lambdaexpressions

http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html

http://ifeve.com/lambda/

http://www.cnblogs.com/figure9/archive/2014/10/24/4048421.html

lambdaexpressions

 

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);
            block.accept(data);
        }
    }
}

To print the e-mail address of members who are eligible for Selective Service, invoke the processElements method as follows:

processElements(
    roster,
    p -> p.getGender() == Person.Sex.MALE
        && p.getAge() >= 18
        && p.getAge() <= 25,
    p -> p.getEmailAddress(),
    email -> System.out.println(email)
);

This method invocation performs the following actions:

1. Obtains a source of objects from the collection source. In this example, it obtains a source of Person objects from the collection roster. Notice that the collection roster, which is a collection of type List, is also an object of type Iterable.
2. Filters objects that match the Predicate object tester. In this example, the Predicate object is a lambda expression that specifies which members would be eligible for Selective Service.
3. Maps each filtered object to a value as specified by the Function object mapper. In this example, the Function object is a lambda expression that returns the e-mail address of a member.
4. Performs an action on each mapped object as specified by the Consumer object block. In this example, the Consumer object is a lambda expression that prints a string, which is the e-mail address returned by the Function object.

You can replace each of these actions with an aggregate operation.

Approach 9: Use Aggregate Operations That Accept Lambda Expressions as Parameters

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:

roster
    .stream()
    .filter(
        p -> p.getGender() == Person.Sex.MALE
            && p.getAge() >= 18
            && p.getAge() <= 25)
    .map(p -> p.getEmailAddress())
    .forEach(email -> System.out.println(email));

The following table maps each of the operations the method processElements performs with the corresponding aggregate operation:

processElements Action	Aggregate Operation
Obtain a source of objects	Stream<E> stream()
Filter objects that match a Predicate object	Stream<T> filter(Predicate<? super T> predicate)
Map objects to another value as specified by a Function object	<R> Stream<R> map(Function<? super T,? extends R> mapper)
Perform an action as specified by a Consumer object	void forEach(Consumer<? super T> action)

The operations filter, map, and forEach are aggregate operations. Aggregate operations process elements from a stream, not directly from a collection (which is the reason why the first method invoked in this example is stream). A stream is a sequence of elements. Unlike a collection, it is not a data structure that stores elements. Instead, a stream carries values from a source, such as collection, through a pipeline. A pipeline is a sequence of stream operations, which in this example is filter- map-forEach. In addition, aggregate operations typically accept lambda expressions as parameters, enabling you to customize how they behave.

For a more thorough discussion of aggregate operations, see the Aggregate Operations lesson.