overview lambda streams


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Overview of Java 8 Lambda and Streams Stuart W. Marks Principal Member of Technical Staff

Twitter: @stuartmarks


What is a Lambda?

 A lambda is a function.

 A function is a computation that takes parameters and returns a value.

 Until Java 8, functions could only be implemented using methods.

 A lambda enables functions to be passed around or stored like data.


Example: Sorting a List of Objects

class Person { String name; int age; String getName() { return name; } int getAge() { return age; } } List<Person> list = ... ; Collections.sort(list, ??? );



List<Person> list = ... ; public int compare(Person p1, Person p2) { return p1.getName().compareTo(p2.getName()); } Collections.sort(list, ??? );



List<Person> list = ... ; class ComparePersonsByName implements Comparator<Person> { @Override public int compare(Person p1, Person p2) { return p1.getName().compareTo(p2.getName()); } } Collections.sort(list, ??? );



List<Person> list = ... ; class ComparePersonsByName implements Comparator<Person> { @Override public int compare(Person p1, Person p2) { return p1.getName().compareTo(p2.getName()); } } Collections.sort(list, new ComparePersonsByName());



List<Person> list = ... ; Collections.sort(list, new Comparator<Person>() { @Override public int compare(Person p1, Person p2) { return p1.getName().compareTo(p2.getName()); } });



List<Person> list = ... ; Collections.sort(list, (Person p1, Person p2) { return p1.getName().compareTo(p2.getName()); } );



List<Person> list = ... ; Collections.sort(list, (Person p1, Person p2) -> p1.getName().compareTo(p2.getName()) );



List<Person> list = ... ; Collections.sort(list, (p1, p2) -> p1.getName().compareTo(p2.getName()) );


Functional Interfaces

interface Runnable { public void run(); } new Thread(new Runnable() { @Override public void run() { System.out.println("Hello, thread!"); } }); new Thread(() -> System.out.println("Hello, thread!"));


New APIs Using Functional Interfaces

List<Person> list = ... ; for (Person p : list) { System.out.println(p); } list.forEach(p -> System.out.println(p));



List<Person> list = Collections.synchronizedList(...); for (Person p : list) { System.out.println(p); } list.forEach(p -> System.out.println(p));



List<Person> list = ... ; for (Person p : list) { if ("Jones".equals(p.getName())) { ??? how to remove this Person ??? } }



List<Person> list = ... ; for (Iterator<Person> iter = list.iterator(); iter.hasNext(); ) { Person p = iter.next(); if ("Jones".equals(p.getName())) { iter.remove(); } } list.removeIf(p -> "Jones".equals(p.getName()));



List<String> list = Arrays.asList("Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot", "Golf"); for (ListIterator<String> iter = list.listIterator(); iter.hasNext(); ) { iter.set(iter.next().toUpperCase()); } list.replaceAll(s -> s.toUpperCase()); list.replaceall(String::toUpperCase); // method reference


Default Methods Added to Collections

Iterable.forEach(lambda) Collection.removeIf(lambda) List.replaceAll(lambda) List.sort(lambda) // replaces static method Collections.sort()


Default Methods Added to Collections

Collections.sort(list, (p1, p2) -> p1.getName().compareTo(p2.getName()) ); list.sort( (p1, p2) -> p1.getName().compareTo(p2.getName()));


Combining Operations

 New collection operations include forEach, removeIf, replaceAll  Each operates on a collection, possibly modifying it  What if we want to combine these operations?

–  for example, operate only certain elements that meet some criterion  Using collections, we'd need to copy and create temporary collections  Solution: streams


Streams

 Streams: a multiplicity of values –  no storage –  might or might not be ordered –  sort-of like an iterator –  affords serial or parallel processing

 Stream pipeline: –  a source –  zero or more intermediate operations –  a terminal operation


Streams

 Sources –  collections, array, varargs parameters, numeric range, lines of file

  Intermediate operations –  filter, map, distinct, sorted, skip, limit, flatMap

 Terminal operations –  forEach, toArray, collect, reduce, count, min, max –  findFirst, findAny, anyMatch, allMatch, noneMatch


Streams Example

// Given a list of persons, generate a sorted list of // names that longer than longer than 3 characters. List<Person> persons = ... ; persons.stream() .map(p -> p.getName()) .filter(s -> s.codePointCount(0, s.length()) > 3) .sorted() .forEach(s -> System.out.println(s));


Streams Example

// Given a list of persons, generate a sorted list of // names that longer than longer than 3 characters. List<Person> persons = ... ; persons.stream() .map(p -> p.getName()) .filter(s -> s.codePointCount(0, s.length()) > 3) .sorted() .forEach(System.out::println);


Streams Example

// Given a list of persons, generate a sorted list of // names that longer than longer than 3 characters. List<Person> persons = ... ; List<String> sortedNames = persons.stream() .map(p -> p.getName()) .filter(s -> s.codePointCount(0, s.length()) > 3) .sorted() .collect(Collectors.toList());


Streams Example

// Calculate pi using the Taylor Series // pi = 4 * (1 – 1/3 + 1/5 – 1/7 + ...) System.out.println( LongStream.range(0L, 1_000_000_000L) .map(i -> ((i & 1) == 0 ? 1 : -1) * (2 * i + 1)) .mapToDouble(i -> 4.0 / i) .sum());


Streams Example

// Calculate pi using the Taylor Series // pi = 4 * (1 – 1/3 + 1/5 – 1/7 + ...) System.out.println( LongStream.range(0L, 1_000_000_000L) .parallel() .map(i -> ((i & 1) == 0 ? 1 : -1) * (2 * i + 1)) .mapToDouble(i -> 4.0 / i) .sum());


Summary

 Lambdas: anonymous functions  Default methods  Method references  Streams: allow pipelined operations on multiple values

–  programmed using lambda –  enable parallelism

Lambdas and streams make code read more like the problem statement, resulting in code that is clear, concise, and easy to maintain.


Java 8 Resources

 Download: java.oracle.com  Documentation & Tutorials: docs.oracle.com/javase  Source code: openjdk.java.net/projects/jdk8  FAQ: lambdafaq.org  Technical documents: openjdk.java.net/projects/lambda

–  State of the Lambda –  State of the Lambda: Libraries Edition


The preceding is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle.

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