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.

Advantages of ImmutableList

• 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.

Note that it is an immutable collection, not collection of immutable objects, so the objects inside it can be modified.

Class Declaration:

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

Class hierarchy:

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

Creating ImmutableList
ImmutableList can be created by various methods. These include:

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

   
   

   
   
   

   // 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

   
   

   
   
   

   // 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 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 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 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 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]
     

Try to change ImmutableList

As mentioned earlier, the below program will throw 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.unmodifiableList()?

Collections.unmodifiableList creates a wrapper around the same existing List such that the wrapper cannot be used to modify it. However we can still change original List.

Output:

[Geeks, For, Geeks]

If we create an ImmutableList from an existing List and change the existing List, the ImmutableList does not change because a copy is created.

Output :

[Geeks]