What is the use of the Optional.stream() method in Java 9?

In Java 9, the stream() method has been added to the Optional class to improve its functionality. The stream() method can be used to transform a Stream of optional elements to a Stream of present value elements. If the Optional contains a value, then return a Stream containing the value. Otherwise, it returns an empty stream.

Syntax

public Stream<T> stream()

Example

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class StreamMethodTest {
   public static void main(String[] args) {
      List<Optional<String>> list = Arrays.asList(
            Optional.empty(),
            Optional.of("TutorialsPoint"),
            Optional.empty(),
            Optional.of("Tutorix"));
      // If optional is non-empty, get the value in stream, otherwise return empty
      List<String> filteredListJava8 = list.stream()
            .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
            .collect(Collectors.toList());
      // Optional::stream method can return a stream of either one or zero element if data is present or not.
      List<String> filteredListJava9 = list.stream()
            .flatMap(Optional::stream)
            .collect(Collectors.toList());
      System.out.println(filteredListJava8);
      System.out.println(filteredListJava9);
   }
}

Output

[TutorialsPoint, Tutorix]
[TutorialsPoint, Tutorix]

Raja

Published on 21-Feb-2020 22:37:57