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2017-04-21
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Collection Factory Methods

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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/ ParameterDescription
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.

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