JavaOne 2016 - Learn Lambda and functional programming

© Henri Tremblay 2015
Henri Tremblay
Senior Software Engineer
Terracotta, a Software AG company
Learn Java 8: Lambdas and Functional
Programming [TUT6198]
@henri_tremblay

4
Henri Tremblay
•  More or less made possible class mocking and proxying
•  Coined the term “partial mocking”

5
Henri Tremblay
•  More or less made possible class mocking and proxying
•  Coined the term “partial mocking”

9
8 minutes break (please do come back)

14
JAVA 5 GAVE THE GENERIC
TYPES TO THE WORLD
(and also annotations, concurrent collections, enum types, for
each, static imports and so on and so on)

15
Type witness
MyClass.<List<String>> anyObject()
because you can’t
(List<String>) MyClass.anyObject()

16
Java 6
(2006, last from Sun)

17
JAVA 6 BROUGHT.. PRETTY
MUCH NOTHING
(a bunch of performance improvements under the hood, better xml
parsing and the first scripting api)

18
Java 7
(2011, first from Oracle)

19
JAVA 7 BROUGHT A LOT OF
SYNTACTIC SUGAR
(plus invokeDynamic, forkJoin, better file IO)

20
Switch for strings
switch(s) {
case "hello":
return "world";
case "bonjour":
return "le monde";
}

21
Diamond operator
List<String> list = new ArrayList<>();

22
Binary integer literals and underscores
int i = 0b1110001111;
int i = 1_000_000;

23
Multiple catches
try {
// ... do stuff
}
catch (IOException | SerializationException e) {
log.error("My error", e);
}
Instead of
try {
// ... do stuff
} catch (IOException e) {
} catch (SerializationException e) {
}

24
Auto Closeable
Before
InputStream in = new FileInputStream("allo.txt");
try {
// … do stuff
} finally {
try { in.close(); } catch(IOException e) {}
}
After
try(InputStream in = new FileInputStream("allo.txt")) {
// … do stuff
}

25
File IO API
List<String> lines =
Files.readAllLines(
Paths.get("path", "to", "my", "file.txt"));
// also: file watcher, symbolic links, file locks,
// copy … Look in java.nio.file

27
JAVA 8: LAMBDA!
(and also a new date API, default methods, metaspace, Nashorn,
JavaFX and CompletableFuture)

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Base64 ;-)
import java.nio.charset.StandardCharsets;
import java.util.Base64;
public class Base64s {
public static void main(String[] args) {
final String text = "Base64 finally in Java 8!";
final String encoded = Base64
.getEncoder()
.encodeToString( text.getBytes( StandardCharsets.UTF_8 ) );
System.out.println( encoded );
final String decoded = new String(
Base64.getDecoder().decode( encoded ),
StandardCharsets.UTF_8 );
System.out.println( decoded );
}
}

29
Date / Time API
Core ideas:
  Immutable
  A time is a time, a date is a date. Not always both (like java.util.Date)
  Not everyone uses the same calendar (the same Chronology)
LocalDate, LocalTime, LocalDateTime à Local. No time zone
OffsetDateTime, OffsetTime à Time with an offset from
Greenwich
ZonedDateTime à LocalDateTime with a time zone
Duration, Period à Time span
Instant à Timestamp
Formatting à Easy and thread-safe formatting

30
Date / Time API (example)
LocalDateTime now = LocalDateTime.now();
String thatSpecialDay = now
.withDayOfMonth(1)
.atZone(ZoneId.of("Europe/Paris"))
.plus(Duration.ofDays(5))
.format(DateTimeFormatter.ISO_ZONED_DATE_TIME);
System.out.println(thatSpecialDay);
Output
2016-09-06T17:45:22.488+01:00[Europe/Paris]

31
Nashorn
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
public class Nashorn {
public static void main(String[] args) throws ScriptException {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName( "JavaScript" );
int i = (Integer) engine.eval( "function f() { return 1; }; f() + 1;");
System.out.println("Result: " + i);
}
}
Rhino Nashorn
Command
line: jjs

33
Lambda: 11th letter of the Greek alphabet

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(also written as λ-calculus) is a formal system in
mathematical logic for expressing computation based
on function abstraction and application using variable
binding and substitution. It is a universal model of
computation that can be used to simulate any single-
taped Turing machine and was first introduced by
mathematician Alonzo Church in the 1930s as part of an
investigation into the foundations of mathematics.
Lambda calculus

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This is a function:
This is a lambda:
Lambda calculus

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c = sqrt(add(pow(a, 2), pow(b, 2)))
Functional programming

38
Lambda
// Classic
list.forEach(e -> System.out.println(e));
// Typed
list.forEach((String e) -> System.out.println(e));
// Multiline
list.forEach((String e) -> {
System.out.println(e);
});
// With closure
String greeting= "Hello ”;
list.forEach(e -> System.out.println(greeting + e));

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Implicit final
String greeting = "Hello "; // no final required
list.forEach(s -> System.out.println(greeting + s));
list.forEach(new Consumer<String>() {
@Override public void accept(String s) {
System.out.println(greeting + s);
}
});
list.forEach(s -> greeting = “Hi”); // won’t compile

40
Everything is a lambda
public interface MyInterface {
int foo();
}
public void bar(MyInterface i) {
System.out.println(i.foo());
}
bar(() -> 4);
bar(new MyInterface() {
@Override public int foo() {
return 4;
}
});
JButton btn = new JButton();
btn.addActionListener(e -> {});
But it’s better to ﬂag them
with
@FunctionalInterface

41
Method references
public static class Passenger {
public void inboard(Train train) {
System.out.println("Inboard " + train);
}
}
public static class Train {
public static Train create(Supplier< Train > supplier)
{
return supplier.get();
}
public static void paintBlue(Train train) {
System.out.println("Painted blue " + train);
}
public void repair() {
System.out.println( "Repaired " + this);
}
}
Train train = Train.create(Train::new); // constructor
List<Train> trains = Arrays.asList(train);
trains.forEach(Train::paintBlue); // static
method
trains.forEach(Train::repair); // instance
method
Passenger p = new Passenger();
trains.forEach(p::inboard); // instance
method taking this in param
trains.forEach(System.out::println); // useful!

42
Upside Down
public class MethodTest {
public class Foo {
static final String ERR_MESSAGE = "bad";
public void doIt() throws IllegalStateException {
throw new IllegalStateException(ERR_MESSAGE);
}
}
@Test
public void test() {
Foo foo = new Foo();
Throwable t = captureThrowable(foo::doIt);
assertThat(t)
.isInstanceOf(IllegalStateException.class)
.hasMessage(Foo.ERR_MESSAGE);
}
public static Throwable captureThrowable(Runnable r) {
Throwable result = null;

43
Streams
try(Stream<String> lines =
Files.lines(Paths.get("src/Streams.java"))) {
System.out.println(lines.findFirst());
}

44
Functional programming
int sum= list
.stream()
.filter(s -> s.startsWith("a"))
.mapToInt(String::length)
.sum();
List<Integer> length = list
.stream()
.map(String::length)
.collect(Collectors.toList());

45
Parallel
int sum = list
.parallelStream()
.filter(s -> s.startsWith("a"))
.mapToInt(String::length)
.sum();
Arrays.parallelSort(array);

46
Optional
try(Stream<String> lines =
Files.lines(Paths.get("src/Streams.java"))) {
System.out.println(lines.findFirst());
}
è  Optional[import java.io.IOException;]
Optional<String> findFirst();
Solution: lines.findFirst().get();
èimport java.io.IOException;

47
Interface default methods
public interface List<E> extends
Collection<E> {
default void replaceAll(UnaryOperator<E>
operator) {
// …
}
}
public interface A {
default void foo() { }
}
public interface B{
default void foo() { }
}
public class C implements A, B {} // forbidden
public class C implements A, B { // allowed
public void foo() { }
}
public static class D implements A, B { //
allowed
public void foo() {
A.super.foo();
}
}

48
Interface static methods
public interface IntStream {
static IntStream empty () {
return …;
}
}
IntStream stream = IntStream.empty ();

51
Who has learned
something today?
?

52
Brian Goetz – State of the lambda
http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-
final.html
Ninja Squad – Lambda Kata
https://github.com/Ninja-Squad/ninjackaton-lambda
Maurice Naftalin’s lambda facts and books
http://www.lambdafaq.org/
  Mastering Lamdbas: Java Programming in a Multicore World
Links

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Questions? http://montreal-jug.org
? http://easymock.org
http://objenesis.org
http:/ehcache.org
Henri Tremblay
http://blog.tremblay.pro
@henri_tremblay

JavaOne 2016 - Learn Lambda and functional programming

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