How to convert between List and Array in Java?

The List interface is a Collection and it stores a sequence of elements. ArrayList is the most popular implementation of the List interface. A list provides user has quite precise control over where an element to be inserted in the List. These elements are accessible by their index and are searchable. ArrayList is the most common implementation of List interface.

An array is a data structure which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

A List interface provides methods to convert a list to an array using toArray() methods. One of the toArray() method accepts another array of object which helps in converting a list to array of specific types. Otherwise we can use toArray() without parameter to get an array of Object. In this article, we're covering the toArray() methods of List with a complete examples to demonstrate the use of toArray() methods to get an array from list.

1. Use toArray() method without parameter.

Object[] toArray()

Notes

• Returns an array containing all of the elements in this list in proper sequence (from first to last element).

• The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this method must allocate a new array even if this list is backed by an array). The caller is thus free to modify the returned array.

• This method acts as bridge between array-based and collection-based APIs.

Returns

An array containing all of the elements in this list in proper sequence.

2. Use toArray() with array.

<T> T[] toArray(T[] a)

Notes

• Returns an array containing all of the elements in this list in proper sequence (from first to last element).

• The returned array will be "safe" in that no references to it are maintained by this list.

• This method must allocate a new array even if this list is backed by an array).

• The caller is thus free to modify the returned array.

• This method acts as bridge between array-based and collection-based APIs.

Type Parameter

• T - the runtime type of the array.

Parameters

• a - the array into which the elements of this list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.

Returns

An array containing the elements of this list.

Throws

• ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this list.

• NullPointerException - if the specified array is null.

Arrays class provides a asList() method to get the list containing all the elements of an array.

public static <T> List<T> asList(T... a)

Type Parameter

• T - the runtime type of the element.

Parameters

• a - the array by which the list will be backed.

Returns

A list view of the specified array.

Example 1

Following is the example showing the usage of toArray() and Arrays.asList() methods −

package com.tutorialspoint;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class CollectionsDemo {
   public static void main(String[] args) {
      String[] array = {"A", "B", "C"};

      // array to List conversion
      List<String> list = new ArrayList<>(Arrays.asList(array));
      System.out.println("List: " + list);

      // list to array conversion
      Object[] items = list.toArray();

      for (Object object : items) {
         System.out.print(object + " ");
      }
   }
}

Output

This will produce the following result −

List: [A, B, C]
A B C

Example 2

Following is another example showing the usage of toArray() with specific typed array and Arrays.asList() methods −

package com.tutorialspoint;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class CollectionsDemo {
   public static void main(String[] args) {
      String[] array = {"A", "B", "C"};

      // array to List conversion
      List<String> list = new ArrayList<>(Arrays.asList(array));
      System.out.println("List: " + list);

      // list to array conversion
      String[] strings = list.toArray(new String[0]);

      for (String string : strings) {
         System.out.print(string + " ");
      }
   }
}

Output

This will produce the following result −

List: [A, B, C]
A B C