This tutorial walks through tons of examples. You will learn everything you need to know about lambdas and functional programming in Java 8. I'm the supplier. You’re the consumer (and you will get the joke after the session).

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

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Henri Tremblay

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Henri Tremblay

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

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

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Java < 8

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Live Coding

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8 minutes break (please do come back)

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More Live Coding

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Questions

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You voting for me

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Java 5
(2004)

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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)

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

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Java 6
(2006, last from Sun)

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

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Java 7
(2011, first from Oracle)

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

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

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

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

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

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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
}

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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

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Java 8
(2014)

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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 );
}
}

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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

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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]

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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

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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

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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
List<String> list = new ArrayList<>();
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

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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

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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!

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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;

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

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

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

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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;

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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();
}
}

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

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Break!
(8 minutes)

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The End

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Who has learned
something today?
?

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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