1. Java 8
Intro to Closures (Laλλbdas)
Kaunas JUG
Dainius Mežanskas · kaunas.jug@gmail.com · http://plus.google.com/+KaunasJUG
{ λ }

2. Dainius Mežanskas
● 16 years of Java
● Java SE/EE
● e-Learning · Insurance · Telecommunications ·
e-Commerce
● KTU DMC · Exigen Group · NoMagic Europe ·
Modnique Baltic

3. Presentation Source Code
https://github.com/dainiusm/kaunasjug2lambdas.git

4. What is Lambda? (aka. Closure)
● Anonymous function in
programming
● Provides a way to write
closures

5. Closure
function adder() {
def x = 0;
function incrementX() {
x = x + 1;
return x;
}
return incrementX;
}
def y = adder(); // Ref to closure
y(); // returns 1 (y.x == 0 + 1)
y(); // returns 2 (y.x == 1 + 1)

6. Why Lambda in Java
● 1 line instead of 5
● Functional programming
● References to methods
● Conciser collections API
● Streams · Filter/map/reduce
...

7. Functional Interface
● Single Abstract Method Interfaces (SAM)
● java.lang.Runnable, java.awt.event.ActionListener, java.util.
Comparator, java.util.concurrent.Callable etc.
● From JavaTM
8 - Functional interfaces

8. @FunctionalInterface (1)
@FunctionalInterface
public interface Runnable {
public abstract void run();
}

9. @FunctionalInterface (2)
@FunctionalInterface
public interface Builder {
public abstract void build();
public abstract String toString();
}

10. @FunctionalInterface (3)
@FunctionalInterface
public interface Builder {
public abstract void build();
public abstract boolean isTrusted();
}

11. @FunctionalInterface (4)
@FunctionalInterface
public interface Builder {
public abstract void build();
public default boolean isTrusted() { return false; }
}

12. @FunctionalInterface (5)
@FunctionalInterface
public interface Builder {
public abstract void build();
public abstract String toString();
public default boolean isTrusted() { return false; }
public static Builder empty() { return () -> {}; }
} Builder.java

13. Lambda Syntax
● Argument List - zero or more
● Body - a single expression or a statement block
Argument List Arrow Token Body
(int a, int b) -> a + b

14. Lambda Examples
(int x, int y) -> x + y
() -> 42
a -> { return a * a; }
(a) -> a * a
(String s) -> { System.out.println(s); }
Comparator<String> c = (s1, s2) -> s1.compareTo(s2);

15. public class RunnableLambda {
public static void main(String[] args) {
Runnable runAnonymous = new Runnable() {
public void run() {
System.out.println("Anonymous Class Thread!");
}
};
Runnable runLmbd = () -> System.out.println("Lambda Thread!");
new Thread(runAnonymous).start();
new Thread(runLmbd).start();
}
}
RunnableLambda
RunnableLambda.java

16. List<String> vegetables =
Arrays.asList( "Carrot", "Watercress", "Dill", "Pea" );
Collections.sort( vegetables,
(String s1, String s2) -> { return -s1.compareTo(s2); } );
Collections.sort( vegetables,
(s1, s2) -> -s1.compareTo(s2)
);
System.out.println( vegetables );
ComparatorLambda
ComparatorLambda.java

17. CustomInterface
Lambda
public static void main(String[] args) {
Artist artist = new Artist();
artist.perform(
() -> System.out.println("Hello, Lambda")
);
}
@FunctionalInterface
interface Action {
void process();
}
class Artist {
public void perform(Action action) {
action.process();
}
} SimpleLambda.java

18. java.util.function
● Consumer<T> / void accept(T t);
● Function<T, R> / R apply(T t);
● blahOperator / ~vary~
● Predicate<T> / boolean test(T t);
● Supplier<T> / T get();
FunctionLambda.java

19. Lambda.this · Object = · final
● () -> { print(this); } // this == outer object
● Object n = () -> { print(“Wow!”)}; // compile fails
● Outer scope variables · final
(“effectively final”)
ThisLambda.java
ObjectLambda.java
FinalLambda.java

20. Default Methods
● Aka. Virtual Extension Methods or Defender Methods
● Inspired by Limitations designing Java 8 Collection API
with Lambdas
● IMHO For very basic/general/specific implementation
● May be inherited
DefaultMethodInheritance.java

21. List.forEach()
public interface Iterable<T> {
.....
default void forEach(Consumer action) {
for (T t : this) {
action.accept(t);
}
}
CollectionsBulkAndStream.java

22. Collections API additions
● Iterable.forEach(Consumer)
● Iterator.forEachRemaining(Consumer)
● Collection.removeIf(Predicate)
● Collection.spliterator()
● Collection.stream()
● Collection.parallelStream()
● List.sort(Comparator)
● List.replaceAll(UnaryOperator)
● Map.forEach(BiConsumer)
● Map.replaceAll(BiFunction)
● Map.putIfAbsent(K, V)
● Map.remove(Object, Object)
● Map.replace(K, V, V)
● Map.replace(K, V)
● Map.computeIfAbsent(K, Function)
● Map.computeIfPresent(K, BiFunction)
● Map.compute(K, BiFunction)
● Map.merge(K, V, BiFunction)
● Map.getOrDefault(Object, V)

23. Method References
Method
String::valueOf
Object::toString
x::toString
ArrayList::new
Lambda
x -> String.valueOf(x)
x -> x.toString()
() -> x.toString()
() -> new ArrayList<>()
->
MethodReferences.java

24. Streams
● Filter-Map-Reduce
● Infinite and stateful
● Sequential or parallel
● One or more intermediate operations
filter, map, flatMap, peel, distinct, sorted, limit, and substream
● One final terminal operation
forEach, toArray, reduce, collect, min, max, count, anyMatch, allMatch, noneMatch, findFirst, and findAny

25. Happy Birthday Java 8!