Java - Collection Factory Methods

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In Java 9, collections were enhanced to have few new methods to create immutable list in an easy and concise way. These new factory methods were added to List, Set and Map interfaces to create immutable instances. These factory methods are mainly convenience factory methods in order to create a collection in less verbose and in concise way.

Before Java 9, following syntax was used to create a immutable list.

List unmodifiableList = Collections.unmodifiableList(arrayList);

Where arrayList is a mutable list instance. So we are required to create a list and then using unmodifiableList() we get an immutable instance from which we cannot add/remove an element.

Now from Java 9, following methods can be used to create a immutable list.

static <E> List<E> of();
static <E> List<E> of(E e1);
static <E> List<E> of(E... elements)
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);

So from Java 9 onwards, following syntax can be used to create a immutable list. of(E... elements) method can be used to have more than 10 elements in an immutable list.

List<String> unmodifiableList = List.of("A", "B", "C");

On similar pattern, Set interface is having these new methods to create a unmodifiable Set to get an instance of a set from which we cannot add/remove an element.

static <E> Set<E> of();
static <E> Set<E> of(E e1);
static <E> Set<E> of(E... elements)
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);

So from Java 9 onwards, following syntax can be used to create a immutable set. of(E... elements) method can be used to have more than 10 elements in an immutable set.

Set<String> unmodifiableSet = Set.of("A", "B", "C");

In case of Map interface, we've ofEntries(...) can be used to accept var args parameter other than of() methods as shown below.

static <K,V> Map<K,V> of();
static <K,V> Map<K,V> of(K k1, V v1);
static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries)
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 k3, V v3);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8,K k9, V v9);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8,K k9, V v9, K k1, V v10);

So from Java 9 onwards, following syntax can be used to create a immutable map.

Map<String, String> unmodifiableMap = Map.of("A","Apple", "B", "Boy", "C", "Cat");

Example - Creating unmodifiable list before Java 9

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Tester {
   public static void main(String[] args) {
      List<String> list = new ArrayList<>();

      list.add("A");
      list.add("B");
      list.add("C");
      list = Collections.unmodifiableList(list);
      System.out.println(list);
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

[A, B, C]

Example - Creating unmodifiable list from Java 9

package com.tutorialspoint;

import java.util.List;

public class Tester {
   public static void main(String[] args){
	   List<String> list =  List.of("A","B","C");
	   System.out.println(list);
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

[A, B, C]

Example - Creating unmodifiable set before Java 9

package com.tutorialspoint;

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

public class Tester {
   public static void main(String[] args){
      Set<String> set = new HashSet<>();
      set.add("A");
      set.add("B");
      set.add("C");
      set = Collections.unmodifiableSet(set);
      System.out.println(set);
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

[A, B, C]

Example - Creating unmodifiable set from Java 9

package com.tutorialspoint;

import java.util.Set;

public class Tester {
   public static void main(String[] args){
	   Set<String> set =  Set.of("A","B","C");
	   System.out.println(set);
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

[A, B, C]

Example - Creating unmodifiable map before Java 9

package com.tutorialspoint;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class Tester {
   public static void main(String[] args){
      Map<String, String> map = new HashMap<>();

      map.put("A","Apple");
      map.put("B","Boy");
      map.put("C","Cat");
      map = Collections.unmodifiableMap(map);
      System.out.println(map);
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

{A=Apple, B=Boy, C=Cat}

Example - Creating unmodifiable map from Java 9

package com.tutorialspoint;

import java.util.Map;

public class Tester {
   public static void main(String[] args){
      Map<String, String> map = Map.of("A","Apple","B","Boy","C","Cat");
      System.out.println(map);
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

{C=Cat, A=Apple, B=Boy}