Java 9 – Immutable Collections with Factory Methods

Learn to create immutable collections such as immutable list, immutable set and immutable map using new factory methods in Java 9.

Table of Contents

Create Immutable List
Create Immutable Set
Create Immutable Map

Create Immutable List

Use List.of() static factory methods to create immutable lists. It has following different overloaded versions –

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 e3)
static <E> List<E>	of(E e1, E e2, E e3, E e4)
static <E> List<E>	of(E e1, E e2, E e3, E e4, E e5)
static <E> List<E>	of(E e1, E e2, E e3, E e4, E e5, E e6)
static <E> List<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)
static <E> List<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)
static <E> List<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)
static <E> List<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)
//varargs
static <E> List<E>	of(E... elements)

The List instances created by these methods have the following characteristics:

1. These lists are immutable. Elements cannot be added, removed, or replaced in these lists. Calling any mutator method (i.e. add, addAll, clear, remove, removeAll, replaceAll) will always cause UnsupportedOperationException to be thrown.
2. They do not allow null elements. Attempts to add null elements result in NullPointerException.
3. They are serializable if all elements are serializable.
4. The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.

Lets see few examples of usage of immutable list.

package com.howtodoinjava;

import java.util.List;

public class ImmutableCollections 
{
    public static void main(String[] args) 
    {
        List<String> names = List.of("Lokesh", "Amit", "John");
        
        //Preserve the elements order
        System.out.println(names);
        
        //names.add("Brian"); //UnsupportedOperationException occured
        
        //java.lang.NullPointerException
        //List<String> names2 = List.of("Lokesh", "Amit", "John", null); 
    }
}

Output:

[Lokesh, Amit, John]

Create Immutable Set

Set behave very similar to List with only few differences. e.g.

1. Set do not allow duplicate elements as well. Any duplicate element passed will result in IllegalArgumentException.
2. The iteration order of set elements is unspecified and is subject to change.

All Set factory methods have the same signature as List.

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 e3)
static <E> Set<E>	of(E e1, E e2, E e3, E e4)
static <E> Set<E>	of(E e1, E e2, E e3, E e4, E e5)
static <E> Set<E>	of(E e1, E e2, E e3, E e4, E e5, E e6)
static <E> Set<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)
static <E> Set<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)
static <E> Set<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)
static <E> Set<E>	of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)
//varargs
static <E> Set<E>	of(E... elements)

Let’s see few examples of immutable sets.

import java.util.Set;

public class ImmutableCollections {
    public static void main(String[] args) {
        Set<String> names = Set.of("Lokesh", "Amit", "John");
        
        //Elements order not fixed
        System.out.println(names);
        
        //names.add("Brian"); //UnsupportedOperationException occured
        
        //java.lang.NullPointerException
        //Set<String> names2 = Set.of("Lokesh", "Amit", "John", null); 
        
        //java.lang.IllegalArgumentException
        //Set<String> names3 = Set.of("Lokesh", "Amit", "John", "Amit"); 
    }
}

Create Immutable Map

Map factory methods are same as List or Set overloaded factory methods. Only difference is that the signatures of the of methods take alternating keys and values as arguments. e.g.

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>	ofEntries(Map.Entry<? extends K,? extends V>... entries)

Java 9 also provide a special method for creating Map entry instance.

static <K,V> Map.Entry<K,V>	entry​(K k, V v)

Let’s take an example of creating immutable Map in java 9.

import java.util.Map;

public class ImmutableCollections {
    public static void main(String[] args) {
        Map<String, String> names = Map.ofEntries(
                Map.entry("1", "Lokesh"),
                Map.entry("2", "Amit"),
                Map.entry("3", "Brian"));
        
        System.out.println(names);
        
        //UnsupportedOperationException
        //names.put("2", "Ravi");
    }
}

Output:

{1=Lokesh, 2=Amit, 3=Brian}

Clearly, new factory methods to create immutable collections in java 9 are very much readable and easy to use.

Drop me your opinion on these immutable collections in comments section.

Happy Learning !!