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("GeeksforGeeks");
          s.add("Practice");
  
          // An immutable copy of s
          Set<String> is  = ImmutableSet.copyOf(s);
  
          System.out.println(is);
      }
  }

  Output :

  [GeeksforGeeks, 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

// Java code illustrating of() method
import java.util.*;

class GfG
{
    public static void main(String args[])
    {
        // empty immutable set
        Set<String> is1 = Set.of();

        // non-empty immutable set
        Set is2 = Set.of("ide", "contribute", "support");

        // Lets try adding element in these set
        is1.add(null);
        is2.add("set");
    }
}

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.

// Java program to demonstrate that a set created using
// Collections.unmodifiableSet() can be modified indirectly.
import java.io.*;
import java.util.*;

class GFG {
public
    static void main(String[] args)
    {
        Set<String> s = new HashSet<String>();
        s.add("Geeks");
        Set<String> us = Collections.unmodifiableSet(s);

        // We change s and the changes reflect in us.
        s.add("Practice");
        s.add("Contribute");
        System.out.println(us);
    }
}

Output:

[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.

// Creating an immutable set using copyOf()
// and modifying original set.
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("GeeksforGeeks");
        s.add("Practice");

        // An immutable copy of s
        Set<String> is  = ImmutableSet.copyOf(s);

        // Now if we change 's', 'is' does not change
        s.add("Contribute");
        System.out.println(is);
    }
}

Output :

[GeeksforGeeks, Practice]

Reference
Creating Immutable Lists, Sets, and Maps