Functional Interface

functional interface – an interface with a single abstract method. The Java API has many one-method interfaces such as Runnable, Callable, Comparator, ActionListener and others. They can be implemented and instantiated using anonymous class syntax.

Labmda expression

public interface Hello {
	public void sayHello(String who);

public interface Print<T> {
	public void print(T msg);

public static void main(String[] args) {
	Hello h = (String s) -> System.out.println(s);

	Print<Float> p = (f) -> {
	p.print(new Float(3.1415926f));


Aggregate operations

The following is a generic version of it that accepts, as a parameter, a collection that contains elements of any data type:

public static <X, Y> void processElements(
    Iterable<X> source,
    Predicate<X> tester,
    Function <X, Y> mapper,
    Consumer<Y> block) {
    for (X p : source) {
        if (tester.test(p)) {
            Y data = mapper.apply(p);

The following example uses aggregate operations to print the e-mail addresses of those members contained in the collection roster who are eligible for Selective Service:

        p -> p.getGender() == Person.Sex.MALE
            && p.getAge() >= 18
            && p.getAge() <= 25)
    .map(p -> p.getEmailAddress())
    .forEach(email -> System.out.println(email));
Aggregate Operation
Stream<E> stream()
Stream<T> filter(Predicate<? super T> predicate)
<R> Stream<R> map(Function<? super T,? extends R> mapper)
void forEach(Consumer<? super T> action)