This document provides information about Java 8 lambdas and closures presented by Dainius Mežanskas. It introduces lambda expressions and closures, explains why they were added to Java, describes functional interfaces and the @FunctionalInterface annotation. It provides lambda syntax examples and examples using lambda expressions with the Collections API and streams. It also discusses method references, default methods, and new methods added to the Collections framework in Java 8.

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!