Which factory methods have added for collections in Java 9?

Factory methods are a special type of static methods that can be used to create unmodifiable instances of collections. It means that we can use these methods to create a list, set, and map of a small number of elements.

List.of()

The List.of() is a static factory method that provides a convenient way to create immutable lists.

Syntax

List.of(elements...)

Example

import java.util.List;
public class ListTest {
   public static void main(String[] args) {
      List<String> list = List.of("item 1", "item 2", "item 3", "item 4", "item 5");
      for(String l : list) {
         System.out.println(l);
      }
   }
}

Output

item 1
item 2
item 3
item 4
item 5

Set.of() method

The Set.of() is a static factory method that provides a convenient way to create immutable sets.

Syntax

Set.of(elements...)

Example

import java.util.Set;
public class SetTest {
   public static void main(String[] args) {
      Set<String> set = Set.of("Item 1", "Item 2", "Item 3", "Item 4", "Item 5");
      for(String s : set) {
         System.out.println(s);
      }
   }
}

Output

Item 5
Item 1
Item 2
Item 3
Item 4

Map.of() and Map.ofEntries() methods

The Map.of() and Map.ofEntries() are static factory methods that provide a convenient way to create immutable maps.

Syntax

Map.of(k1, v1, k2, v2)
Map.ofEntries(entry(k1, v1), entry(k2, v2),...)

Example

import java.util.Map;
public class MapTest {
   public static void main(String[] args) {
      Map<Integer, String> map = Map.of(101, "Raja", 102, "Adithya", 103, "Jai");
      for(Map.Entry<Integer, String> m : map.entrySet()) {
         System.out.println(m.getKey() + " " + m.getValue());
      }
   }
}

Output

103 Jai
102 Adithya
101 Raja