Java 9 Collection Literals

An outline of some of the Java 9 changes to Collection literals and other areas of the core API.

List.of and Set.of


List<Integer> digits = List.of(3, 1, 4, 5, 9, 2);

Set<Integer> digits = Set.of(2, 7, 31, 127, 8191, 131871);

Points to note:

Similarities to Java 8 Streams

We already came across the of function in Java 8 when we seen Stream.of.

Null not supported

It’s also not allowed to insert a null value into the of function as this is the preferred coding-style these days.

Immutable object returned

It should also be noted here that while similar to Arrays.asList the objects returned by the of functions here are immutable. As a result if you try to sort or modify the collection you will get an java.lang.UnsupportedOperationException.


The new Map.of function is follows the api structure static <E> List<E> of(E... elements).

The syntax for using the new Map.of function is as follows:

jshell> Map.of("Jan", 1, "Feb", 2, "March", 3)
$1 ==> {Feb=2, March=3, Jan=1}

Points to note:

Varags syntax instead of Map<K, V>

Since the type of Key or Value can change we need to use a flat varags structure to represent the data. This can seem odd when looking at data that is difficult to separate such as: Map.of(1,1,2,2,3,3).

Null not supported

Again null is not supported here.

Maximum of 11 entries

Over 11 entries will produce an error. To get around this you will need to use Map.ofEntries(Map.Entry ...)


Java 8 introduced the concept of Optional as an operator to help prevent constantly checking for nulls. With Java 9 we now see more enhanced Optional support in the form of:

  • New API methods yielding optional values:
    • HttpClient.authenticator/cookieManager/proxy/sslParameters
    • ServiceLoader.findFirst()
    • Runtime.version().build/pre/post
    • ProcessHandle.of
    • ProcessHandle.Info.command/commandLine/arguments/startInstant/totalCpuDuration/user
  • New Optional.or() method:
    • If a value is present, returns an Optional describing the value, otherwise returns an Optional produced by the supplying function.
    • Optional<T> or(Suppplier<? extends Optional<? extends T>> supplier)
    • Example:
  • New method
    • Yields a stream of length 0 or 1
    • Can be useful in place of a stream filter to drop empty results
    • Example: Stream<User> users =

As a result of the above inclusions it is more common now to see code that no longer requires null checks all over the place. The idea here is to assist development teams to create and maintain cleaner code bases but as a result it is necessary to understand the core Java API better. Making better use of functional programming techniques like Java Streams will help to make the above API changes easier to use in the long run.


As you can see Java 9 has introduced some useful new functions but in many cases they appear to come with them a number of technical intricacies.

While the above features do provide syntactical sugar we can only hope that in time they will improve. Perhaps they will begin to serve a foundation for future changes to the core Java language but only time will tell. For the time being they remain useful when programming in a reactive style.