Core Java

Java 9 JShell Examples: Collections Static Factory Methods

This post continues my exploration of Java9 features from my My Top Java 9 Features blog post. Here we are experimenting with Java9 Collections Static Factory Methods in the List, Set and Map interfaces.

Collections Static Factory Methods

Java9 makes it easier to create immutable lists using its new static Factory Methods

List and Set

There are 12 Set.of and List.of methods –

  • List.of() or Set.of()
  • List.of(E e1) or Set.of(E e1) to E e10
  • List.of(E… elements) or Set.of(E… elements)


jshell> Set.of()
$1 ==> []
| created scratch variable $1 : Set<Object>

Note the inference as a List object

To static


of (E e1, E e2, E e3) –

jshell> List.of("one","two","three")
$2 ==> [one, two, three]
| created scratch variable $2 : List<String>

Note the inference as a List


The number of arguments keeps increasing until E e10, at which point we can use vararg –



of (E… elements)


Similarly Map defines –

  • staticMapof ()
  • staticMapof (K k1, V v1) to (K k10, V v10)
  • staticMapofEntries (Map.Entry
    … entries) – Note the use of


jshell> Map.of()
$12 ==> {}
jshell> Map.of("key1", "value1", "key2", "value2")
$13 ==> {key1=value1, key2=value2}
| created scratch variable $13 : Map<String,String>

Characteristics of Collections Static Factory Methods

Common characteristics of these static Factory Methods Lists, Sets and Maps are –

  • Structurally Immutable – UnsupportedOperationException is thrown, although the elements themselves are immutable
jshell> Set<String> immutableSet = Set.of("one","two","three")
immutableSet ==> [three, two, one]
| created variable immutableSet : Set<String>
jshell> immutableSet.add("four")
| java.lang.UnsupportedOperationException thrown:
  • No Nulls – NullPointerException thrown
jshell> List<Object> notNullList = List.of(null)
| Warning:
| non-varargs call of varargs method with inexact argument type for last parameter;
| cast to java.lang.Object for a varargs call
| cast to java.lang.Object[] for a non-varargs call and to suppress this warning
| List<Object> notNullList = List.of(null);
| ^--^
| java.lang.NullPointerException thrown:
| at List.of (
| at (#10:1)
  • Serialized – Serialized if elements Serializable

List Specific Characteristics

  • Order – Order is maintained the same as elements input
jshell> List<String> immutableList = List.of("one","two","three")
immutableList ==> [one, two, three]
| created variable immutableList : List<String>

Set Specific Characteristics

  • Reject Duplicates – The Set will also reject duplicates at creation time with an IllegalArgumentException –
jshell> Set.of("one","one")
| java.lang.IllegalArgumentException thrown: duplicate element: one

Map Specific Characteristics

  • Reject Duplicate Keus – The Map will reject duplicate keys with IllegalArgumentException –
jshell> Map.of("key1", "value1", "key1", "value2")
| java.lang.IllegalArgumentException thrown: duplicate key: key1
| at ImmutableCollections$MapN.<init> (
| at Map.of (
| at (#15:1)
  • Iteration is also not guaranteed


These are a useful and quick method for creating immutable collections, and jshell provides a good test ground to learn about the new methods and their associated characteristics

Published on Java Code Geeks with permission by Martin Farrell, partner at our JCG program. See the original article here: Java9 JShell Examples : Collections Static Factory Methods

Opinions expressed by Java Code Geeks contributors are their own.

Martin Farrell

Martin is a Software Developer and Consultant specializing in Java and the Spring Framework. When not coding he spends time with his wife and two children, or cycling his bike.
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