How can we create an unmodifiable Set in Java 9?

The immutable static factory method Set.of() can provide a convenient way to create unmodifiable sets in Java 9. 

An instance of a set created by using the Set.of() method has the following characteristics.

  • The set returned by a factory method is conventionally immutable. It means that elements can't be added, removed, or replaced from a Set. Calling of any mutator method on the Set causes UnsupportedOperationException.
  • If the contained elements of Set are mutable, it may cause the Set's contents to appear to change.
  • An immutable set can be created using static factory methods that don't allow null elements. If we are trying to create with null elements, it throws NullPointerException.
  • It rejects duplicate elements at the time of immutable set creation. The duplicate elements passed to a static factory method results in IllegalArgumentException.
  • The order of iteration of set elements is unspecified and subject to change.


Set.of(E... elements)


import java.util.Set;
public class SetOfMethodTest {
   public static void main(String args[]) {
      Set<String> names = Set.of("Adithya", "Bhavish", "Chaitanya", "Jai");
      System.out.println("Names - " + names);
      names.add("Raja");       // throws UnsupportedOperationException


Names - [Bhavish, Adithya, Jai, Chaitanya]
Exception in thread "main" java.lang.UnsupportedOperationException
 at java.base/java.util.ImmutableCollections.uoe(Unknown Source)
 at java.base/java.util.ImmutableCollections$AbstractImmutableSet.add(Unknown Source)
   at SetOfMethodTest.main(

Updated on: 21-Feb-2020


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