Java List.of() Method With Examples

Java List.of() Method With Examples | The of() method of the List interface works very similarly to the Arrays.asList() method. Both Arrays.asList() and List.of() methods can be used to get a list representation of an array.

Arrays.asList() method returns a fixed-size list backed by the specified array. The changes made to the array will be visible in the returned list. Whereas the List.of() method returns an unmodifiable list i.e. the modification done in the array will not be available to the list. Also see:- How to convert list to an array using toArray() method.

Following are the overloaded forms of the Java List.of() method:-

1. static List of()
2. static List of(E e1)
3. static List of(E e1, E e2)
4. static List of(E e1, E e2, E e3)
5. static List of(E e1, E e2, E e3, E e4)
6. static List of(E e1, E e2, E e3, E e4, E e5)
7. static List of(E e1, E e2, E e3, E e4, E e5, E e6)
8. static List of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)
9. static List of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)
10. static List of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)
11. static List of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)
12. static List of(E… elements)

The of() method in the List interface was introduced in the Java 9 version. Therefore to use the List.of() method Java 9 or the later version is required.

Examples of Java List.of() Method

import java.util.Arrays;
import java.util.List;

public class Main {
   public static void main(String[] args) {
      Integer[] array = { 10, 15, 20, 25, 30 };
      List<Integer> numbers = List.of(array);

      System.out.println("Array: " + Arrays.toString(array));
      System.out.println(array.getClass().getName());

      System.out.println("List: " + numbers);
      System.out.println(numbers.getClass().getName());
   }
}

Output:-

Array: [10, 15, 20, 25, 30]
[Ljava.lang.Integer;
List: [10, 15, 20, 25, 30]
java.util.ImmutableCollections$ListN

If we have multiple values and want to create a list from them then there is not necessary to create the array. We can directly pass those values to the Java List.of() method and we will have a list representing those values. See the below example.

import java.util.List;

public class Main {
   public static void main(String[] args) {
      List<Integer> numbers = List.of(11, 22, 33, 44, 55);
      System.out.println("List: " + numbers);
   }
}

Output:-

List: [11, 22, 33, 44, 55]

Program to convert String Array to List of String using Java List.of() Method

import java.util.List;

public class Main {
   public static void main(String[] args) {
      String[] arr = { "JavaScript", "HTML", "CSS" };
      List<String> list = List.of(arr);
      System.out.println("List = " + list);
   }
}

Output:-

List = [JavaScript, HTML, CSS]

Unlike the Arrays.asList() method, while using the List.of() method if we modify the array then the change will be available to the list.

import java.util.Arrays;
import java.util.List;

public class Main {
   public static void main(String[] args) {
      String[] arr = { "JavaScript", "HTML", "CSS" };
      List<String> list = List.of(arr);

      arr[0] = "Java";
      System.out.println("Array: " + Arrays.toString(arr));
      System.out.println("List: " + list);
   }
}

Output:-

Array: [Java, HTML, CSS]
List: [JavaScript, HTML, CSS]

UnsupportedOperationException By Java List.of() Method

Even though List.of() method introduced as an enhancement to the Arrays.asList() method. But the List.of() method doesn’t create an except copy of the given array. It only prevents array modification from affecting the list representation.

The growable nature of the List and the fixed size of the array again got conflict with each other. In Java, the array has a fixed size once it is initialized. We can’t increase or decrease the size of the array after initialization. Whereas the List is growable in nature and we can modify its size. Elements can be added or removed from the list.

After using the Java List.of() method to convert the array to a list if we try to modify the size of the list i.e try to add an element or remove an element from the list then we will get java.lang.UnsupportedOperationException.

String[] arr = { "JavaScript", "HTML", "CSS" };
List<String> list = List.of(arr);
list.add("Java");

Output:-

Exception in thread “main” java.lang.UnsupportedOperationException

String[] arr = { "JavaScript", "HTML", "CSS" };
List<String> list = List.of(arr);
list.remove(0);

Output:-

Exception in thread “main” java.lang.UnsupportedOperationException

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