Factory Method to Create Immutable List in Java SE 9

With Java 9, new factory methods are added to List interface to create immutable instances. These factory methods are convenience factory methods to create a collection in less verbose and in concise way.

Old way to create collections

Example

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Tester {
   public static void main(String []args) {

      List<String> list = new ArrayList<>();

      list.add("A");
      list.add("B");
      list.add("C");
      List<String> readOnlylist = Collections.unmodifiableList(list);
      System.out.println(readOnlylist);
      try {
         readOnlylist.remove(0);
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

Output

It will print the following output.

[A, B, C]
java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableList.remove(Collections.java:1317)
at Tester.main(Tester.java:15)

New Methods

With java 9, following methods are added to List interface along with their overloaded counterparts.

static <E> List<E> of(); // returns immutable list of zero element
static <E> List<E> of(E e1); // returns immutable list of one element
static <E> List<E> of(E e1, E e2); // returns immutable list of two elements
static <E> List<E> of(E e1, E e2, E e3);
//...
static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10);
static <E> List<E> of(E... elements);// returns immutable list of arbitrary number of elements.

Points to Note

• For List interface, of(...) method is overloaded to have 0 to 10 parameters and one with var args parameter.

• These methods returns immutable list and elements cannot be added, removed, or replaced. Calling any mutator method will always cause UnsupportedOperationException to be thrown.

New way to create immutable collections

Example

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Tester {
   public static void main(String []args) {
   
      List<String> list = List.of("A","B","C");
      System.out.println(list);
      try {
         list.remove(0);
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

Output

It will print the following output.

[A, B, C]
java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableList.remove(Collections.java:1317)
at Tester.main(Tester.java:10)