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java functional programming Lambda Expression Functional Interface Method Reference Constructor Reference Interface Streams The java.time Package

admin 11月 11, 2020 0

Working with streams, lambda expressions, and method references in Java SE8 and beyond

Lambda Expression

  • Assigned to Single Abstract Method (SAM) interface (functional interface) (ex: Runnable, file)
  • Parameter types inferred from context
  • Method Reference

Functional Interface

JavaDoc

  • see java.util.function package
  • Functional Interfaces (Not Required but useful)
  • Use Pure Function
  • Consumer -> single arg, no result (not a Pure function)
    void accept(T t)
    Ex : print or log
    Also IntConsumer, LongConsumer, BiConsumer<T, U>

Ex :

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import java.util.function.Consumer;
import java.util.logging.Logger;
import java.util.stream.Stream;

public class  {
    public static void main(String[] args) {
        Consumer<String> cPrinter = System.out::println;
        Logger logger = Logger.getLogger(ConsumerCompositionDemo.class.getName());

        Consumer<String> cLogger = logger::info;

        Consumer<String> composed = cPrinter.andThen(cLogger);

        Stream.of("This", "is", "a", "string").forEach(composed);

    }    
}

  • Predicate -> returns boolean
    boolean test(T t)

  • Supplier -> no arg, returns single result
    T get()

    Also BooleanSupplier…

  • Function -> single arg, returns result
    R apply(T t)

Also IntFunction, LongFunction, ToDoubleFunction, ToDoubleBiFunction<T,​U>, ToDoubleFunction, ToIntBiFunction<T,​U>, ToIntFunction, ToLongBiFunction<T,​U> Represents a function that accepts two arguments and produces a long-valued result.
ToLongFunction…

Method Reference

Method references uses :: notation

System.out::println equivalent to x -> System.out.println(x)
Math::max equivalent to (x, y) -> Math.max(x, y)
String::compareToIgnoreCase equivalent to (x, y) -> x.compareToIgnoreCase(y)

Constructor Reference

Constructor references uses :: notation

Ex: 

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import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class UsePerson {
    ({"UnusedAssignment", "Convert2MethodRef"})
    public static void main(String[] args) {
        List<String> names = Arrays.asList("John", "Paul", "George", "Ringo");

        List<Person> people = names.stream()    
                .map(name -> new Person(name))  // Stream<Person>
                .collect(Collectors.toList());  // Converts Stream<Person> to List<Person>


        people = names.stream()
                .map(Person::new)
                .collect(Collectors.toList());

        System.out.println(people);

        Person[] peopleArray = names.stream()
                .map(Person::new)
                .toArray(Person[]::new);

        System.out.println(Arrays.toString(peopleArray));
    }
}

Interface

Default methods

Use keyword default

If conflict :

  • Class vs Interface => Class always wins
  • Interface vs Interface => Child overrides parent, otherwise compilation error

If class implements multiple Interface with same method

  • Must call InterfaceName.super.method
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public class Pegasus implements Horse, Bird {
    @Override
    public String speak() {
        return Horse.super.speak() + " " + Bird.super.speak();
    }
}

Streams

Definition

A sequence of elements

  • Does not store the elements
  • Does not change the source
  • Operations are lazy when possible
  • Closed when terminal expression reached

A stream carries values

  • from a source
  • through a pipeline

Java Doc

Pipeline

A pipeline is

  • a source
  • zero or more intermediate operations
  • a terminal operation

Reduction Operations

Terminal operation that produce one value for a stream
Ex: average, sum, max, min, count, ....

The java.time Package

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