Java Optional stream()

In this guide, you will learn about the Optional stream() method in Java programming and how to use it with an example.

1. Optional stream() Method Overview

Definition:

The stream() method of the Optional class in Java is used to transform the Optional instance into a Stream. If a value is present inside Optional, this method returns a sequential Stream containing only that value, otherwise, it returns an empty Stream.

Syntax:

public Stream<T> stream()

Parameters:

- The method does not take any parameters.

Key Points:

- It is a terminal operation, meaning it doesn’t return a new Optional, but a Stream.

- It is useful when working with a collection of Optional objects and you want to perform Stream operations on the values present inside them.

- This method is available from Java 9 onwards.

2. Optional stream() Method Example

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

public class OptionalStreamExample {

    public static void main(String[] args) {

        // Create a list of Optional objects
        List<Optional<String>> listOfOptionals = Arrays.asList(
                Optional.empty(),
                Optional.of("Hello"),
                Optional.of("World"),
                Optional.empty()
        );

        // Transform the list of Optionals into a Stream and filter out the empty Optionals
        List<String> filteredValues = listOfOptionals.stream()
                .flatMap(Optional::stream)
                .collect(Collectors.toList());

        System.out.println(filteredValues);
    }
}

Output:

[Hello, World]

Explanation:

In this example, we created a list containing several Optional objects, some of which are empty. 

Using the stream() method along with flatMap(), we transformed each Optional in the list into a Stream, thus filtering out the empty Optional objects. 

The result is a list containing the values present in the original Optional objects, demonstrating how Optional::stream() can be used to process values in a collection of Optional objects.