Immutable List in Java

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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:

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]

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]

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]

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.

// 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");
    }
}

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.

// 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);
    }
}

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.

// 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);
    }
}