Collection Factory Methods

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>

List Factory Method Examples

`List immutableEmptyList = List.of();`

- Initializes an empty, immutable `List`.

• Initializes an immutable List<Integer> with five initial elements.

• Initializes a mutable List<Integer> from an immutable List<Integer>.

Set Factory Method Examples

• `Set immutableEmptySet = Set.of();`
  - Initializes an empty, immutable `Set`.

  • Initializes an immutable Set<Integer> with five initial elements.

  • Initializes a mutable Set<Integer> from an immutable Set<Integer>.

  Map<K, V> Factory Method Examples

  • `Map immutableEmptyMap = Map.of();`
    - Initializes an empty, immutable `Map`.

    • Initializes an immutable Map<Integer, Integer> with two initial key-value entries.

    • Initializes an immutable Map<Integer, Integer> with two initial key-value entries.

    • Initializes a mutable Map<Integer, Integer> from an immutable Map<Integer, Integer>.

    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.