Java Programing laungage

new Thread(new Runnable() { 
@Override public void run() {
System.out.println("It runs !");
}
} 
start(); 
new Thread(() -> { System.out.println("It runs !"); } ).start();

Lambdas:

Examples of such functional interfaces:

java.lang.Runnable -> run()
java.util.concurrent.Callable -> call()
java.security.PrivilegedAction -> run()
java.util.Comparator -> compare(T o1, T o2) 
java.awt.event.ActionListener ->
actionPerformed (ActionEvent e)
java.lang.Iterable -> forEach(Consumer<? super T> action)

Lambdas java.lang.Iterable -> forEach(Consumer<? super T> action) ?!? Isn't this breaking backward compatibility ?!?

Lambdas java.lang.Iterable -> forEach(Consumer<? super T> action) ?!? Isn't this breaking compatibility ?!? No - because forEach is an extension method.

Lambdas Extension methods provide a mechanism for extending an existing interface without breaking backward compatibility.

public interface Iterable<T> { 
	default void forEach(Consumer<? super T> action)
	{
		Objects.requireNonNull(action);
		for (T t : this) { action.accept(t); } } }

Lambdas: This essentially means that lambdas can be used to replace all functional interfaces used in a gazillion of libraries

Moreover lambdas increase performance when used instead of anonymous inner classes because they are implemented with the invokedynamic instruction

In this regard many of the standard JDK class are being refactored to use lambdas

Lambdas Additional functional interfaces are provided by the java.util.function package for use by lambdas such as: o Predicate<T> - one method with param of type T and boolean return type o Consumer<T> - one method with param T and no return type o Function<T, R> - one method with param T and return type R o Supplier<T> - one method with no params and return type T

Stream API: Databases and other programming languages allow us to specify aggregate operations explicitly

The streams API provides this mechanism in the Java platform

The notion of streams is derived from functional programming languages

Stream API : The stream API makes use of lambdas and extension methods

Streams can be applied on collections, arrays, IO streams and generator functions

Stream API: Streams can be finite or infinite

Streams can apply intermediate functions on the data that produce another stream (e.g. map, reduce)

Stream API:

java.util.stream.Stream<T>
collection.stream(); 
java.util.stream.Stream<T> 
collection.parallelStream(); 
Useful methods: o filter(Predicate), map(Function), 
reduce(BinaryOperator), collect() o sum()
, min(), max(), count() o anyMatch(), 
allMatch(), forEach(),

Stream internals int sum = widgets.stream() .filter(w -> w.getColor() == RED) .mapToInt(w -> w.getWeight()) .sum(); • Stream operations are composed into a pipeline • Streams are lazy: computation is performed when the terminal operation is invoked

Method references : Intended to be used in lambda expressions, preventing unnecessary boilerplate

Example: books.stream().map(b -> b.getTitle())  books.stream().map(Book::getTitle) • Lambda parameter list and return type must match the signature of the method

Method references: static methods public class Printers

{ 
	public static void print(String s) {...} 
	} 
	Arrays.asList("a", "b", "c").forEach(Printers::print)

Method references :instance methods (1) public class Document

 {
	 public String getPageContent(int pageNumber) 
	 {
		 return this.pages.get(pageNumber).getContent();
} }
public static void printPages(Document doc, int[] pageNumbers)
{ 
	Arrays.stream(pageNumbers) .map(doc::getPageContent) .forEach(Printers::print);
	}

Method references : instance methods (2)

public class Document {
	public String getPageContent(int pageNumber) {
		return this.pages.get(pageNumber).getContent();
		} }
		public static void printDocuments(List<Page> pages) {
			pages.stream() .map(Page::getContent) .forEach(Printers::print);
			}