Streams or Loops? Java 8 Stream API by Niki Petkov - Proxiad Bulgaria

Stream or LoopJava 8 Stream API

Nikolay Petkov

Java 8 Goodies

● Lambda expressions aka functional interfaces

● Method references● Default methods

Functional Interface

Lambda Expressions

Lambda Expressions

Method Reference

● Static method● Instance method of particular instance● Instance method of particular type● Constructor

Static method reference

Instance method reference

Default Method

public interface Developer {void writeCode();

default void writeTest() {throw new ImPerfectException(

“Why bother? My code is perfect”);}

}

External vs Internal Iteration

for (Car car : cars) {if (“BMW”.equals(car.getBrand()) {

car.setOwner(“Alex”) ;}

}

cars.forEach((car) -> {if (“BMW”.equals(car.getBrand()) {

car.setOwner(“Alex”) ;}

});

Stream API

Classes to support functional-style operations on streams of elements, such as map-reduce transformations on collections.

int oldestBmwAge = cars.stream().filter((car) -> “BMW”.equals(car.getBrand())).mapToInt((car) -> car.getAge()).max();

Streams vs Collections

● Not a storage● Functional by design● Possibly unbounded● Lazy by nature● Use and discard

Stream pipeline

A stream pipeline consists of:● a source (such as a Collection, an array, a

generator function, or an I/O channel);● followed by zero or more intermediate

operations such as Stream.filter or Stream.map;

● and a terminal operation such as Stream.forEach or Stream.reduce.

Execution begins when the terminal operation is invoked, and ends when the terminal operation completes

Source

● Collection.stream()● Arrays.stream(Object[])● Stream.of(Object[])● IntStream.range(int, int)● BufferedReader.lines();● Files.list(Path dir)● etc.

Intermediate - filter

Returns a stream consisting of the elements of this stream that match the given predicate.

Stream<T> filter(Predicate<? super T> predicate)

filter((String s) -> s.lenght() < 20)filter((String s) -> String::isEmpty))

Intermediate - sorted

Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.

sorted(Comparator<? super T> comparator)

sorted(comparing(Person::getLastName)sorted((s1, s2) -> s1.lenght() > s2.lenght())

Intermediate - map

Returns a stream consisting of the results of applying the given function to the elements of this stream. The map operation is kind of transformation of the stream elements to another elements.

map(Function<T,R> mapper)

mapToInt((String s) -> s.lenght())map((User u) -> User::getRole())

Terminal – max/min/sum/count

Returns the maximum/minimum element of this stream according to the provided Comparator

max(Comparator<? super T> comparator)

max(comapring(Persone::getAge))stream.mapToInt(Persone::getAge).sum()

A lot more follows

Questions?