Immutable List in Java

• ImmutableList, as suggested by the name, is a type of List which is immutable. It means that the content of the List are fixed or constant after declaration, that is, they are read-only.
• If any attempt made to add, delete and update elements in the List, UnsupportedOperationException is thrown.
• An ImmutableList does not allow null element either.
• If any attempt made to create an ImmutableList with null element, NullPointerException is thrown. If any attempt is made to add null element in List, UnsupportedOperationException is thrown.

• They are thread safe.
• They are memory efficient.
• Since they are immutable, hence they can be passed over to third party libraries without any problem.

@GwtCompatible(serializable=true,
               emulated=true)
public abstract class ImmutableList
extends ImmutableCollection
implements List, RandomAccess

java.lang.Object
  ↳ java.util.AbstractCollection
      ↳ com.google.common.collect.ImmutableCollection
          ↳ com.google.common.collect.ImmutableList 

1. From existing List using copyOf() function of Guava Java

   // Below is the Java program to create ImmutableList
   
   import com.google.common.collect.ImmutableList;
   import java.util.*;
   
   class GFG {
   
       // Function to create ImmutableList from List
       public static <T> void iList(List<T> list)
       {
           // Create ImmutableMap from Map using copyOf()
           ImmutableList<T> immutableList =
                             ImmutableList.copyOf(list);
   
           // Print the ImmutableMap
           System.out.println(immutableList);
       }
   
       public static void main(String[] args)
       {
           List<String> list = new ArrayList<>(
               Arrays.asList("Geeks", "For", "Geeks"));
   
           iList(list);
       }
   }
   

   Output:

   [Geeks, For, Geeks]
   

2. New ImmutableList using of() function from Guava Java

   // Below is the Java program to create ImmutableList
   
   import com.google.common.collect.ImmutableList;
   import java.util.*;
   
   class GFG {
   
       // Function to create ImmutableList
       public static void iList()
       {
           // Create ImmutableList using of()
           ImmutableList<String> immutableList = 
                  ImmutableList.of("Geeks", "For", "Geeks");
   
           // Print the ImmutableMap
           System.out.println(immutableList);
       }
   
       public static void main(String[] args)
       {
           iList();
       }
   }
   

   Output:

   [Geeks, For, Geeks]
   

3. Using Java 9 Factory Of() method In Java, use of() with Set, Map or List to create an Immutable List. Please Note: The programs below are of Java 9. Hence you would need a Java 9 compiler to run them. Java

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

   Output:

   [Geeks, For, Geeks]
   

4. Using Builder() from ImmutableList In Guava, ImmnutableList class provides a function Builder(). Through this function, a new ImmutableList can be created, or an ImmutableList can be created from an existing List or both.
   • Creating a new ImmutableList Java

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

     Output:

     [Geeks, For, Geeks]
     

   • Creating an ImmutableList from existing List Java

     // Java code illustrating of() method to
     // create a ImmutableList
     import java.util.*;
     import com.google.common.collect.ImmutableList;
     
     class GfG {
         public static void main(String args[])
         {
             // non-empty immutable set
             List<String> list = List.of("Geeks", "For", "Geeks");
             ImmutableList<String> iList = ImmutableList.<String>builder()
                                               .addAll(list)
                                               .build();
     
             // Let's print the List
             System.out.println(iList);
         }
     }
     

     Output:

     [Geeks, For, Geeks]
     

   • Creating a new ImmutableList including the existing List Java

     // Java code illustrating of() method to
     // create a ImmutableList
     import java.util.*;
     import com.google.common.collect.ImmutableList;
     
     class GfG {
         public static void main(String args[])
         {
             // non-empty immutable set
             List<String> list = List.of("Geeks", "For", "Geeks");
             ImmutableList<String> iList = ImmutableList.<String>builder()
                                               .addAll(list)
                                               .add("Computer", "Portal", )
                                               .build();
     
             // Let's print the set
             System.out.println(iList);
         }
     }
     

     Output:

     [Geeks, For, Geeks, Computer, Portal]
     

// Java code to show that UnsupportedOperationException
// will be thrown when ImmutableList is modified.
import java.util.*;

class GfG {
    public static void main(String args[])
    {
        // empty immutable map
        List<String> list = List.of();

        // Lets try adding element in  List
        List.add("Geeks");
    }
}

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

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

class GFG {
    public static void main(String[] args)
    {
        List<String> list = new ArrayList<>();
        list.add("Geeks");

        // Create ImmutableList from List using copyOf()
        List<String> iList = Collections.unmodifiableList(list);

        // We change List and the changes reflect in iList.
        list.add("For");
        list.add("Geeks");

        System.out.println(iList);
    }
}

[Geeks, For, Geeks]

// Below is a Java program for
// Creating an immutable List using copyOf()
// and modifying original List.
import java.io.*;
import java.util.*;
import com.google.common.collect.ImmutableList;

class GFG {
    public static void main(String[] args)
    {
        List<String> list = new ArrayList<>();
        list.add("Geeks");

        // Create ImmutableList from List using copyOf()
        ImmutableList<String> iList = ImmutableList.copyOf(list);

        // We change List and the changes wont reflect in iList.
        list.add("For");
        list.add("Geeks");

        System.out.println(iList);
    }
}

[Geeks]