Immutable Set in Java

• As the name suggest these Set are immutable.
• If any attempt made to add, delete and update elements in the set we will have UnsupportedOperationException.
• An ImmutableSet does not allow null element either.
• If any attempt made to create an ImmutableSet with null element, we will have NullPointerException. If any attempt made to add null element in set, we will have UnsupportedOperationException.
• One advantage of any immutable collection (Set, Map, List) is thread safety. These are automatically thread sage as they are immutable.
• Note that it is an immutable collection, not collection of immutable objects, so the objects inside it can be modified.

Creating an ImmutableSet in Java

• Using copyOf() method in Guava We use an existing Set to create an ImmutableSet.

  
  

  
  
  

  // Creating an immutable set using copyOf() import java.util.*; import com.google.common.collect.ImmutableSet; import java.io.*;    class GfG {     public static void main(String args[])     {         // creating empty set         Set<String> s = new HashSet<String>();         s.add("Lazyroar");         s.add("Practice");                    // An immutable copy of s         Set<String> is  = ImmutableSet.copyOf(s);                      System.out.println(is);     } }

  Output :

  [Lazyroar, Practice]

• Using Of() method in Guava

  
  

  
  
  

  // Java code illustrating of() method to // create a ImmutableSet import java.util.*; import com.google.common.collect.ImmutableSet;    class GfG {     public static void main(String args[])     {                   // non-empty immutable set         ImmutableSet<String> is =                   ImmutableSet.of("ide", "code");                      // Lets try adding element in these set         System.out.println(is);                  } }

  Output :

  [ide, code]

• Using Java 9 Factory Of() method
  In Java, if we use of with Set, Map or List, an Immutable Set is created.

  
  

  
  
  

  // Java code illustrating of() method to // create a ImmutableSet import java.util.*; import com.google.common.collect.ImmutableSet;    class GfG {     public static void main(String args[])     {                   // non-empty immutable set         Set<String> is = Set.of("ide", "code");                      // Let's print the set         System.out.println(is);                  } }

  Output :

  [ide, code]

What if we try to change an ImmutableSet?
It throws UnsupportedOperationException

Output :

Exception in thread "main" java.lang.UnsupportedOperationException
    at com.google.common.collect.ImmutableCollection.add(ImmutableCollection.java:218)
    at ImmutableListDemo.main(Main.java:16)

How is it different from Collections.unmodifiableSet()?
Collections.unmodifiableSet creates a wrapper around the same existing set such that the wrapper cannot be used to modify it. However we can still change original Set.

[Geeks, Practice, Contribute]

If we create an ImmutableSet from an existing set and change the existing set, the Immutable Set does not change because a copy is created.

Output :

[Lazyroar, Practice]

Reference
Creating Immutable Lists, Sets, and Maps