Java Arrays.asList() Method

Java Arrays.asList() Method | Arrays class contains useful methods for the array manipulation operations like sorting, searching, copying, comparing two arrays, converting to string, and e.t.c. It also contains the asList() method which is used to return a list representation of the given 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, and changes made to the list will be visible in the array. The list returned by the Arrays.asList() method is modifiable. Also see:- How to convert list to an array using toArray() method.

The returned list is Serializable and implements RandomAccess. This method acts as the bridge between array-based and collection-based APIs, in combination with Collection.toArray(). The Arrays.asList() method runs in O(1) time. The Arrays.asList() method is given as follows:-

public static <T> List<T> asList(T... a) {
   return new ArrayList<>(a);
}

The Arrays.asList() method will throw NullPointerException if the specified array is null. Let us see some examples of converting different types of arrays as Lists using the Arrays.asList() method.

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

public class Main {
   public static void main(String[] args) {
      // Integer array
      Integer[] arr = { 11, 12, 13, 14, 15 };
      System.out.println(arr.getClass().getName());

      // convert Integer array to list
      List<Integer> list = Arrays.asList(arr);

      // display list
      System.out.println("List = " + list);
      System.out.println(list.getClass().getName());
   }
}

Output:-

[Ljava.lang.Integer;
List = [11, 12, 13, 14, 15]
java.util.Arrays$ArrayList

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 asList() method and we will have a list representing those values. See the below example.

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

public class Main {
   public static void main(String[] args) {
      List<Integer> numbers = Arrays.asList(10, 20, 30, 40, 50);
      System.out.println("List: " + numbers);
   }
}

Output:-

List: [10, 20, 30, 40, 50]

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

// String array
String[] arr = {"Java", "Python", "C++"};
// convert String array to list
List<String> list = Arrays.asList(arr);
// display list
System.out.println("List = " + list);

Output:-

List = [Java, Python, C++]

Limitation of Arrays.asList()

Be careful while using the Arrays.asList() method because the changes made to the array will be visible in the returned list, and changes made to the list will be visible in the array.

Integer[] array = { 11, 12, 13, 14, 15 };
List<Integer> numbers = Arrays.asList(array);

// modifying array
array[0] = 90;

System.out.println("Array: " + numbers);
System.out.println("List: " + numbers);

Output:-

Array: [90, 12, 13, 14, 15]
List: [90, 12, 13, 14, 15]

It is not recommended to convert the array to a list using the Arrays.asList() method if we are going to modify the array/list later. In that case, we can use List.of() method which creates ImmutableCollections i.e once created can not be modified.

In the case of primitive data types, the type of the specified array must be a Wrapper Class(Integer, Double, etc). If we pass int a[ ] to Arrays.asList() method, then it will return a List and not List, as “autoboxing” doesn’t happen in this case. The int a[ ] is itself identified as an object, and a List of int arrays is returned, instead of the list of integers, which will give an error in various Collection functions.

int[] array = { 11, 12, 13, 14, 15 };
List<?> numbers = Arrays.asList(array);

System.out.println(Arrays.toString(array));
System.out.println(numbers);

Output:-

[11, 12, 13, 14, 15]
[[I@aec6354]

UnsupportedOperationException By Java Arrays.asList() Method

As we know, In Java, the array size is fixed and we can’t modify the array size once initialized. After initialization, we can increase or decrease its size. In order to add or remove elements from the existing array we have to create another array.

Whereas we can modify the size of the list after initialization. We can add more elements to the list and we can remove elements from it. The modification is applied to the same list.

Arrays.asList() method act as a bridge between array and list but this is the situation where they got conflict with each other. Array size can’t be modified whereas the list size can be modified. Arrays.asList() has given us a list presentation of the array elements.

After converting the array to a list using the Arrays.asList() method if we try to add or remove elements from the list then we will get java.lang.UnsupportedOperationException.

Integer[] array = { 11, 12, 13, 14, 15 };
List<Integer> numbers = Arrays.asList(array);
// modifying list
numbers.add(100);

Exception:-

Exception in thread “main” java.lang.UnsupportedOperationException

Integer[] array = { 11, 12, 13, 14, 15 };
List<Integer> numbers = Arrays.asList(array);
// modifying list
numbers.remove(0);

Exception:-

Exception in thread “main” java.lang.UnsupportedOperationException

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