JAVA 8 - what is Functional Interface and Lambda Expression, etc

Functional Interface is an interface with only one method.
- default methods do not count
- static methods do not count
- methods from the Object class do not count

A Lambda Expression is an instance of a functional interface.

public interface Predicate<T> {
    boolean test(T t);
}

Implementation would be:

Predicate<String> predicate = s -> s.length() < 20;
System.out.println(predicate.test("Hello world!"));//expicite way of using it

Four categories of java.util.function: 
- The Consumers 

public interface Consumer<T> {
    public void accept(T t);
}

Consumer<String> printer = s -> System.out.println(s);
Consumer<String> printerAlt = System.out::println;

- The Supplier 

public interface Supplier<T> {
    public T get();
}

Supplier<Person> personSupplier = () -> new Person();
Supplier<Person> personSupplierAlt = Person::new;

- The Functions 

public interface Function<T, R> {
    public R apply(T t);
}

Function<Person, Integer> ageMapper = person -> person.getAge();
Function<Person, Integer> ageMapperAlt = Person::getAge;

- The Predicates

public interface Predicate<T> {
    boolean test(T t);

}

Predicate<String> predicate = s -> s.length() < 20;
System.out.println(predicate.test("Hello world!"));

Map, Filter, Reduce over collection

MAP changes the TYPE of the collection, not the size.

FILTER changes the SIZE of the collection and preserves its type.

REDUCE completely changes the TYPE of the collection.

Java 7 implementation:

Parallel Reduce computation on computer with 2 Cores Processor

Streams

Intermediate and Terminal calls of Stream;
Intermediate call return Stream (e.g. Stream.peek)
Terminal call returns something else or is void and thus trigger the processing (Stream.forEach)

Stream skip() and limit()

Stream Match Reduction - short-circuiting terminal operation 
Stream.anyMatch, allMatch, NoneMatch (all return boolean)
Short-circuiting operations of Stream may not to evaluate the predicate for all the elements in the stream.

Stream Find Reduction - short-circuiting terminal operation
Stream.findFirst, findAny (all may return Optional that can be empty is there no such item)
Optional<Person> opt = people.stream().findAny(p -> p.getAge() > 20);