types, e.g. mapToInt) flatMap maps each element in a Stream to possibly multiple elements e.g. flatMap(line -> Stream.of(line.split(REGEXP)); List<String> names = Arrays.asList(“Bob”, “Alice”, “Charlie”); System.out.println(names. stream(). filter(e -> e.getLength() > 4). findFirst(). get());

java.util.function Package Predicate<T> Determine if the input of type T matches some criteria Consumer<T> Accept a single input argumentof type T, and return no result Function<T, R> Apply a function to the input type T, generating a result of type R Plus several more

The iterable Interface Used by most collections One method forEach() The parameter is a Consumer wordList.forEach(s -> System.out.println(s)); wordList.forEach(System.out::println);

Files and Lines of Text BufferedReader has new method Stream<String> lines() HINT: Test framework creates a BufferedReader for you

Maps and FlatMaps Map FlatMap Map Values in a Stream 1 to 1 mapping Input Stream Output Stream Map 1 to many mapping Input Stream Output Stream FlatMap

Useful Stream Methods collect (terminal) filter (intermediate) count (terminal) skip, limit (intermediate) max (terminal) getAsInt (terminal)

Getting Started Open the LambdasHOL project in NetBeans The exercises are configured as tests Edit the tests Remove the @Ignore annotation Run the tests (Ctrl F6, or from the menu) Make the tests pass Simple!

Let’s Go!