Java8

first taste of Java 8
Sunil Kumar | Wiley London | 22-01-2014

Java 8 features
● Lambda
● Interface improvement
● Streams
● new packages
Keywords : Big data, Parallelism, Cloud computing

1. Lambda (anonymous function)
In mathematics and computing generally, a lambda expression is a function: for some
or all combinations of input values it specifies an output value. Lambda expressions in
Java introduce the idea of functions into the language
Ex: 0 param
Runnable r = () -> System.out.println("hello world");
r.run(); // prints "hello world"
Object o = () -> System.out.println("hello world"); //won’t compile
Ex: 1 param
FileFilter filter = f -> f.getAbsolutePath().endsWith(".txt");
return filter.accept(new File("myfile.txt")); // returns true

Lambda - syntax
Lambda expression has three part : parenthesized params, arrow and body
● (parameters) -> expression
● (parameters) -> {statements…}
Type inference
Comparator<String> c = (lhs, rhs) -> {
System.out.println("comparing : " + lhs + " to " + rhs);
return lhs.compareTo(rhs);
};
int result = c.compare(“hello”, “world”)

More examples
List<Apple> inventory = Arrays.asList(new Apple(80,"green"),
new Apple(155, "green"),
new Apple(120, "red"));
// [Apple{color='green', weight=80}, Apple{color='green', weight=155}]
List<Apple> greenApples2 = filterApples(inventory, (Apple a) -> "green".equals(a.getColor()));
System.out.println(greenApples2);
// [Apple{color='green', weight=155}]
List<Apple> heavyApples2 = filterApples(inventory, (Apple a) -> a.getWeight() > 150);
System.out.println(heavyApples2);
public static List<Apple> filterApples(List<Apple> inventory, Predicate<Apple> p){
List<Apple> result = new ArrayList<>();
for(Apple apple : inventory){
if(p.test(apple)){
result.add(apple);
}}
return result;
}

Method References
Method references are compact lambda expressions for methods that already have a
name.
● String::valueOf x -> String.valueOf(x)
● Object::toString x -> x.toString()
● x::toString () -> x.toString()
● ArrayList::new () -> new ArrayList<>()

2. Interface improvement
(Functional interface)
A functional interface is one whose type can be used for a method parameter when a lambda is to
be supplied as the actual argument.
For example, the forEach method on collections could have the following signature:
public void forEach(Consumer<? super T> consumer);
More precisely, a functional interface is defined as any interface that has exactly one explicitly
declared abstract method.
● interface can now define static methods.
● interfaces can now provide default methods
(The qualification is necessary because an interface may have non-abstract default methods.) This
is why functional interfaces used to be called Single Abstract Method (SAM) interfaces, a term
that is still sometimes seen.

3. Streams
A stream is a sequence of values. Allows functional-style operations on streams of
values.
Stream<T> stream = collection.stream();
Stream<T> stream = collection.parallelStream();

Steams type
Operations Interface Used lambda signature return type return value
Sample lazy/intermediate operations
filter Predicate<T> T -> boolean Stream<T> stream containing input
elements that satisfy the
Predicate
map Function<T, R> T -> R Stream<R> stream of values, the result
of applying the Function to
each input element
sorted Comparator<T> (T, T) -> int Stream<T> stream containing the input
elements, sorted by the
Comparator
Sample eagar/terminal operation
reduce BinaryOperator<T> (T, T) -> T Optional<T>
forEach Consumer<T> T -> void void

4. java.util.function
Function<T, R> - take T as input, return R as output
Predicate<T> - take T as input, return a boolean as output
Consumer<T> - take T as input, don't return anything
Supplier<T> - take no input, return T
BinaryOperator<T> - take two T's as input, return one T as output

Reference:
● http://www.lambdafaq.org/
● http://www.infoq.com/news/2013/08/everything-about-java-8
● https://blogs.oracle.com/thejavatutorials/entry/jdk_8_documentation_developer_preview
● http://sett.ociweb.com/sett/settFeb2013.html
● http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
● https://github.com/java8/LambdasInAction

Java8

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