Collection Factory Methods

Introduction#

The arrival of Java 9 brings many new features to Java’s Collections API, one of which being collection factory methods. These methods allow for easy initialization of immutable collections, whether they be empty or nonempty.

Note that these factory methods are only available for the following interfaces: List<E>, Set<E>, and Map<K, V>

Syntax#

static <E> List<E> of()
static <E> List<E> of(E e1)
static <E> List<E> of(E e1, E e2)
static <E> List<E> of(E e1, E e2, ..., E e9, E e10)
static <E> List<E> of(E... elements)
static <E> Set<E> of()
static <E> Set<E> of(E e1)
static <E> Set<E> of(E e1, E e2)
static <E> Set<E> of(E e1, E e2, ..., E e9, E e10)
static <E> Set<E> of(E... elements)
static <K,V> Map<K,V> of()
static <K,V> Map<K,V> of(K k1, V v1)
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2)
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, ..., K k9, V v9, K k10, V v10)
static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries)

Parameters#

Method w/ Parameter	Description
`List.of(E e)`	A generic type that can be a class or interface.
`Set.of(E e)`	A generic type that can be a class or interface.
`Map.of(K k, V v)`	A key-value pair of generic types that can each be a class or interface.
`Map.of(Map.Entry<? extends K, ? extends V> entry)`	A `Map.Entry` instance where its key can be `K` or one of its children, and its value can be `V` or any of its children.
## List Factory Method Examples

List<Integer> immutableEmptyList = List.of();
- Initializes an empty, immutable List<Integer>.
List<Integer> immutableList = List.of(1, 2, 3, 4, 5);
- Initializes an immutable List<Integer> with five initial elements.
List<Integer> mutableList = new ArrayList<>(immutableList);
- Initializes a mutable List<Integer> from an immutable List<Integer>.

Set Factory Method Examples

Set<Integer> immutableEmptySet = Set.of();
- Initializes an empty, immutable Set<Integer>.
Set<Integer> immutableSet = Set.of(1, 2, 3, 4, 5);
- Initializes an immutable Set<Integer> with five initial elements.
Set<Integer> mutableSet = new HashSet<>(immutableSet);
- Initializes a mutable Set<Integer> from an immutable Set<Integer>.

Map<K, V> Factory Method Examples

Map<Integer, Integer> immutableEmptyMap = Map.of();
- Initializes an empty, immutable Map<Integer, Integer>.
Map<Integer, Integer> immutableMap = Map.of(1, 2, 3, 4);
- Initializes an immutable Map<Integer, Integer> with two initial key-value entries.
Map<Integer, Integer> immutableMap = Map.ofEntries(Map.entry(1, 2), Map.entry(3, 4));
- Initializes an immutable Map<Integer, Integer> with two initial key-value entries.
Map<Integer, Integer> mutableMap = new HashMap<>(immutableMap);
- Initializes a mutable Map<Integer, Integer> from an immutable Map<Integer, Integer>.