I. New Optional API

1. stream()

With Java 8, this is a way for getting a List of Values from a Stream of Optionals:

List<String> strings = streamOptional()
				.map(Optional::get)
				.collect(Collectors.toList());

But, if streamOptional() returns a Stream that contains an empty Optional instance. It will throw a java.util.NoSuchElementException. To handle this case, we can add filter() method:

// streamOptional(): [(Optional.empty(), Optional.of("one"), Optional.of("two"), Optional.of("three")]
List<String> strings = streamOptional()

.filter(Optional::isPresent)

.map(Optional::get)

.collect(Collectors.toList());
// Result: strings[one, two, three]

Java 9 provides new Optional::stream to work on Optional objects lazily, it returns a stream of either zero or one/more elements. It also checks empty element automatically and removes it.

// streamOptional(): [(Optional.empty(), Optional.of("one"), Optional.of("two"), Optional.of("three")]

List<String> newStrings = streamOptional()
				.flatMap(Optional::stream)
				.collect(Collectors.toList());
				
// Result: newStrings[one, two, three]

2. ifPresentOrElse()

Java 8 Optional provides ifPresent() method to perform the given action if a value is present, otherwise do nothing.

More at:

https://grokonez.com/java/java-9/java-9-optional-improvements

Java 9 Optional Improvements