Streams Post Java 8 Updates

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Since their initial release with Java 8, Streams, and supporting classes including; Predicate and Collectors, have continued to see updates improving usability and helping to fill in functionality gaps. Let’s take a look at a few key changes.

Collectors.teeing

The teeing method was added to Collectors with Java 12. Teeing allows the merging of the results of two down stream collectors. Teeing can be useful from when processing a stream and needing two discrete types of values from it. In the below example the minimum and maximum value from a stream is being retrieved and put into a list.

record Employee(String name, 
	int monthsEmployed, boolean executive) {}

public static void main(String[] args) {

	Stream<Employee> streamOfEmployees = Stream.of(
		new Employee("Homer", 187, false),
		new Employee("Lenny", 122, false),
		new Employee("Carl", 93, false),
		new Employee("Montgomery", 552, true));

	streamOfEmployees.collect(
			Collectors.teeing(
			Collectors.maxBy(
				Comparator.comparing(Employee::monthsEmployed)),
			Collectors.minBy(
				Comparator.comparing(Employee::monthsEmployed)), 
            (e1, e2) -> {
                return List.of(e1.get(), e2.get());
            }
			)).forEach(System.out::println);
}

Output

Employee[name=Montgomery, monthsEmployed=552, executive=true]
Employee[name=Carl, monthsEmployed=93, executive=false]

Predicate.not()

The public static not(Predicate<T>) was added to Predicate with Java 11. This allows for a more concise definition of a negation predicate than what was available before Java 11. Below is an example of using not() with Java 11:

Java 11 Syntax

Stream<Employee> streamOfEmployees = 
		Stream.of(new Employee("Homer", 187, false),
		new Employee("Lenny", 122, false), 
		new Employee("Carl", 93, false),
		new Employee("Montgomery", 552, true));

streamOfEmployees.filter(Predicate.not(Employee::executive))
	.forEach(System.out::println);

And what the equivalent functionality would like like pre-Java 11:

Pre-Java 11 Syntax

Stream<Employee> streamOfEmployees = 
		Stream.of(new Employee("Homer", 187, false),
		new Employee("Lenny", 122, false), 
		new Employee("Carl", 93, false),
		new Employee("Montgomery", 552, true));
Predicate<Employee> isExecutive = Employee::executive;

streamOfEmployees.filter(isExecutive.negate())
	.forEach(System.out::println);

Stream.ofNullable()

Nulls are a source of frequent headaches to Java developers. Added in Java 9, Stream.ofNullable provides for null safety when processing a single element as a Stream. If the passed in value is null an empty Stream is returned. If the value is non-null the passed in value is wrapped as a single element stream. Below is an example of using a ofNullable to handle a potentially null List:

record Employee(String name, 
	int monthsEmployed, boolean executive) {}

public static void main(String[] args) {
	List<Employee> streamOfEmployees = null;
	System.out.println("streamOfEmployees is null");
	Stream.ofNullable(streamOfEmployees).
	flatMap(Collection::stream).forEach(System.out::println);

	streamOfEmployees = List.of(new Employee("Homer", 187, false),
	new Employee("Lenny", 122, false), 
	new Employee("Carl", 93, false),
	new Employee("Montgomery", 552, true));
	
	System.out.println("streamOfEmployees is not null");
	Stream.ofNullable(streamOfEmployees).
	flatMap(Collection::stream).forEach(System.out::println);
}

Output

streamOfEmployees is null
streamOfEmployees is not null
Employee[name=Homer, monthsEmployed=187, executive=false]
Employee[name=Lenny, monthsEmployed=122, executive=false]
Employee[name=Carl, monthsEmployed=93, executive=false]
Employee[name=Montgomery, monthsEmployed=552, executive=true]

Stream.iterate with Predicate

Also in Java 9 an additional Stream.iterate was added that takes a Predicate. The Predicate behaves as a “hasNext” and is processed before the “next” which is a UnaryOperator<T>:

System.out.println("Remaining days of the week:");
Stream.iterate(
		LocalDate.now().getDayOfWeek().getValue(), 
		d -> d <= DayOfWeek.SUNDAY.getValue(), 
		d -> ++d)
		.forEach(
		d -> System.out.println(DayOfWeek.of(d)));

Output

Remaining days of the week:
FRIDAY
SATURDAY
SUNDAY

Further Reading

Predicate Javadoc: https://docs.oracle.com/en/java/javase/16/docs/api/java.base/java/util/function/Predicate.html

Stream Javadoc: https://docs.oracle.com/en/java/javase/16/docs/api/java.base/java/util/stream/Stream.html

Collectors Javadoc: https://docs.oracle.com/en/java/javase/16/docs/api/java.base/java/util/stream/Collectors.html

Happy Coding!